# How does concentration affect rate of reaction?

The rate of reaction increases as the concentration of reactant particles increases by increasing the frequency of collisions..

Concentration is one of the factors that affect rates of reaction and there is a sequence on how concentration affects rates of reaction.

We have established before that there are about six factors that affect rates of reaction and these factors are dependent on some crucial parameters.

For example we mentioned that activation energy. Frequency of collisions and average kinetic energy of particles are the parameters that govern the factors affecting rates of reaction.

Concentration is a major factor that affects rates of reaction and there is a way it does that while depending of the frequency of collisions of particles.

Again frequency of collision is the number of times the reactant particles collide or hit each other in a given unit of time.

## How does concentration affect rate of reaction?

Let’s assume we have a 20cm3 reactant vessel capacity container and we have different concentrations of reactant particles.

Please stay with me so I can explain the details very well. I am going to talk about concentrations of solids and liquids or aqueous.

Concentration for solids

Concentration for solids is in terms of mass of the solids

So when we talk about concentration for solids we mean mass of the solids

So let’s get down more to specific example on how concentration affects reaction rate

If the reaction of magnesium with hydrochloric acid occurs in a 20cm3 vessel capacity.

Mg + 2HCl ====MgCl2 + H2

Magnesium is a solid

Case 1

We are using 40g of magnesium with same HCl in case 1

Case 2

We are using 20g of magnesium with same HCl in case 2

You can see that in case 1, there are more magnesium particles reacting with same HCl.

So we can boldly say that the magnesium particles in case 1 is more concentrated than the reactant particles of magnesium in case 2.

Now recall that the volume of the container which is 20cm3 is same in both cases.

Concentration for liquids/Aqueous solution

I am going to illustrate concentration for aqueous solutions in terms of real concentration i.e amount of undiluted liquid solute dissolved in solvent.

So let’s get down more to another specific example on how concentration affects reaction rate

If the reaction of calcium carbonate with hydrochloric acid occurs in a 30cm3 vessel capacity.

CaCO3 + 2HCl ====CaCl2 + H2O + CO2

Calcium carbonate is a solid while hydrochloric acid is in aqueous form.

Case 1

We are using 40g of Calcium carbonate with 3.5 mol/dm3 HCl in case 1

Case 2

We are using 40g of Calcium carbonate with 2.5 mol/dm3 HCl in case 2

So we can see from the illustration above that the case 1 is more concentrated not in terms of the solid calcium carbonate particles but in the direction of concentration of the aqueous hydrochloric acid.

Because the mass of the solid particles are the same but the concentrations of the aqueous solutions are different.

The 3.5 mol/dm3 of HCl is more concentrated than the 2.5 mol/dm3 since we have more hydrochloric acid dissolved and reacting with calcium carbonate.

So we can really say that the higher the concentration (more 3.5mol/dm3) of Hydrochloric acid the hiher the rates of reaction.

In conclusion, the rate of reaction increases as the concentration increases and decreases as the concentration decreases because the higher the concentration the higher the frequency of collision and the lower the concentration the lower the frequency of collision.