Experiment to investigate how changing the concentration of hydrochloric acid affects the rate of its reaction with the reactant

Mohamed Maamir

Aim: Is to investigate how concentration of hydrochloric acid affects the rate of its reaction with the reactant. To decide which reactant I will use for my investigation, I will do preliminary experiments to decide this.

Preliminary work

I will be doing some different preliminary experiments. The preliminary will allow me to decide which reactants to use for my investigation and how much of that reactant to use in each experiment.

Experiment 1a: Which reactant reacts best?

Display 3 beakers. In:

Beaker 1 place some chalk
Beaker 2 place some marble chips
Beaker 3 place some strips of magnesium

In each beaker pour some hydrochloric acid and observe the reaction.
Estimate the rate of the reaction on a scale of 0-5 (0= no reaction, 1= very slow, 2= slow, 3= medium, 4= fast, 5= very fast)

I noted down the results, however, I realized that they are too inaccurate for me to base my decisions on them. Therefore, I will set a volume of hydrochloric acid to pour into the beakers and also I will weigh the reactants and make sure they all have the weight.

Experiment 1b: Which reactant reacts best?

Display 3 beakers. In:

Beaker 1 place 2g of chalk
Beaker 2 place 2g of marble chips
Beaker 3 place 2g of strips of magnesium

In each beaker pour 40ml of hydrochloric acid and observe the reaction.
Estimate the rate of the reaction on a scale of 0-5 (0= no reaction, 1= very slow, 2= slow, 3= medium, 4= fast, 5= very fast)

I noted my results and I will keep these because they are fairly accurate. However, I decided to extend my experiment and test marble chips and magnesium again but in a different form. This is because the surface area of the marble chips and magnesium is very random. As we will later figure out, surface area affects the rate of reaction. If the surface area of these two reactants is random then it will affect my results. Therefore I will use magnesium powder and grounded marble chips which will allow me to have more control over the reaction.

Experiment 1c: Which reactant reacts best?

Display 2 beakers. In:

Beaker 1 place 2g of grounded marble chips
Beaker 2 place 2g of magnesium powder

Then follow steps 2 and 3 of experiment 1b.

Observation and Results: Experiment 1b and 1c

Scale of reaction rates:

0= no reaction, 1= very slow, 2= slow, 3= medium, 4= fast, 5= very fast

Decisions

I am looking for a reactant with a rate of reaction of 4 because it would be neither too fast nor too slow. However, I also have to keep in mind that I would need as much control over the experiment as possible. For example: being able to keep control of the surface area and concentration. Although there are three reactants with a rate of reaction of 4, I decide to use chalk for my investigation. This is because out of the three reactants, the only reactant that I will be able to control its surface area is chalk. Chalk is in the form of a cylinder, therefore using the formula; , I will be able to work out the surface area. As for the concentration, I will weigh the cylinders of chalk and make sure that they all have approximately the same weight.

Experiment 2:

Display 1 beaker and place 2g of chalk in it. Pour 40ml of hydrochloric acid in the beaker and observe:

If it is an endothermic or exothermic reaction
What happens to the chalk
What do I see
Rate of reaction

Observation and results

If the chalk dissolves then it is not re-usable after the reaction. If a lot of bubbles come out of the reaction then I will have to come up with a solution to make sure that the bubbles do not get out of the beaker.

Background Knowledge

A chemical reaction is a process by which one or more substances called reactants changes into or more other substances, called products. Reactions occur at different speeds, they have different rates of reaction. In the experiment of this investigation, the two reactants will be chalk and hydrochloric acid.

Hydrochloric acid, HCl, is a solution of hydrogen chloride in water. The concentrated acid is about 35% hydrogen chloride and is corrosive. HCL has a pH of less than 7 and is a common laboratory acid.

Chalk is a sedimentary rock consisting mainly of the skeletons of microscopic organisms composed of calcite (CaCO3). It is distinguished by its white color, crumbly nature, and emission of carbon dioxide gas when in contact with hydrochloric acid. Chalk, when strongly lithified, becomes limestone. Chalk is mainly composed of calcite. Calcite will always bubble when it is in contact with acid. The reason for the bubbling is shown in the formula below:

Calcium Carbonate + Hydrochloric acid Carbon Dioxide + Calcium Chloride + Water

CaCO3 (s) + 2HCL (aq) CO (g) +CaCl2(aq) +H2O(I)

This is the formula for the reactions that will take place in my experiments.

