An investigation into how changing one variable influences the rate of reaction between marble chips and hydrochloric acid.

Ross Bowden Chemistry SC1

CaCO3(s) + 2HCl(aq) CaCl2(aq) + H2O(l) + CO2

Planning

List of variables which could affect the rate of reaction

Temperature: - Temperature will affect the rate of reaction because the higher the temperature of the solution of hydrochloric acid, water and marble chips, the more kinetic energy the presents particles will have and they will therefore collide with one another more and more successfully.
Mass of Marble Chips: - The mass of the marble chips could affect the rate of reaction because on average, if you increase the mass of the marble chips, you increase the amount of chips in there and will therefore increase the surface area of the present chips. An increased surface area could affect the rate of reaction because if the surface area is increased, the solution of hydrochloric acid and water will have more area to work on and would therefore have more reactions more quickly thereby increasing the rate of reaction.
Concentration of solution: - The concentration of the solution of distilled water and hydrochloric acid will affect the rate of reaction because the higher the concentration, the faster the rate of reaction because they are directly proportional to each other. If you double the strength of the solution, the rate of reaction will double.

A list of variables I will attempt to keep constant.

I will attempt to keep the temperature constant by doing my experiment in the same area and doing all the experiments at a certain time of day so there will not be many or any temperature fluctuations. I made sure of this by keeping a thermometer present at all times to check whether there were any temperature fluctuations.
I will try to keep the surface area of the marble chips constant in every experiment by keeping the mass of the marble chips constant in every experiment. The surface area of the marble chips plays a great part in this experiment and when you select your chips there might be some powder on them that must be removed. This is done by washing the chips in hydrochloric acid and then rinsing them in water to stop them reacting. I then kept my cleaned chips separate from the others that were not cleaned to avoid any dust or power being transferred onto the clean chips.

The variable which I have decided to investigate

I have decided to investigate the affect on the rate of reaction when changing the concentration of the hydrochloric acid. I will vary the molarity of the acid by diluting it with distilled water. I will see how long it will take to collect 20cm³ of CO2.

Preliminary work

For my preliminary work, I have decided to investigate whether or not to keep the same marble chips after every experiment or replace them with new ones. I decided to use 3 grams of marble chips because anything over 3 grams seems too much and any thing under seems too little. I discovered that after every experiment, the mass of the marble chips decreased as they were reacting with the acid and therefore their surface area decreased so the next experiment took longer to produce the same amount of gas.

My results ...

This is a preview of the whole essay

Preliminary work

My results show that the mass of the marble chips decreased after every experiment which then made the following experiment slower because the solution of hydrochloric acid and distilled water had less surface area of the chips to work on because the chips were being broken down and dissolved.

I will therefore use fresh chips every time I do an experiment to ensure accuracy and reliability. If I used the same chips after every experiment, I would use more chips because the percentage change in the mass of the chips would be less using more chips.

Prediction

The concentration of the solution of distilled water and hydrochloric acid will affect the rate of reaction because the higher the concentration, the faster the rate of reaction because they are directly proportional to each other. If you double the strength of the solution, the rate of reaction will double.

Anticipated Range of Measurements

The range of measurements that I plan to take will be the rate of reaction, which means I will have to record the time it takes to collect a certain amount of Carbon Dioxide providing I have enough hydrochloric acid and enough marble chips to product a certain amount of gas.

Number of reading I intend to take

I will do three repeats of each concentration, so I will get four results for each concentration to prevent me using anomalies. However, if I get a big range between two or more results I will repeat the experiment one or two more times to avoid anomalies. Each experiment will be done using new, clean chips and fresh acid and water solution.

Method

Apparatus I intend to use: -

2x 50cm³ measuring cylinders
2x 250cm³ beakers
Trough
Conical Flask
Delivery tube
Clamp
Stop watch
2.0 molar Hydrochloric Acid
Distilled Water
Top Pan Balance
Paper towels
Safety Goggles
Marker Pen or Elastic Band

Diagram:

Step by Step Bullet Pointed Method

Wear safety goggles in case any acid splashes into your eyes.

Weight out 60 grams of marble chips and wash in hydrochloric acid for no longer than 30seconds to get rid of dust. Rinse with water to stop the chips reacting and leave to dry on paper towels.

Separate the 60grams of chips into 20 lots of 3grams each, four lots for every concentration.

Place the conical flask in the clamp.

Place marble chips in the conical flask and make sure the upturned section of the delivery tube is in the open end of the measuring cylinder.

Fill the measuring cylinder to capacity by submerging it in the water in the trough and displacing the air in the cylinder with water by upturning it in the water until there are no air bubbles present.

Using your marker pen or elastic band place a mark around the 20cm³ mark on the cylinder.

Measure out your desired quantities of acid and water solution into a 50cm³ cylinder. Either 50cm³ of acid; 40 cm³ of acid and 10cm³ of distilled water; 30cm³ of acid and 20cm³ of distilled water; 20cm³ of acid and 30cm³ of distilled water; 10cm³ of acid and 40cm³ of distilled water.

