How does changing the concentration of hydrochloric acid affect the rate of reaction between the acid and calcium carbonate chips?

GCSE Chemistry Coursework

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Introduction

The rate of reaction is a measure of a change that happens in a unit of time.

My aim is to measure the rate of reaction between hydrochloric acid and calcium carbonate chips by using different concentrations of hydrochloric acid. The equation for this experiment is

I plan to measure the amount of carbon dioxide gas given off every five seconds.

Theory

According to the collision theory, for molecules to react together they need

To collide
Collide with enough energy to achieve an activated complex. This energy that they need is activation energy.
The right orientation for the reaction to go ahead.

Anything that increases the above factors increases the rate of reaction i.e. Temperature, surface area, concentration, pressure and catalysts.

The effect of Temperature

A reaction can also be made to go faster or slower by changing the temperature of the reactants. The higher the temperature the faster the rate of reaction. This is because heat gives the particles more energy so as to make more successful collisions. This is the reason why the temperature in this experiment is kept constant.

The effect of concentration

A reaction can also be made to go faster or slower by changing the concentration of the reactant. A reaction goes faster when the concentration of a reactant is increased. It is on this theory that the whole experiment is based on.

The effect of Surface area

A reaction can also be made to go faster or slower by changing the surface area. The rate of a reaction increases when the surface area of a solid reactant is increased. This is the reason why the surface area of the chips is similar.

The effect of a catalyst

A catalyst is a substance that changes the rate of a chemical reaction but remains chemically unchanged itself. For this reason a catalyst is not used in the experiment.

The effect of Pressure

A reaction can also be made to go faster or slower by changing the pressure at which the reaction is taking place. This mainly increases the rate of reaction of gases more than solids.

Hypothesis and Preliminary Work

I predict that if the concentration of the acid is increased, it will have a much faster rate of reaction than the weaker solution. I have predicted this from my knowledge of a previous experiment and scientific knowledge. Recently I did a similar experiment involving magnesium and hydrochloric acid instead of the calcium carbonate and hydrochloric acid, from this I found that the higher concentration solutions the faster the reaction, therefore I believe that I will obtain a similar graph of results. I think that this happened because the more HCl in the solution the greater the concentration of Hcl particles and there will be more particles colliding and therefore a faster rate of reaction. Also in the 2.0M solution compared with the 0.5M solution there are much more HCl particles in the same volume so they are going to collide with each other more often.

The diagram below compares the particle structure of a strong acid with that of a weak acid.

Diagram 1: A comparison of the particle structure of a Weak Acid and Strong Acid

Below are the results of the experiment I carried out with magnesium ribbons and hydrochloric acid. These results will justify my hypothesis as I believe that the marble chips will work in a similar way.

The equation for this experiment is: -

Mg + 2HCL ➔ MgCl2 + H2

The gas produced in this experiment is hydrogen not carbon dioxide. I did this experiment with 1mol and ...

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The diagram below compares the particle structure of a strong acid with that of a weak acid.

Diagram 1: A comparison of the particle structure of a Weak Acid and Strong Acid

The equation for this experiment is: -

Mg + 2HCL ➔ MgCl2 + H2

The gas produced in this experiment is hydrogen not carbon dioxide. I did this experiment with 1mol and 0.5mol of HCl acid (25ml). Below is a table of results for both experiments

1mol HCL+Mg

0.5 HCL+Mg

From the table of result above it can be noted that the rate of reaction is higher with a 1 mol concentration compared to a 0.5 mol concentration. The graphs shown below can be compared to see the rate of reaction. It can be noted that the graph of the 1 mol concentrated acid has a steeper curve than the one of the 0.5 mol acid.

Below is a graph showing the amount of H2 given off when Mg (25ml) reacts with 1mol of HCL acid. The rate of reaction is 0.79 cm3/s

Below is a graph showing the amount of H2 given off when Mg (25ml) reacts in 0.5mol of HCL acid. The rate of reaction is 0.23 cm3/s

Apparatus: -

Flask
100cm3 Gas syringe
50cm3 of 2 mol/dm3
50cm3 of water
Medium Calcium Carbonate chips
Delivery tube
Clamp stand
Stop watch
Electronic Pan Balance
Water bath

Safety: -

Safety goggles
Apron
Tie long hair back
Remove any loose jewellery
Do not leave work unattended
Wipe out any acid spills

Method: -

Below is a diagram of how the apparatus should be set up while carrying out the experiment.

Diagram 2: Apparatus Set-Up

Follow this method step by step

Firstly make sure that you are wearing a lab coat and goggles to protect your eyes.
Set up the gas syringe on to the clamp stand. (Make sure that the gas syringe is not closed too tight on the clamp as this may cause your experiment not to be very reliable.)

Connect a delivery tube to the gas syringe and a bung.
Use medium size chips for this experiment as the rate of reaction will not be either too slow or too fast.
Using the electronic pan balance, measure 2 grams of the medium size marble chips. Measure four more batches of marble chips each with a mass of 2 grams. Keep each batch of chips separately.
Make sure that the sizes of the chips are similar as surface area affects the rate of reaction.
Using the table given below prepare the concentration of the acids needed for the experiment.