In order for a chemical reaction to occur, molecules of different substances must collide with each other at a certain speed. This is the collision theory. When the particles collide, their bonds breakdown to form new ones. However, most collisions are not successful. This is because a minimum amount of kinetic energy is needed to break down the bonds. The minimum energy needed to start any chemical reaction is called the activation energy. When collisions are successful they are called 'fruitful collisions.' All rate controlling factors have to do with collision frequency and energy of reactant particle collision. I will explain activation energy when I write about the effect of temperature on the rate of reaction because they are very well linked.

There are many factors that can affect the rate of reaction. I will explain most of these factors and see how they are relevant to my investigation and if these factors will affect the specific reaction of chalk and hydrochloric acid.

Effect of temperature

Temperature is an important factor in all reactions. As the temperature increases, the rate of reaction increases. When a mixture of substances is heated, the particles move faster. This has two effects:

Since the particles are moving faster, the collision frequency increases between reactant molecules. Thus, the rate of reaction increases. However, this is not the main reason for the increase of the rate of reaction.
As most molecular collision do not result in chemical change, the colliding molecules must have a minimum kinetic energy called the activation energy which is shown in Figure 1.

Energy Level Diagram

The purple arrow represents this minimum energy needed to break bonds, which is Activation energy.
Going down the other side represents the new bonds formed in the reaction. The red arrow represents the energy released- exothermic reaction. In a reaction, it is the activation energy which is important in terms of temperature and rate of reaction.

When the substances are heated, the molecules have a greater average kinetic energy, therefore, a greater proportion of the particles have the required activation energy to react. This means that there is an increased chance of successful collisions that greatly increases the rate of reaction. In this experiment, I am investigating how concentration affects the rate of reaction. Therefore, the temperature will be kept at room temperature throughout all the experiments.

Effects of pH

The pH does affect the reaction because chalk reacts to acids and the reaction is with an acid. The stronger the acid, the bigger the reaction.

Effect of Concentration of Substrate

Increasing the concentration of the substrate means that there will be more substrate molecules in dm3. The more molecules of the substrate there are in the same amount of volume means that it will collide more frequently with the calcium carbonate and react with it. Hence, increasing the rate of reaction. Of course, I am investigating how substrate concentration affects its rate of reaction with chalk. Therefore, I will be changing the concentration of the substrate to investigate this. As we know that the reaction between hydrochloric acid and chalk produces three different substances: Calcium chloride, Carbon dioxide and water. In my experiment I will measure how mass is lost during the first four minutes of the reaction.

Effects of Surface Area

An increase in surface area means that there is a larger area and therefore larger number of exposed reacting molecules to act. This increase in collisions causes an increase in rate of reaction. This is a factor that can optimize reaction rates. I only want the reaction rate to be affected by the concentration of Hydrochloric Acid. Therefore, the surface area will not change because I will use the same cylinders of chalk in every experiment.

Effects of Amount of Substrate

Increasing the amount of substrate means that there will ...

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Effects of Surface Area

Effects of Amount of Substrate

Increasing the amount of substrate means that there will be more substrate molecules reacting with the molecules quicker and the mass will reduce. This will make the test unfair as I am measuring the amount of mass lost during the first four minutes of the reaction. Also, I only want the reaction to be affected by the concentration of hydrogen peroxide. Therefore I will use a measuring cylinder to make sure that I will use only 100ml of hydrochloric acid every time.

Effects of Reactant Concentration

If the reactant concentration increases then more reacting molecules are exposed. Hence, making the reaction faster. I will keep the reactant concentration constant throughout my experiments by weighing the cylinder of chalk and make sure that it weighs between 2 to 3 grams.

Effects of Pressure

When one or more of the reactants are gases, an increase in pressure also means an increase in rate of reaction. The increase in pressure forces the particles closer together. This causes more collisions and increases the rate of reaction. This will not affect my experiment as none of the reactants are gases.

