Changing Concentration

Concentration is a measure of how much solute is dissolved in a given volume of a solution.

Look at the example above. Both beakers contain the same volume of solution once the solid solute has been dissolved. Let's call the volume 100cm³. But in the beaker on the left, we only dissolved a really small mass of the solute (let's say 0.5g). On the beaker on the right, there was much more solute dissolved (let's say 2.0g)

We would say that the beaker on the left has a LOW CONCENTRATION. We could even give it a numerical value.... 0.5 g / 100cm³

The beaker on the right has a HIGH CONCENTRATION.... 2.0 g / 100cm³

Now let's look at how this can affect the rate of a chemical reaction...

Mg(s) + 2 HCl(aq) --> MgCl2(aq) + H2(g)

Experiment 1: Using 2.0 mol/dm3 HCl(aq)

Experiment 2: Using 0.5 mol/dm3 HCl(aq)

(Note: Do not worry about the units for concentration given here. You just need to be happy that experiment 1 is using HCl(aq) that is more concentrated than in experiment 2 (4x more concentrated in fact!)

To make this a fair test, we have to use the same mass and surface area of Mg in each test, the same temperature and also the same volume of acid.

In each experiment the volume of gas was collected at a set time interval, using a gas syringe to collect the gas.

Remember, the gradient of the line tells us the rate of reaction. In experiment 1 the gradient is much steeper, telling us the higher concentration of acid reacts at a faster rate. Both lines finish at the same 'final volume' of gas, because in each experiment, the same mass of Mg ribbon is used! This means in the end we produce the same volume of gas...one reaction just produces it faster than the other.

Why are the results like this, using collision theory to explain?

If you look at the diagram above, the red particles represent the HCl. Because the concentration of HCl is higher in the right diagram, this means there are more HCl particles in a given volume of solution. This means there will be more frequent collisions with the Mg particles, therefore there will be more frequent successful collisions.

Pressure for gases?

When we talk about the concentration of a substance, we are usually talking about a solution. However, we can use the same type of idea when talking about gases... but we don't usually talk about a gas in terms of a concentration... we talk about the pressure of a gas. It means roughly the same thing... if a gas is at a high pressure, it's particles are much closer together and bouncing into the walls of the container a lot more. A gas at high pressure has a high concentration.

INCREASING THE CONCENTRATION OF A SOLUTION INCREASES THE RATE OF A CHEMICAL REACTION.