First measure 50 cm3 of 2 molar acid using a measuring cylinder.
Pour the acid into a flask.
Add the first batch of the 2 gram medium size marble chips to the acid.
Close the flask by connecting it to the bung which is connected to the gas syringe.
Immediately start timing the experiment

Note down the amount of gas produced every 5 seconds in a table similar to the one shown below.

Time the experiment for 90 seconds
Once the experiment is over, empty the flask out and rinse it with water.
Carry out the experiment according to the numbering on the table of concentrations.
Repeat the above step once more so that the results can be averaged. This makes the results more reliable.
Draw a graph for all the averaged results obtained from this experiment.

I am sure that any variations in the results are due to the changes in the concentration of acid because I have eliminated any other factors which could speed up or slow down the rate of reaction. The table below indicates the variables that changes and the reasons why they change; it also indicates the non-changing variables.

Fair testing

I have made sure that the temperature of this reaction is constant by checking the temperature constantly while carrying out the experiment. This is important because heat gives the particles more energy thus allowing more successful collisions to occur more often. This is all done to make sure that the reaction is happening due to the concentration of the acid. This makes sure that the test is fair and there are no other factors affecting the rate of reaction.

I am sure that my results are reliable and accurate because I have made sure that the surface area of the calcium carbonate chips is the same, I have also made sure that the temperature is constant and I have also averaged out all my results thus giving me a reliable and accurate result.

Below is what the graph of the reaction will look like. (NB: The graph is only a prediction. It is not the real graph.)

Table of Results

In this section I present my results.

Rate of reaction: 0.63 cm3/s

Rate of reaction = 0.49 cm3/s

Rate of reaction = 0.28 cm3/s

Rate of reaction = 0.18 cm3/s

Rate of reaction = 0.06 cm3/s

Analysis

Analysis of the results

From the results above and the graph, it can be seen that the rate of reaction is higher in the experiments involving higher concentrations. This proves my hypothesis, which is in the plan. Furthermore it can be seen that the experiment involving 2-mol acid made an average of 57 cm3 carbon dioxide gas in 90 seconds, whereas the experiment involving 0.5 mol acid made an average of 5 cm3 only. From the graph at the time of 55 seconds, the 2 mol acid had made 35 Cm3 of CO2 gas where as the 1.5 mol acid had made approximately 28.5 Cm3 of CO2 gas where as the 1 mol acid had made 26 Cm3 of CO2 gas where as the 0.8 mol acid had made 9 Cm3 of CO2 gas and finally the 0.5 mol s acid had only made approximately 4.5 Cm3 of CO2 gas. This proves the collision theory quoted in my hypothesis which states for molecules to react together they need:-

To collide
Collide with enough energy to achieve an activated complex. This energy that they need is activation energy.
The right orientation for the reaction to go ahead.

This proves my hypothesis furthermore. I noticed from the graph that the rate of all the reactions was faster in the start as opposed to the end. I think this is because at the start of the experiment there are more particles than at the end so they are colliding more often into each other at the start. The graph has a positive correlation which means as the time increases the volume increases.

As I drew a curve of best fit in my graph for my averaged results, I found that I had a couple of anomalies. These anomalies are justified in the evaluation. The graph below shows the rate of reaction against the concentration of HCl acid.

Evaluation:-

After carrying out the experiment I am fairly sure that my results are reliable. I think so because as I repeated the experiment twice I found the results to be very similar though I cannot trust my results fully. I say this because I am sure that it was not only concentration that affected the rate of reaction. I mentioned in my plan that I wanted to use a water bath to control the temperature and also make sure that it was not a factor affecting the rate of reaction. Unfortunately, the water bath was not available for use. Furthermore I am sure that the surface area also affected the rate of reaction. This is because when I was carrying out the experiment I estimated the size of the chip as I was not sure in anyway of ensuring that the surface area of the marble chip was the same. This experiment could have been more accurate and more reliable if the temperature and the surface area had been kept the same.

As I mentioned in the analysis of the results, I had a couple of anomalous results on my graph. I think this happened while I was carrying out the experiment. I noticed that the gas syringe jerked as the reaction took place which made it difficult to read the actual volume made. The other reason is that there could be human error usually known as the parallax error. This is when the results from the syringe are not read accurately. I noticed that when I used the bung to close the flask the syringe moved a little bit making my results inaccurate. This ‘bung effect’ was inevitable as I tired to correct it but was in vain.

I could do further work on this experiment to improve my prediction by using more accurate equipment. I would try to keep the temperature constant by using a water bath. I if had more time I would have carried out the experiment more times and compare the results. This would make the experiment results more reliable and also accurate if the results compare to be similar. I would also use a gas syringe which can handle a greater volume of gas so I could finish the whole experiment. I could use a larger range of acid concentrations so as to have more graphs from which to compare.

Bhavin Patel - -