Effect of Catalyst

A catalyst is a substance, which can alter the reaction but remains chemically unchanged at the end of the reaction. Catalysts speed up reactions by providing an alternative pathway. With a catalyst, less activation energy is needed. Hence, increasing the rate of reaction. This is shown in figure 2.

Energy Level diagram

Plan

The aim of this investigation is to investigate how changing the concentration of hydrochloric acid affects the rate of its reaction with the reactant. Therefore, the only factor that will change throughout the experiments will be the concentration of the hydrochloric acid. Chalk will be used as the reactant as it gave me the most positive results from my preliminary work: It reacts well with hydrochloric acid and we can control its surface area. For every trial I will use one cylinder of chalk which has a surface area of 8.6cm2 I will weigh the chalk every time and make sure that it weighs around 1.5g. This is because the concentration of the chalk could change but I want to keep it constant. In the preliminary work I used 40ml of hydrochloric acid of molar 3. I decided to use 100ml of hydrochloric acid in every trial for two reasons:

In the preliminary I used 40ml of molar 3. I will be using molar 1 and 2 as well. In case the reaction between the chalk and hydrochloric acid will be too weak at molar 1 and 2 then I use a larger amount to keep up a good reaction. If the reaction would be too weak then my graph would be hard too analyze as the difference between each concentration would be too little.
If I use 100ml of hydrochloric acid it would be much easier for me to change its concentration. I will explain how to do this later.

As mentioned earlier, the hydrochloric acid breaks down the calcium carbonate into three different substances: Carbon dioxide, Calcium chloride and water. Carbon dioxide is a gas; therefore weight is lost throughout the reaction. Therefore, in this investigation, I will measure how much weight is lost throughout the first four minutes of the reaction. This will allow me to work out the rate of reaction.

To keep this a fair test several factors will have to stay constant due to the collision theory. The rate of reaction depends on how often and how hard the reacting particles collide with each other. Therefore, more collisions increase the rate of reaction. The following increases the number of collisions and that is why they have to be kept constant:

Surface area of chalk- Changing the surface area will change the speed of reaction. As we increase the surface area, we increase the rate of reaction. Increasing the surface area leads to an increase in collisions which causes an increase in rate of reaction. I only want the reaction rate to be affected by the concentration of Hydrochloric Acid. Therefore, the surface area will be the same throughout the experiments.

I will not have to do anything to keep the surface area of the chalk the same because it is provided in equal pieces of cylinders. So they all already have the same surface area.

Mass of chalk- If the reactant concentration increases then more reacting molecules are exposed. Hence, making the reaction faster.

Although the chalk is provided in equal pieces, I realized that sometimes that the mass is different from one another. Therefore I will have to weigh every cylinder of chalk that I will use and make sure that it weighs approximately 1.5g.

Temperature- As the temperature increases, the rate of reaction increases. This is because heat increases the random movement of molecules. This means that when a lot of energy is transferred into kinetic energy, a greater proportion of the particles have the required activation energy to react and there is an increase in molecule collisions. This means that there is an increased chance of successful collisions that greatly increases the rate of reaction.

In this experiment, I am investigating how concentration affects the rate of reaction. Therefore, the temperature will be kept at room temperature throughout all the experiments.

As I am investigating how concentration of hydrochloric acid affects the rate of reaction, I will be changing the concentration throughout the reactions. I will test the reaction with 5 different concentrations. This will allow me to have a good display of results on my graph. These are the five different concentrations that I will use for my experiment:

1M, 1.5M, 2M, 2.5M, 3M

The reason why I couldn't use a wider range of concentrations is because we are not allowed to use hydrochloric acid that is above 3M. This is because the acid would be too corrosive. However, this is not the problem; I have been provided hydrochloric acid of concentration 2M and 3M. I will need to produce the other concentrations by adding a calculated amount of distilled water to the acid. The table below shows the calculations to get the other concentrations.

