Investigation into the effect of concentration on the rate of reaction between calcium carbonate and hydrochloric acid

Charlotte Bell 10A

Investigation into the effect of concentration on the rate of reaction between calcium carbonate and hydrochloric acid.

Introduction

Equation: Calcium carbonate + hydrochloric acid carbon dioxide + water + calcium chloride

CaCO + HCl CO + H O + CaCl

During this reaction Carbon dioxide is formed. We measured the rate of reaction by collecting the Carbon dioxide in a gas syringe and timing how long it takes to collect 30cm of gas in the gas syringe.

Variables that Affect the Rate of Reaction:

-Concentration- if there is a higher concentration of acid then there will be a faster rate of reaction because there will be more reactant particles per unit volume, so there will therefore be more successful collisions occurring.

-Temperature- if the temperature is higher there will be a faster reaction rate. This is because the average kinetic energy of the particles will be higher and therefore the collisions will have more energy so successful collisions will increase.

-Size of chips (surface area)- If the marble chips are larger then there will be more surface area exposed and so there will be a faster reaction rate due to more successful collisions occurring.

-Catalyst- A catalyst lowers the energy needed for successful collisions. So the reaction rate will therefore be faster.

The variable that I am going to study is the concentration of acid.

What is a catalyst?

A catalyst speeds up the rate of reaction by lowering the activation energy (the minimum energy that molecules collide with to bring about a successful reaction). This means that more particles will have a successful reaction.

Examples of catalysts:

Iron: used in making ammonia in Haber process.

N + 3H 2NH

Rhodium + Platinum: used in catalytic convertors in cars to convert harmful pollutant gases into less harmful ones.
Enzymes: Biological catalysts found in living organisms. E.g: amylase converts starch into maltose (sugar). Enzymes are very specific, the substrate fits into the active site like lock and key. Enzymes are temperature sensitive – 37 C is the optimum temperature, if it is any higher than this the protein structure is denatured so the substrate and active site don’t fit together any more. Enzymes are also specific about what pH they work best at. For example, pepsin in the stomach works best in acid conditions.

Preliminary Results

This is a preview of the whole essay

We decided to choose to collect 30cm of gas because the results were closer together and we believed that this would give us more accurate results. We had already decided to use the range of 20-10 for the concentration of acid and 0-10 for the concentration of water, this is because we believe that this range will provide enough information for us to investigate the effect of concentration on rate of reaction. We decided that we would have one person putting the bung into the boiling tube and another starting the stop clock, this way we hoped to make our results as accurate as possible. These results informed me as to which amount of gas we should collect by showing that to collect 50cm of gas it would take much longer to collect the gas, and also the results were further apart, also the rate of reaction may have slowed down towards the end of collecting the gas.

Method

Apparatus

Gas syringe
Boiling tube
Bung
Calcium carbonate (marble chips)
Hydrochloric acid
Boiling tube holder
Stop clock
Scales
Measuring cylinder
Stand and clamps

Controlling the Variables

Temperature: We will keep the experiment at room temperature, to ensure is does not change, we could measure the temperature using a thermometer, however as the reaction occurs the temperature could change according to it as energy is used.

Size of chips: We could make sure that the chips are the same size by checking them before we start the experiment.

Catalyst: We will not add a catalyst.

Method

1. Set up the experiment as shown in the diagram above

2. Using the measuring cylinder, pour 20cm of 1 mole dm hydrochloric acid into the boiling tube.

3. Measure out 5g of calcium carbonate (marble chips) on the scales and then tip them into the boiling tube.

4. Quickly put the bung into the boiling tube, as soon as possible, otherwise the loss of gas increases.

5. Start the stop clock at the same time that you put the bung in and measure how long it takes to collect 30cm of gas in the gas syringe.

6. Take out the bung and pour the contents of the boiling tube into a sieve in a sink and wash out the boiling tube.

7. Repeat the previous steps but with the different concentrations. Make sure that you push the gas syringe back in.

Prediction

Low concentration Higher Concentration

I predict that the higher the concentration of acid, the faster the rate of reaction will be, this is due to the collision theory. For a reaction to occur, particles must collide. There are different factors that affect the collision theory.

The one that is affecting this experiment is the concentration. If the concentration is higher then there will be more particles per unit volume.

Observations

The only possible anomaly that I can see from the preliminary results table is the result for time taken to collect 30cm of gas, and the concentration of acid being 12.

