To investigate the effect of change concentration on the rate of reaction between marble chips and excess hydrochloric acid

3. Factors affecting rate of reaction

There are certain factors that affect the rate of reaction. These are:

1. Temperature. An increase in temperature speeds up the reaction, thus increasing the rate of reaction. This is because the particles in the reaction get more energy and so they collide more often and with more energy so the reaction is faster.

2. Pressure. An increase in pressure increases the rate of reaction. This is because by increasing the pressure, the particles will collide more often. This is because there is less space so there are more chances of the particles colliding against each other. There will be more successful collisions between each other. This way the rate of reaction increases.

3. Catalyst. A catalyst is any material or a compound that helps to speed up the reaction. It helps it by lowering its activation energy. By lowering the activation energy, the reaction starts at a lower temperature and so the reactions takes place faster. The catalyst allows a surface where the ions can react together.

4. Surface Area. By increasing the surface area the rate of reactions increases. This is because there are more particles to collide and the reactions will go faster if the mass is kept the same. When there is a solid reactant, it can be reacted as a lump, granules or powder. Using a lump will take a long time because not all the particles are exposed form the substance, so the reaction is slow. When granules will be used, the reaction will go faster but still it can be broken down to expose more particles. In granular form, not all the particles are exposed. But in powder form, all the particles will be exposed and so the reaction will go the fastest. This is because there are more particles being exposed. But in laboratory use, granules are used, so that they can be seen reacting. Powder also reacts but it is more difficult to see and make observations. In a powder the reactant is spread and so the gas that is produced will not be in one place, it will be all throughout the powder. Because of this powder is not usually used in reaction although it does make the reaction go fastest. The powder has the largest surface area and thus makes the reaction go fastest.

5. Concentration. By increasing the concentration of the reactants, the rate of reaction increases. This is because there are more particles in the same volume so there are more collisions and thus more reactions. By doubling the concentration, the rate of reaction doubles. By tripling the concentration, the rate of reaction also triples. They are in proportion. Therefore the rate of reaction is directly proportional to the concentration of the reactant used. If the concentration of Hydrochloric acid is doubled, then the reaction should theoretically take place twice as fast. This is because there will be twice as many number of H+ ions and thus the reacting atoms has doubled in number and increases the rate. The rate of collision between the hydrogen and the carbonate ion increases and thus the reaction between the hydrochloric acid and the calcium carbonate doubles. The same would be the case if the concentration is tripled. When the concentration is doubled, the time taken to complete the reaction is half and the time is one third of the original time when the concentration is tripled.

           

  Concentration                Rate of Reaction          

We will see on the next page how the rate of reaction changes with the concentration.

CO32-(s)    +  2H+(aq)          CO2 (g) + H2O(l)

 

 

 

 

 

 

 

 

 

0.5 M HCl        1 M of HCl        1.5 M of HCl

From this diagram we can see that the reaction taking place between the carbonate ions and the hydrogen ions according to the concentration. We can see that the number of hydrogen ions changes from each beaker according to the concentration. The number of ions in 1M doubles than that in 0.5 M because the concentration also doubles. Therefore the reaction in 1 M is twice as fast as that in the 0.5 M solution because the number of ions have doubled and so the rate also has doubled. So we can understand form this that since the number of ions doubles, the rate of reaction also doubles. The rate of reaction for 1.5 M will triple because it has three times the number of ions that 0.5 M has. So this proves that concentration is directly proportional to the rate of reaction. The time for the reaction to take place for the 1 M solution is half and one third for the 1.5 M solution.

Calculation for volume of HCl and mass of CaCO3

Here we will use the syringe method because a gas is a product. The gas produced here is Carbon dioxide as we have already seen from the equation. The capacity of the syringe is 100cm3. So we take the volume of Carbon dioxide produced as 100 cm3 also.

CaCO3(s)  + 2HCl (aq)                  CaCl2(aq) + H20(l) + CO2(g)

Volume of CO2 produced = 100 cm3

Moles of CO2 produced = volume

                                             

                                           24000

moles =   100

               24000

         

           = 0.004166 moles of CO2 produced

According to the equation 1 mole of CaCO3                                 1 mole of CO2

                                           y moles of CaCO3                                    0.004166 moles of CO2

therefore that is 0.004166 moles of CaCO3 

 

moles = mass

           R.F.M

R.F.M of CaCO3 = 40 + 12 + 16 x 3

                              = 100

mass = moles x R.F.M

                = 0.004166 x 100

                =0.416 g

Now we will find out the volume of HCl required

2 moles of HCl give                                             1 mole of CO2

y moles give           0.004166 moles

                           

y= 2 x 0.004166

  = 0.00833 moles of HCl

volume of HCl = moles

                             concentration

The concentration here is 0.5 M

Volume =  0.00833

                    0.5

Volume = 16.66 cm3 

Concentration for the next will be 1 M

Volume = 0.00833

                 1.0

Volume = 8.33 cm3

 Concentration will be 1.5 M for the next

Volume = 0.00833

                 1.5

 Volume = 5.553 cm3

So for a concentration of 0.5 M we will have to take 16.66 cm3 of Hydrochloric acid

For a concentration of 1 M we will have to take 8.33 cm3 of Hydrochloric acid

For a concentration of 1.5 M we will have to take 5.33 cm3 of Hydrochloric acid

But excess acid will be taken to ensure that all of the Calcium Carbonate reacts. During the reaction, the concentration also has to be maintained. The volume taken will be 16.66 x 2 = 33.32 cm3

Factors remaining constant

Surface area of Calcium Carbonate. Calcium Carbonate will be in the form of granules

Temperature will be constant at room temperature

Mass of Calcium Carbonate will be constant. It will be 0.416g

List of Chemicals / Apparatus

• Calcium Carbonate – 0.416 g
• Hydrochloric acid – 33.32 cm3
• Syringe of capacity of 100 cm3
• Conical Flask
• Stand
• Burette
• Stop Watch
• Weighing bottle (plastic)
• Tongs
• Rubber Bung
• Rubber tube
• Tongs

Note: Tongs are used to lower the test tube containing Hydrochloric acid into the conical flask.