Apparatus List

At least 15 cylinders of chalk
A beaker
A 200ml graduated cylinder
Stop watch
Hydrochloric acid ( 2M and 3M)
Funnel
Dilute water
Balance with readings to 2 decimal places
Safety glass

Diagram of set up experiment

Method

I will display a balance and plug it to the electricity. Next to the balance I will display a beaker that can hold 500ml.
I also have to get the hydrochloric acid of 2M which is in a big bottle and the cylinders of chalk. Before starting the experiment I have to do two things:

Weigh the chalk on the balance and make sure that it weighs approximately 1.5g. I have to do this because the mass can vary between the equal pieces of chalk. This shows that some are more concentrated than others. As we know, if the reactant concentration increases then the rate of reaction of reaction increases. I am investigating how concentration of the substrate affects the rate of reaction. Therefore, that is why I have to weigh the cylinders of chalk.
Produce the concentration of hydrochloric acid needed for that particular experiment if necessary. Only hydrochloric acid concentration 2M and 3M are provided. I need a wider range of concentrations in order for me to get a good set of results that I can analyze. I will use five different concentrations: 1M, 1.5M, 2M, 2.5M and 3M.

1M- Use the funnel to pour 50ml of 2M hydrochloric acid into the 300ml graduated cylinder. Then pour 50ml of distilled water into the same measuring cylinder and that will make 100ml of 1M hydrochloric acid.
1.5M- Use the funnel to pour 50ml of 3M hydrochloric acid into the 300ml graduated cylinder. Then pour 50ml of distilled water into the same measuring cylinder and that will make 100ml of 1.5M hydrochloric acid.
2.5M- Use the funnel to pour 83ml of 3M hydrochloric acid into the 300ml graduated cylinder. Then pour 17ml of distilled water into the same measuring cylinder and that will make 100ml of 2.5M hydrochloric acid.

Pour 100ml of hydrochloric acid concentration 1M into the beaker. I have to make sure that the beaker is dry and has no other substance in it. If the beaker is wet it can change the concentration of the hydrochloric acid, although only a little bit. However, I want to keep my experiment as fair as possible in order for me to get reliable results.
Get my stopwatch and set it to zero. Place the beaker onto the balance as carefully as possible (not to confuse the balance as it is very sensitive)
Re-set the balance to zero. Then slowly drop the chalk cylinder into the beaker and as soon as it is in the beaker start the stopwatch. I have to put the cylinder of chalk into the beaker as slowly as possible to avoid too much movement in the hydrochloric acid that is already in the beaker. Mixing the two substances is a factor that would affect the rate of reaction. I don't want this factor to be introduced into my experiments. When the chalk gets in contact with the acid it starts reacting and I need to start the stopwatch as soon as the reaction starts. Therefore, I have to be very quick and precise.
Every 10 seconds I have to write down the readings on the balance. I have to do this for 4 minutes.
Then I have to repeat this experiment again two times with no changes, but I will need to use another beaker or wash the beaker and dry it, a new cylinder of chalk and unused hydrochloric acid.

Afterwards, I will repeat all of the above for every different concentration of hydrochloric acid that I will use; which is 1M, 1.5M, 2M, 2.5M and 3M. For each concentration I need to do the experiment three times. From the three set of results for each concentration I will calculate the total and average so my results will be as accurate and reliable as possible.

Prediction

I predict that the higher the concentration, the higher the rate of reaction. This is due to the effects of concentration on the rate of reaction. Concentration increases the number of collisions as the higher the concentration the more molecules of the substrate there will be in the same volume. Therefore, the reacting particles will react more often: The calcium carbonate will meet the substrate molecules quicker which means that the carbon dioxide will be released from the reaction quicker. Therefore, mass will be lost in a shorter period of time and this makes the rate of reaction faster.

I also predict the increase in concentration and mass loss is proportional with the ratio of 1:1. If the concentration doubles, so does the rate of reaction.

Fair Test

In order to keep the experiment a fair test and get accurate and reliable results I need to:

Use a dry beaker for every experiment. If it is wet it could affect the concentration of the hydrochloric acid, although only a little bit. However, I want to keep my experiment as fair as possible to get reliable results.
When I use the hydrochloric acid from the bottle I have to take off the lid and put it back on as quickly as possible. This has to be done for two reasons:

Hydrochloric acid evaporates
Hydrochloric acid decomposes when in contact with sunlight.

