# Investigation: How does channging the concentration of a substance affect the rate of reaction?

Essay by clare_101Junior High, 7th gradeC, May 2004

A chemical reaction is the process by which atoms or groups of atoms are redistributed, resulting in a change in the molecular composition of substances.

The reaction rate is how quickly a reaction occurs. I will be using the reaction rate during my investigation to see how the concentration of acid effects the reaction rate. To do this I will be using the following reaction

Hydrochloric + Calcium Ü Calcium + Carbon + Water

Acid Carbonate Chloride Dioxide

powder

2HCl + CaCO3 Ü CaCl2 + CO2 + H2O

I will be using these substances because I know from my research and previous experiment that these two substances react well together. So, I will be able to have quick and efficient experiments.

I will be using the following equipment and setting the equipment up as shown in the diagram.

3. Rubber plug

4. Stop Clock

5. Top Pan Balance

6. Heat Proof mat

7. Clamp and Boss

8. Clamp Stand

I will be testing to see how the concentration of the hydrochloric acid effects the reaction rate. I will be able to see this by measuring the gas using the gas syringe. As carbon dioxide is released during the experiment, I will measure how much carbon dioxide will be produced in certain amounts of time. I will then draw a graph to show how much carbon dioxide had been produced in several time intervals. I will then take the gradient at one point in the graph where each line is straight.

This will show me the reaction rate of each experiment. I will use five different concentrations of acid. Below are the different concentrations I will be using.

Quantity of HCl (ml) Quantity of H20 (ml)

50 0

45 5

40 10

35 15

30 20

25 25

From this information, I can work out what molar, the acid is. I will do this by using a special formula, the formula is:

Volume of acid X 2

Total volume of solution

Dividing the volume of acid by the volume of gas will show how much of the solution is made up of acid. However you have to multiply this answer by two, this is because I will be using a two molar acid as my original hydrochloric acid, which I will then add water to.

I can now work out the molars of each solution. They will be:

Quantity of acid (ml) Quantity of Water (ml) Molar (M)

50 0 2.0

45 5 1.8

40 10 1.6

35 15 1.4

30 20 1.2

25 25 1.0

I have decided to use 50 ml of acid as my upper limit, because it should give me a relatively quick reaction, so the experiment will take only a short amount of time. I will use 25 ml of acid as my lower limit if I start taking readings below that measurement, the reactions will start to get very slow and so take up a long amount of time. I will be able to see the general pattern, as I would already have six readings. I am using six readings because it will be enough to draw a good line of best fit.

This should hopefully give me a clear result of what effect the concentration of acid has on the reaction rate. For each experiment I will use 5 grams of calcium carbonate. I will take readings from the gas syringe every 30 seconds. The amount of gas produced every 30 seconds shows how quickly the reaction has taken place.

Before my main experiment I will perform a trial experiment to make sure I can complete the experiment in time, make sure I am using the right quantities of each substance, so that I can see any other problems that might occur during my experiment. For the trial experiment I will test the two extreme quantities to check that they are suitable for the experiment. So I will be using

Quantity of HCl (ml) Quantity of H2O (ml)

50 0

25 25

I hope for each experiment to last over one minute, so my results are more accurate. After my trial I will change the quantities if they are wrong so that each experiment lasts over one minute.

In order to make my experiment safe I will use goggles when I handle acids or alkalis. I will also make sure that if I decide to use any heating equipment I will use goggles and a heatproof mat and tongues if I am required to handle any hot equipment.

In order to make my experiment fair, I will make sure that each experiment will be performed under the same conditions, with hopefully only the concentration of acid being altered for each experiment. There will be a problem in getting the calcium carbonate into the acid and placing the rubber bung on top of the conical flask before any gas is released. However I have decided to tip the calcium carbonate powder into the acid and then place the bung on as soon as it is all fully inside the conical flask. This will obviously allow some gas to escape, however as the time between the calcium carbonate starting to react with the acid and placing the rubber bung on top of the conical flask will be roughly the same for each experiment, I think that my experiment will be fair.

Here is the method for the experiment I will perform.