Procedure:

We have to accurately weigh 0.416 g of Calcium carbonate. Calcium carbonate is usually find in the earth in the form of rocks as limestone and chalk. So we have to basically collect 0.416 g of this limestone. The conical flask then has to be rinsed with Hydrochloric acid. This removes any impurities or any other substance that may take part in the reaction. Then we remove this hydrochloric acid. The limestone will be placed in the form of granules into the conical flask. After this is done, the hydrochloric acid also has to be added. But it should not be added directly. It will be measured correctly using a burette. This burette gives the precise volume of acid. Once the volume reaches approximately 32 cm3, it is emptied into a weighing bottle that will be placed inside the conical flask which has also been rinsed with hydrochloric acid. When rinsing, a few droplets of the acid remain inside the test tube. This does not make any difference to the reaction taking place. Once we add this, we can insert it in the conical flask. The calcium carbonate can then be put in the conical flask in the form of granules. Then a rubber bung is placed on the top of the conical flask. This prevents the escaping of carbon dioxide. However, our results have to be found using the volume of Carbon dioxide so a syringe is placed. This syringe is placed on a stand and is on the same level as the flask. This has a rubber tube so that it does not react with the gas and this is connected inside the flask. So when a gas is produced, it will be collected into the syringe instead of escaping. This we can use to find the reading of the amount of volume of gas. Once the cork has been fitted and the rubber tube also fixed, the flask is shaken. It is very important to see that the cork and the tube are fitted properly. If the cork is not fitted, the gas will escape and if the tube is not fitted, the gas will not be able to escape and the pressure will build inside the flask and the flask will collide. So it is necessary to check this. Once the flask is shaken, the hydrochloric acid will fall out of the test tube and onto the granular calcium carbonate. The reaction taking place can be denoted by the bubbles of gas formed. At the time when the hydrochloric acid is dropped, the stop watch should be switched on. The plunger in the syringe moves from right to left. This shows us the exact amount of gas accumulating. Every 15 seconds the volume can be checked till it ends. We will come to know if the reaction is over when we do no see any more bubbles and when the volume reading remains constant. This reading has to be taken every 15 seconds to get the rate of reaction. This is done for one concentration. We should repeat the experiment for the same concentration and also for other concentrations to make sure that our reading are correct and we can get more accurate and reliable results.

On the next page are graphs that show how the rate of reaction has changed over time.

Expected Graphs

 

                                              Time in seconds

This sketch shows how we are to predict the rate of reaction This graph is general for all reactions. It is a smooth curve. On the graph the x axis is the time and the y axis is the volume of product formed.

A shows part of the graph when the reaction is just started and the rate of reaction is the fastest. We came to know this because it is steeper at the beginning than at the end. This shows that is the rate of reaction is faster at the beginning than at the end because the particles can react easily because there is no other product between them.

B shows the part of the graph where the rate of reaction slows down. This slowing down is caused because there are less particles or reactants to react. The curve is less steep here and we can understand that the reaction is slowing down.

C shows the part where the reaction has come to a stop. The dotted line shows the time taken for the reaction to be completed. This part is constant because there is no reaction taking place as the reaction is finished and the maximum amount of product is formed. The corresponding volume is the final volume produced and when we have to find the rate we take the point where the reaction is finished, that is dotted line. C also shows that we would wait for some time after the reaction is finished to make sure that the reaction is finished.

                               T1

                              T2

                              T3

This graph shows the reaction in our experiment. The curves show the difference. A shows the concentration of hydrochloric acid is to be 1.5 m and C shows 0.5 M.

A shows the highest concentration. This we know because it takes the least time to complete the reaction and is steepest. The time taken for it to complete is denoted as T1 on the graph.

B shows the concentration to be less than that of A. This is because the time taken is more than the time taken by A. The volume however was the same. It does not change, only the time changes. In this graph the time is known to be T2. T2 is twice the that of T1 so we can assume that the concentration is half of that of A. The time increases because there are not many particles to react with the calcium carbonate.

C shows the concentration to be less than that of A and B. This is because the graph is not as steep and it takes more time. The time taken for it is T3. T3 is shown to be three times than that of T1. So that means that the concentration of C is one third of A.

From this graph we can come to know the following.

T2 = 2 x T1

T3 = 3 x T1

Observation Table

Precautions

1. There should be no parallex error while measuring the volume of acid.
2. Plunger of the syringe should be at 0.
3. Stop watch should be at 0.
4. There should be no leakage in the rubber tube before starting.
5. The acid should be handled with care.
6. The measurements related to the mass of the calcium carbonate have to be accurate.
7. Flask should be rinsed with hydrochloric acid.