Weigh the chalk as accurately as possible because if the concentration of the chalk changes, then there would be another factor affecting the rate of reaction. Remember, I only want the concentration oh hydrochloric acid to affect the rate of reaction. If another factor affects the rate of reaction then my results will not allow me to see how the concentration of hydrochloric acid affects the rate of reaction.
Make sure that the cylinder of chalk used is new and is not cracked or dented. Otherwise, the surface area will affect the rate of reaction. As I have explained earlier, I don't want any other factor to affect the reaction.
When mixing the hydrochloric acid with dilute water, measure the amounts accurately in order to get the correct concentration. If I do not get the correct concentration my results will be incorrect.
When placing the cylinder of chalk into the beaker with hydrochloric acid, I have to do it as softly as possible without creating too much movement within the hydrochloric acid.
When the chalk is placed in the beaker I have to start the stopwatch as soon as possible because I have to start the time as soon as the reaction starts, therefore, I have to do this as quickly as possible for me to get accurate timings.
When doing the experiment I have to turn off the air conditioner. I have to do this for two reasons:

The AC affects the readings on the balance and random numbers appear. For me to get correct results I need the balance to function properly.
I want the temperature to be at room temperature and the AC can make the room too cold which would slow down the rate of reaction.

Obtaining Results

Every ten seconds I recorded the mass during the first four minutes of the reaction. I did this for several different concentrations. The following tables display my results and calculations.

I have highlighted the anomalies in red. I have realized that they weren't normal. The range in the total in mass loss is too wide therefore, the result is unreliable. In the analysis I will explain the cause of these anomalies.

Now I need to gather all the results that I need into a table to plot my graph. For my graphs I only need the average loss mass for each concentration.

Main table of results

From the above results I need to calculate the rate of reaction for each concentration because I will make a graph that will show concentration and rate of reaction.

Rate of reactions is how much reactant is used up or how much product is formed in a certain amount of time. Therefore, I need to divide mass loss by time taken. Time taken is 4 minutes because I recorded the results for the first four minutes of the reaction.

0.5M- 0.33/240= 0.001375g/s
1.0M- 0.56/240= 0.002333g/s
1.5M- 0.86/240= 0.003583g/s
2.0M- 1.12/240= 0.004666g/s
2.5M- 1.10/240= 0.00491g/s
3.0M- 1.28/240= 0.005333g/s

Now I need to gather all the rate of reactions that I have calculated for each concentration and put it into a table.

Analysis

From my table of results, I made two graphs. The first graph shows mass loss during the first four minutes of the reaction against the concentration of hydrochloric acid. The second graph shows the rate of reaction against the concentration.

From the first graph we can see that the higher the concentration of hydrochloric acid, the higher the mass loss during the first four minutes of the reaction. The mass loss indicates that CO2 is being released from the reaction. This is because:

Calcium Carbonate + Hydrochloric acid Carbon Dioxide + Calcium Chloride + Water

The higher the mass loss the more CO2 is released. Therefore, the faster the reaction. From what we see on the graph, it seems that the mass loss and concentration are directly proportional. As one increases, the other increases.

We realize that there is an anomaly on the graph. I have circled it to show that that results isn't accurate as it doesn't follow the general trend. The graph shows that the mass loss for a reaction with concentration 2.5M is 1.10g. Whilst with concentration 2M it is 1.12g. This goes against the pattern of the results and contradicts my prediction as there is a higher mass loss with concentration 2M than with concentration 2.5M. According to my prediction and the rest of the results, it should be the inverse. I will ignore this result as it is a most probably an anomaly produced due to an error or an inconsistency in the procedure. I might have made a mistake while making the concentration. Remember, I had to mix hydrochloric acid with distilled water to produce the concentration I needed. I used hydrochloric acid 3M to produce the 2.5M; perhaps I have mixed the acid with too much distilled water. Then as a result, the acid of 2.5M was maybe even weaker than the 2M acid.