Results for actual experiment.

Rate of reaction:

1/time

Analysing

I have found from my results that my prediction was correct and that as the concentration of acid increases, the rate of reaction increases, and as the concentration of acid decreases, the rate of reaction decreases. This is due to the collision theory. A higher concentration means a higher number of particles in a given volume. If there are more particles, then there will be more collisions and therefore more successful collisions, so the rate of reaction is faster.

Looking at the results that show all three tests, there seems to be an anomaly in the second test for when the concentration of solution is 1.2 moles dm 3. The other two results for this concentration are both close to each other, however the second result is not, it is 10, whereas the other two are 15 and 14.

The relationship between concentration and time is that the higher the concentration of the acid, the less time it takes to react. This is due to the collision theory. If the concentration is higher then there are more acid particles to react with the marble chips and so the rate of reaction is faster.

The relationship between concentration and rate is that as the concentration increases, as does the rate, this is also due to the collision theory; there are more acid particles to react with the marble chips and so the rate of reaction is faster. This is basically the same as time and concentration as the rate is worked out from the time.

I plotted an average graph and a graph that shows the results from all three tests.

The Average time taken graph shows a smooth curve in a negative correlation, although there are a few anomalies. This graph shows the relationship between concentration and time, because we can tell that as the concentration increases the line on the graph goes down, proving that it takes less time to react when the concentration is higher.

The Average rate of reaction graph shows a straight line of best fit in a positive correlation. This graph shows the relationship between concentration and rate of reaction because we can tell that as the concentration increases the line of best fit goes up, proving that the rate of reaction is faster when the concentration is higher. There are some anomalies, which are circled. These could have been because I rounded the results to two decimal places or because some of the gas escaped when the bung was put into the boiling tube.

The graph that shows results from all three tests shows smooth curves in a negative correlation. There are some anomalies and these have been circled. The bung not being put into the boiling tube fast enough caused these, as some of the gas will have escaped. The third test had quite a few anomalies.

Evaluation

I believe that my results are valid, this is because during the experiment we did all that was possible to keep them valid, such as recording them in a clear way, having one person on the stop clock, starting and stopping it and one person putting the bung into the boiling tube and watching the gas syringe and seeing how much gas has been collected. Also, the graphs agree with the prediction and the collision theory in that as the concentration increases the time taken to collect 30cm of gas and the rate of reaction both increase.

The anomalies are highlighted or circled on the graphs and also on the results table. The anomaly that is highlighted on the results table is due to human error. As this measurement was being taken, the stop clock was started late and this is the reason for the lower result.

Any anomalies could be due to design faults in the experiment, for example when we put the bung into the boiling tube some of the gas could have escaped. Using a thistle funnel and a gas syringe could amend this. Although, at the time and with the equipment given to us this was beyond our control.

This would work better because you would not have to put a bung in, as you can see from the diagram above, the gas syringe is already connected to the flask. You would switch a tap and let the acid into the flask that contained the marble chips. The gas would go straight to the gas syringe.

Mini Method for this version

Set up the apparatus as shown above.
Put 5g of calcium carbonate (marble chips) into the flask
Measure out the correct amount of acid or acid and water and put it into the thistle funnel, making sure the tap is switched off.
Switch the tap on and as soon as the gas begins to go into the gas syringe start the stop clock.
Repeat the experiment with the remaining volumes of acid and water.

The method that we used is not the best that we could have used as gas could easily have escaped and there could be human error involved, such as starting or stopping the stop clock.

There are still some factors that could cause an anomaly using this method, for example, human error. Reactions could be delayed when starting the stop clock.

Other anomalies could have been because the gas syringe could have stuck, also, even though we did our best to keep the chips the same size, we could not guarantee they would be the correct size. This would alter the rate of reaction to make it faster or slower. We could possibly have ruled this out by getting a machine to cut the marble chips into precise sizes, but this would take a long time and would be impractical, and is therefore beyond our control. The temperature could also have changed, even though we attempted to keep it the same, although we did not measure the temperature with a thermometer. This was also beyond our control as we did not have the equipment to keep the temperature at a constant level. Another problem that we found was that one of the boiling tubes broke during the experiment and a small bit of glass had fallen out of the bottom of it. This was faulty equipment.

Other than these possible reasons I can conclude that the experiment as fairly and accurately as possible with the equipment and conditions that we were operating in, and that the results are valid.