Start the stopwatch and let it run while making sure the apparatus is all in order.

When the stopwatch reaches 8 seconds add the hydrochloric acid and distilled water solution to the conical flask and quickly and firmly place the stopper into the top of the conical flask.

When 20cm³ of gas has been collected, stop the stopwatch and record the time. Remove the stopper and wash out the conical flask and throw away the used chips. Repeat as many times as needed.

Quantities of chemicals used

50cm³ of hydrochloric acid (2.0 molar)

40cm³ of hydrochloric acid and 10cm³ of distilled water (1.6 molar)

30cm³ of hydrochloric acid and 20cm³ of distilled water (1.2 molar)

20cm³ of hydrochloric acid and 30cm³ of distilled water (0.8 molar)

10cm³ of hydrochloric acid and 40cm³ of distilled water (0.4 molar)

Accuracy

I used a clamp to hold my conical flask in place during the experiment. This was done in order to prevent any one from bumping or shaking the conical that would have increased the speed at which gas is produced because more solution would have been in contact with the marble chips.
I used two separate measuring cylinders to measure out the volumes of hydrochloric acid and distilled water because it’s quicker and easier than using a burette. It can be just as accurate as long as you avoid the error of parallax.

Reliability

Each experiment for every concentration of acid will be repeated four times to make sure that any anomalies are highlighted and I am aware of them so I don’t take them into account when averaging out results for drawing a graph.

The marble chips will be “washed” in hydrochloric acid before the experiment and before being weighed out to ensure they contain no calcium carbonate powder or dust which could affect the results.

Safety

Safety Goggles must be worn at all time because we are working with hydrochloric acid, which is an irritant.
Ties should be tucked away.
Apparatus should be kept away from the edge of the desk to prevent it from falling off.

Obtaining Evidence

I obtained this evidence using the method I stated in the planning section.

* Anomalies (excluded from average time and rate of reaction)

Analysing and Considering Evidence

My Comparison

From my results, I can see that my prediction was accurate. The concentration of distilled water and hydrochloric acid did affect the rate of reaction; this is evident in my results, as shown in my graph. The higher the concentration was, the fast the rate of reaction was when it reacted with the marble chips. My results prove that the concentration of the acid and the rate of reaction are directly proportional to one another because as shown from the line of best fit in my graph, the rate of reaction increases steadily with the increase in concentration. My results show that as the concentration of the solution increases, the time it takes to collect 20cm³ decreases significantly.

Conclusion

This investigation has shown me that the higher the concentration of hydrochloric acid and distilled water the faster the rate of reaction when it comes into contact with the marble chips. The greater the number of hydrochloric acid particles there are present in the solution, the faster the reaction will occur because there are more particles to collide and therefore react with the marble chips at one time. This is proved by the collision theory which states “the more collisions between particles in a given time, the faster the reaction”.

Evaluation

My Method

I think my method of carrying out the investigation was good because it was simple and easy to follow and lead to a clear objective that I had to reach. It was straightforward weighing out the masses of the marble chips and setting up the apparatus, as well as recording the time it took to collect the 20cm³ of CO2.

What could have made my results inaccurate and unreliable is that it is very difficult to keep the surface area of the marble chips the same and also the elapsed time when you add the solution of hydrochloric acid and distilled water to the marble chips in the conical flask and you have to place the plug in the top of the conical flask and then start the stop watch at the same time.

Repeating the investigation and extension work

If I had to repeat this investigation, I would use calcium carbonate powder instead of the marble chips because the surface area is more consistent. I would then only have to worry about the mass of the powder that is an easy variable to control using a top pan balance.

I would use a beurette to measure out the volumes of the various liquids instead of the measuring cylinder because a beurette is more accurate and reliable, however it does take a lot more time.

I would repeat the experiment more times, at least over 5. This would give my results a good reliability and good, accurate, average.

I would also extend the limits of my experiment and investigate more concentrations, in divisions of 5 instead of 10. (E.g. 25cm³ Hydrochloric Acid and 25cm³ Distilled Water). This would give me a more reliable line of best fit as I would have more points to plot on my graph.

I would use a gas syringe instead of an upturned measuring cylinder because a gas syringe is infinitely more accurate and would simplify the whole process of having to make sure the plug is in tightly and the delivery tube is connected before starting the stopwatch.

My Results

Most of the points on my graph went through my line of best fit, however, there were 1 or 2 that did not, so I can’t call it an exponential line. There are quite a few anomalies in my table that suggest that my results were unreliable and inaccurate. These could have happened for a number of reasons such as:

Some of the chips I used could have had some power or dust present on their surface, despite my attempts on cleaning them.
I could have taken too long on starting the stopwatch when I placed the plug in the top of the conical flask.
The upturned measuring cylinder could have been inaccurate, or not sitting level when I read off 20cm³.