However, looking at the other results, we see that the reaction with acid 3M had the highest mass loss: 1.28 g. While with acid 0.5M there was only a mass loss of 0.33g.

If we use the above results, you will find that when the concentration grows by 6, the mass loss grows by 4. This means that the increase in concentration and mass loss is proportional with the ratio of 3:2. In my prediction, I predicted that the proportion would be 1:1. My calculated proportion shows that my predicted proportion is incorrect. I will explain later why this doesn't completely disprove the predicted proportion.

My results support my prediction. I predicted that the higher the concentration, the higher the rate of reaction. The mass loss is an indirect way of showing the rate of reaction. As it is the mass loss during the first four minutes of the reaction. Therefore, the higher the mass loss, the higher the rate of reaction. Although my results show that the higher the concentration the higher the mass loss which supports my prediction, it is an indirect way of showing the rate of reaction. Therefore, I will plot another graph showing concentration against the rate of reaction. This will allow me to directly prove or disprove my prediction. Remember that I am investigating how changing the concentration of hydrochloric acid affects the rate of reaction.

From the second graph we can see that the higher the concentration, the higher the rate of reaction. The line of the graph shows that the concentration and rate of reaction are directly proportional.

On the graph we see that the rate of reaction with concentration 0.5M is 0.001375g/s. Whilst with concentration 3M the rate of reaction is 0.005333g/s.

If we use the above results we will find that when the concentration grows by 6, the rate of reaction grows by 4. This means that the increase in concentration and rate of reaction are proportional to the rate of 3:2. In my prediction, I predicted that the proportion would be 1:1. My calculated proportion shows that my predicted proportion is incorrect. However, the proportion that I have calculated is only is only one example out of my set of results. By looking at the other results, I realized that my predicted proportion holds. The table below shows this:

My prediction states that the higher the concentration, the higher the rate of reaction. The graph clearly supports my prediction. However, on this graph as well there is an anomaly. I have explained earlier the cause of this anomaly and why I should ignore it.

In spite of the anomaly, there is a strong pattern in my graph suggesting that my prediction is correct. My results show that the higher the concentration, the higher the rate of reaction. This is exactly like my prediction.

The background information I researched backs up the results on my graph as well as my prediction. The concentration affects the rate of reaction because concentration increases the number of collisions. This is because the higher the concentration the more molecules of the substrate there will be in the same volume. Therefore, the reacting particles will react more often: The calcium carbonate will meet the substrate molecules quicker which means that the carbon dioxide will be released from the reaction quicker. Therefore, mass will be lost in a shorter period of time and this makes the rate of reaction faster.

My results backed up with background information leads me to conclude that as the concentration increases, the rate of reaction increases. As my results are fairly accurate and reliable, and my background information is scientific facts, my prediction is strongly supported. My results from the graph are very similar to my prediction.

I have also noticed that the line gets less steep as it reaches concentration 3M. This means that the rate of reaction increases at a slower pace. I guess this is because another factor is introduced into my experiment: surface area.

The reaction takes place on the surface of the chalk; therefore, as time goes by, the surface area decreases. As I have mentioned in my background information, if the surface area decreases, the rate of reaction decreases. At concentration 3 Molar, the surface area of the chalk decreases too quickly, therefore, affecting my results.

Evaluation

From my results, I have found out that the higher the concentration of hydrochloric acid, the higher the rate of reaction. The method I have used to acquire these results is quite accurate and detailed to avoid any errors or inconsistency in my results. My method may not be perfect, but it gave results that showed strong pattern. However, one anomaly was identified in my analysis. This was the 2 Molar results. This anomaly undermines my conclusion to some extent. However, as the other results form a pattern which supports my prediction, the anomaly does not make my conclusion invalid. The anomaly is due to a mistake I have made during the procedure. Probably, I have made a mistake while making the concentration. Remember, I had to mix hydrochloric acid with dilute water to produce the concentration I needed. I used hydrochloric acid 3M to produce the 2.5M; perhaps I have mixed the acid with too much dilute water. Then as a result, the acid of 2.5M was maybe even weaker than the 2M acid. However, the other results were accurate enough for me to produce a firm conclusion. Therefore, my experiments were a success. The results showed a clear pattern which allowed me to analyze and conclude that as concentration increases, rate of reaction increases. I think that this was the best procedure for my experiment, as the graphs were clear in showing us a trend between concentration and rate of reaction.

