The Effect of Concentration and Pressure
Introduction

We get used to changing the concentration of solutions from when we are very young. When orange squash is too weak, we soon learn to use more orange in the same volume of water, or add less water to the same amount of squash, to get the taste we like.

In chemistry, the concentration of a solution refers to the amount of substance dissolved in a certain volume of the solution. The volume chosen is 1 dm3 – 1 cubic decimetre (which equals 1000 cm3 or 1 litre). In this unit we will see how the concentration of reactants affects the rate of reaction.

Increasing concentration
We can investigate the effect of concentration by looking at the reaction of marble chips and dilute hydrochloric
acid
An acid is a substance that forms a solution with a pH value of less than 7. Acidic solutions contain an excess of hydrogen ions, H+(aq).
acid
.


CaCO3(s)  + 2 HCl(aq)     CaCl2(aq)  +  H2O(l)  +  CO2(g)

We can use different concentrations of hydrochloric acid in each test, keeping all other factors the same.

We measure concentration in moles per cubic decimetre ( or moles per litre), abbreviated to mol dm−3.

A mole is not a mass of a substance. Instead, it is a certain number of particles of a substance. So one
mole
One mole is the number of carbon atoms in exactly 12 g of carbon (6.02 × 1023).
mole
of hydrogen chloride (HCl) dissolved in one cubic decimetre of water will contain the same number of chloride ions as one mole of sodium chloride (NaCl) dissolved in one cubic decimetre of water. And because the volume of water is the same in each case, the solutions have the same concentration: 1 mol dm−3.

A 2 mol dm−3 solution is twice as concentrated as a 1 mol dm 3 solution. It has twice as many dissolved particles in the same volume of solution.

In the experiment in Fig.1 below, we will time how long it takes to collect 20 cm3 of carbon dioxide given off in reactions using 1 mol dm−3 and 2 mol dm−3 solutions of hydrochloric acid and marble chips.

Figure 1.   The effect of concentration on rate of reaction.
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Which solution reacts faster?
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This illustrates a general rule about concentration and reaction rates, which is:

The more concentrated a solution is, the faster the rate of reaction.


Look at the reactions in Fig.2 below between hydrogen peroxide solution and a solution containing iodide ions. Iodine is formed in the reaction. When a set amount of iodine has been produced, the indicator (starch solution) turns black. The experiment was done three times with differing concentrations of hydrogen peroxide solution:

Figure 2.   The 'iodine clock' experiment.
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In the experiment above, the concentration of hydrogen peroxide was halved each time. What happened to the time taken for the solutions to turn black?
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Which statement below is correct?
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In the experiment above, which flask contained the most concentrated solution of hydrogen peroxide?
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Which statement about the experiment is correct?
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Explaining the effect of concentration
Once again, we can use the collision theory (see the unit Introducing Rates of Reaction) to explain why concentration affects the rate of a reaction.

Look at the model in Fig.3 below:

Figure 3.   Collisions in solutions of different concentrations.
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In a more concentrated solution, there are more particles in the same volume of solution. This increases the chances of collisions between
reactant
A reactant is a substance we start with before a chemical reaction.
reactant
particles. Therefore we will get more collisions in any given time, resulting in a faster rate of reaction.

Which of the following solutions of hydrochloric acid will react fastest with marble chips?
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The effect of pressure on gaseous reactions
We don't usually talk about the concentrations of gases. It is more common to refer to their pressure. If you have a syringe containing 100 cm3 of a gas and press the plunger down to the 80 cm3 mark, you have increased the pressure of the gas. You have also increased its concentration, as you have the same number of gas particles, but now they are in a smaller volume.

So when gases react, if we increase the pressure, we increase the rate of the reaction.

Look at the model in Fig.4 below:

Figure 4.   The effect of pressure on a gaseous reaction.
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Which mixture of gases would react fastest with each other?
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Summary


As we increase the concentration of solutions, we increase the rate of reaction.

Increasing the concentration means that we have more particles in the same volume of solution. This increases the chance of collisions between reactant particles, resulting in more collisions in any given time and a faster reaction.

As we increase the pressure of reacting gases, we increase the rate of reaction. Again, we have more reactant particles in the same volume, therefore we have more frequent collisions.

Exercises
1. What two products are formed when magnesium ribbon reacts with dilute hydrochloric acid?
  • Chlorine
    Hydrogen
    Magnesium chloride
    Magnesium hydroxide
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2. Which concentration of hydrochloric acid will result in the fastest reaction with magnesium ribbon?
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3. A test was carried out to compare the effect of varying concentration. Look at the graph in Fig.5 below showing the result for two experiments with magnesium ribbon and dilute hydrochloric acid. Match the concentration of hydrochloric acid to the curve.
  • 1.0 mol dm -3
    0.5 mol dm -3
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Figure 5.  
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4. List three factors that must be controlled in the experiments designed to test the effect of concentration on rate of reaction.
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5. Explain why increasing the concentration of a solution increases the rate of reaction.
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