1. I will first set up the equipment as shown in my diagram.

2. I will then measure out 5 grams of calcium carbonate and the quantity of hydrochloric acid needed for the experiment.

3. I will then pour the calcium carbonate into the beaker.

4. I will then pour the hydrochloric acid into the beaker and as soon as all the acid is in the test tube I will start the stop clock and put the bung on the beaker.

5. I will then take measurements every 30 seconds of how much gas has been produced from the reaction. I will continue this until the reaction has completely stopped.

6. Finally I set equipment up again for the next measurement.

Here are the results from my trial experiment

Time (secs) Amount of gas produced (ml)

50 ml of HCl 25ml of HCl

30 >100 >100

From our results we decided that we used far too much calcium carbonate for the amount of acid we used. So we decided to cut the amount of calcium carbonate down to 0.4 gram. We then decided to do another trial for this quantity. Here are the results from our second trial.

Time (secs) Amount of gas produced (ml)

50 ml of HCl 25ml of HCl

30 25 24

60 25 31

90 34

120 35

150 36

180 37

210 37

After finishing the trial repeats, I decided that I should take readings of how much gas is being produced every 15 seconds. This is because I think that I may not be able to distinguish a difference in reaction rates of the different experiments if I measure the gas produced every 30 seconds. We also realized that during the reaction involving the 2 molar acid, too much gas escaped before I was able to put the bung into the conical flask. As the reaction is a lot quicker than the 1 molar acid reaction, a lot more gas is produced in the first few seconds therefore a lot more gas escaped from the conical flask before I could place the rubber bung on top of it. This meant that the total gas produced in the 2 molar experiment was less than the total gas produced in the 1 molar experiment. This made my results wrong and unfair. So I found a way to get the calcium carbonate into the acid and place the rubber bug on top of the conical flask without letting any gas escape from the conical flask. I did this by using a very small test tube, the test tube was small enough to fit inside the conical flask while the rubber bung was on top. So I tied a piece of string onto the small test tube, placed the calcium carbonate into the test tube, and slowly lowered the test tube into the hydrochloric acid, which was in the conical flask. Making sure I placed the rubber bung on top of the conical flask before any gas escaped. This meant that all my experiments would be fair. I found out how to do this method during my research, it was explained in a book called 'Chemistry For You'.

After research from the book 'The Hutchinson Encyclopedia (1998 Edition)' and trial experiments I am now able to predict what I think my results will be. I predict that as the concentration of acid gets higher, so does the reaction rate. This is because of what is explained in the particle theory about when particles collide, a reaction occurs. It explains that for a reactant to occur, the reactant particles must collide. Only a certain fraction of the total collisions cause chemical change; these are called fruitful collisions. The fruitful collisions have enough energy at the moment of impact to break the existing bonds and form new ones. The new bonds form the products of the reaction. Therefore in order to make a reaction occur more quickly, there need to be more particles involved in the reaction. To do this you have to make the acid more concentrated, as this means there is a higher concentration of particles. This means that the particles will collide with each other more often and so break the existing bonds and form new bonds quicker. This can be seen in my diagram below.

Particles rarely Particles ten collide

collide

Volume of Gas (ml)

Con (M) Time(secs) 2.0 1.8 1.6 1.4 1.2 1.0

1 2 Av 1 2 Av 1 2 Av 1 2 Av 1 2 Av 1 2 Av

15 30 29 29.5 28 29 28.5 22 24 23.0 21 20 20.5 21 21 21.0 18 17 17.5

30 30 30 30.0 29 30 29.5 24 26 25.0 24 25 24.5 24 25 24.5 20 20 20.0

45 30 30 30.0 26 28 27.0 26 27 26.5 26 27 26.5 21 22 21.5

60 28 29 28.5 28 30 29.0 27 28 27.5 23 24 23.5

75 30 31 30.5 30 32 31.0 29 30 29.5 27 25 26.0

90 30 34 32.0 30 31 30.5 28 27 27.5

105 29 28 28.5

120 30 30 30.0

For the second set of results for the 1.6 molar hydrochloric acid, I have inserted repeat results into the table, my original results are shown below:

Time (seconds) 1.6 Molar concentration

15 25

30 43

45 50

60 52

75 52

I think that these results didn't fit the recent trend of the results. I think that they probably went wrong due to the wrong amount of calcium carbonate, or the wrong concentration of hydrochloric acid being used for the experiment. It was