In the procedure, I measured the mass during the first four minutes of each reaction. I did this to find the mass loss which I would then be able to calculate the rate of reaction. I could have found rate of reaction by amount of gas released from reaction in a certain amount of time. I decided not to use this method because many errors could then be introduced into my results, as it is more complicated to set up. My procedure was much simpler; therefore, it would help me to eliminate errors.

In my procedure, there are many factors that I controlled and kept constant. I controlled the surface area of the chalk, amount of substrate, pH of substrate and mass of chalk. My procedure, however, has a disadvantage; there is no way of controlling the room temperature throughout the experiments. There may have been some fluctuations in the temperature which could have affected the reaction rate. I cannot use a water bath because I am using a scale to weigh the mass loss. Also, the air conditioning may have been turned on whilst the reaction took place. The wind could have taken away the carbon dioxide which would affect the rate of reaction. I could have avoided this by having the reaction take place in a glass container.

I mentioned earlier that I controlled the surface area of the chalk. It is true because all the chalk cylinders that I used for the reaction had the same surface area. However, during the reaction I cannot control its surface area. The reaction takes place on the surface of the chalk; therefore, the surface area of the chalk decreases as it dissolves. As we know, surface area is a factor that affects rate of reaction: The larger the surface area, the higher the rate of reaction. I realized that there is nothing I can do to solve the problem because the chalk dissolves. Only when I used concentration 3 molar did this factor affect my results. 3 molar is the strongest concentration that I used and the surface area of the chalk decreased rapidly, therefore, the rate of reaction decreased.

We can see this on the graphs that I analyzed. I can't avoid this factor, but I can reduce its effects by recording the mass for only two minutes instead of four minutes. If I record the results for only two minutes, the surface area wouldn't decrease as much if I recorded the results for four minutes. Therefore, the effects of this factor would be reduced.

The formula for the reaction that takes place between chalk and hydrochloric acid is:

Calcium Carbonate + Hydrochloric acid Carbon Dioxide + Calcium Chloride + Water

I noticed that if water is released from the reaction, then the water would start to dilute the acid. Therefore, the concentration of the acid gradually becomes weaker. This is a factor that affects my results and I should have avoided this. The solution to this problem is the same for the problem of the surface area of the chalk: Reducing the time I take down the results would minimize the effects of this factor. If I record my results in a shorter period of time, the water wouldn't have as much time to dilute the acid.

The chalk I used is a white blackboard chalk. This type of chalk is usually coated to stop it from leaving residue on the fingers. This would reduce the reaction surface of the chalk, lowering the rate of reaction. I should have used pure mineral chalk to avoid this.

My results are based on the experiments I have done. My experiments were done following a detailed method I have written and I have also paid much attention to keeping the experiment a fair test. For each concentration, I have repeated the experiment three times and I calculated the average of the results I got for each concentration. The procedure that I have followed convinces me that my results are reliable. However, the fact that other factors were introduced into my experiment, means that my plan and method have to be more detailed in order to get even more accurate results. I would also consider doing more repetitions for each concentration to get an even more accurate average.

To obtain more evidence to support my conclusion I could have used a wider range of substrate concentration to make sure there is no limit to the pattern I found in my analysis. Another thing I could have done, was to do more repeats for each concentration, this would make my results more accurate when calculating the average.

To extend this investigation I should investigate reactions between hydrochloric acid and other reactants such as magnesium and sodium thiosulphate. This would allow me to see if the concentration does not only affect the reaction with chalk. The reaction between hydrochloric acid and magnesium would be:

2HCL+Mg MgCl2+H2

Hydrogen is released from the reaction. Therefore, I could use the same method to calculate the mass loss. However, if I use a different method, it would allow me to see if the results will be consistent. I would collect the hydrogen gas released in a syringe. I would measure how much hydrogen gas is released in a certain amount of time. This would allow me to calculate rate of reaction and see if concentration affects rate of reaction.