Investigate the rate of reaction between hydrochloric acid and calcium carbonate.

Surface Area

• If a solid reactant is broken down into smaller pieces the rate of reaction increases.
• The speed increase happens because smaller pieces of the same mass of solid have a greater surface area compared to larger pieces of the solid.
• Therefore, there is more chance that a reactant particle will hit the solid surface and react.
• The diagrams below illustrate the acid-marble chip reaction

Acid partcals


Marble Chips (small)                         Marble Chips (large)

The Effect of Temperature

• When gases or liquids are heated the particles gain kinetic energy and move faster (see diagrams below).
• The increased speed increases the chance of collision between reactant molecules and the rate increases.
• Evan small increases in temperature can have large effects on the rate of reaction

However this is not the main reason for the increased reaction speed.

• Most molecular collisions do not result in chemical change.
• Before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the Activation Energy shown on the energy level diagrams below.

• Going up and to top 'hump' represents bond breaking on reacting particle collision.

• The upwards arrow up represents this minimum energy needed to break bonds to initiate the reaction.

• Going down the other side represents the new bonds formed in the reaction products.

• It does not matter whether the reaction is an exothermic or an endothermic energy change.
• Now when heated molecules have a greater kinetic energy, a greater proportion of them have the required activation energy to react.
• The increased chance of 'fruitful' higher energy collisions greatly increases the speed of the reaction.

Effect of Mixing

• In doing rate experiments with a solid reactant (e.g. marble chips-acid solution) it is sometimes forgotten that shaking the mixture is an important rate factor.
• If the reacting mixture is not mixed 'evenly' then the reactant concentration in solution becomes much less near the solid, which tends to settle out.
• At the bottom of the flask the reaction prematurely slows down distorting the overall rate measurement and making the results uneven and therefore inaccurate. The 'unevenness' of the results is even more evident by giving the reaction mixture the 'odd stir'!

Effect of a Catalyst

• The word catalyst means changing the rate of a reaction with some other material 'added to' or in 'contact with' the reaction mixture.
• Catalysts increase the rate of a reaction by helping break chemical bonds in reactant molecules.
• This effectively means the Activation Energy is reduced (see diagram below).
• Therefore at the same temperature, more reactant molecules have enough kinetic energy to react compared to a solution which has not had a catalyst added.

• It is chemically the same at the end of the reaction but it may change a little physically if it's a solid.

• A solid catalyst might change physically by becoming more finely divided, especially if the reaction is exothermic.

Fair Test

To make my experiment fair I will:

· Keep the volume of acid the same (100cm3)

· Keep the same mass of marble chips and also the keep them the same shape (20g)

· Make sure the same person is marking the cylinder (I will be doing the marking)

· Make sure the same person is taking the recordings from the gas cylinder (I am doing this also)

· Make sure the same person is doing the timing on the stop watch (me again)

· Use water from the same supply source to dilute acid (Distilled water container)

· Use water at the same temperature (room temperature)

· Keep the type of acid the same (HCl)

· Keep the temperature at which the experiment takes place the same (room temperature)

· Not disurb the mixture at any time during the experiment

Number of Measurements

I will investigate with 4 different concentrations because this way I will have a larger collection of results to prove a firm conclusion and to hopefully back my prediction. For each concentration I will record 3, therefore I can find an average rate of reaction between the 3. Choosing to record about 4 concentrations will also give me enough points to plot on my graph in order to attain a line of best fit.

Range of Measurements

I am going to use a range of concentrations between 0.5M and 2.0M. I will use concentrations from this range because if I use concentration of less than 0.5 my solution will be too dilute and not much of a reaction will occur. Also if I use a solution that is more than 2.0M, then the solution will become too corrosive to work with and therefore would be unsafe.

The concentrations I will use are 0.5M, 01.0M, 1.5M and 2.0M. Using 4 different concentrations will give me a wider range of results enough to back my prediction and also if one set of results don't match my theory then I always have a back up.

Reliable Results

I will ensure my results are reliable by repeating the experiment 3 times. This will also help me see if my results follow a pattern.

Method

· Measure 25cm3 at a time either  HCl or water depending on the concentration needed (4 lots of 25 will give 100cm3 and this will make it easy to vary the conc. accuratly) and pour into a conical flask

· Set up all the equipment as shown in the diagram

· Measure 20g of calcium carbonate (marble chips) on electronic scales - (the electronic scales are more accurate as their units of measurement are to 1/10g)

o Add calcium carbonate to hydrochloric acid, close bung and start timer after the recation is going steadily.

o Stop the stopwatch when 100cm3 of CO2 is colected and record time

o Re-do the experiment 3 times for each conc. required

Results

Conclusion

From the graph you can see that as the concentration of HCL goes up the rate increases.

Looking at the line of best fit you can see that as the concentration doubles so does the rate, hence the gradient is 1. My hypothesis was right.

Anomalous Results

From looking at my graph, there is no immediate recognition of any anomalous results and it seems that my results are fairly accurate. However after checking through my results tables, I noticed that there is a set of recordings that bear no relation to others for that concentration (they are too high). I have highlighted them in my results table (0.5mol all too high). By comparing the other recordings for that

I cannot recall of any mistakes made during the experiment however the results do show this and the anomalous results must be due to a human error with inacurate timeing or due to the fact that some carbon dioxide dissolved in the solution, esp for lower concentrations. Also the gas syringe had some friction and did not works smoothly for the lower conc. of 0.5 mol so well.

Due to the fact that there is a full set of anomalous results I would have to say that my results are fairly reliable and are able to support a firm conclusion and also back my prediction.

How to make more accurate

There are a number of reasons why this will improve the experiment:

• some of the Co2 gas is dissolved in the water

• The markings on the syringe are not very accurate

• The plunger in the syringe does not move back steadily at low conc.s due to friction so slows down times.

How to make more fair

To make the experiment fairer, I am going to set the conical flask containing the marble chips and acid on top of an electronic heater. This is because there may be a change in temperature of the acid in between each test. I am going to set the heater at 30oC as this is when the acid is at its optimum temperature.

How to make more reliable

To make my results more reliable I could do 5 tests on each acid concentration. By doing this I will increase the accuracy for the average of each concentration.

There is another way of making my results more reliable. I could extend the range of concentrations. Using a concentration below 0.5M would not react effectively enough to improve my experiment so I would be better using concentrations of more than 2.0M. However this would mean that the acids would be corrosive and I would have to work with a facemask and safety gloves under safer conditions in a fume cupboard so i could try using a wider range in the 0.5-2.0mol limit.

Investigating the effect that concentration has on the reaction between hydrochloric acid and calcium carbonate.