Changing Surface Area

Imagine that we want to look at the reaction between hydrochloric acid and calcium carbonate.

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

The calcium carbonate is a solid. Let's say I choose to do this reaction with 1.00g of calcium carbonate. In the first experiment I just use a few large pieces of calcium carbonate, but in a second experiment I use the same mass of calcium carbonate but this time there are a lot of smaller pieces. In both experiments I still use 1.00g though, to keep it a fair test (the mass of calcium carbonate is a control variable).

This time, instead of measuring the volume of gas produced in a given time interval, we will measure the mass lost from the reaction container. If a gas is being produced, the mass of the container contents must go down (conservation of mass!)

First experiment - low surface area of calcium carbonate (a few big chips)

Second experiment - high surface area of calcium carbonate (lots of smaller chips)

If we record the mass of the container and contents for each experiment over time, then convert into "mass loss", we get the following type of graph....

Both graphs start at the origin (because the reaction hasn't started and there is no loss of mass yet from the container!).

But then as time goes on, there is a clear change.

With the first experiment (red line), the graph gradient is shallower! Because the gradient shows the rate of reaction, this means that with the fewer large chips in experiment 1, the smaller surface area makes the reaction slower.

With the second experiment (blue line), the gradient is steeper. This means the higher surface area of the chips speeds up the rate of reaction.

But why do they both level off at the same height? Well think about it... in each reaction we used the SAME MASS (1.00g) of marble chips, so in total we will give off the same volume of gas in both, meaning the mass loss will be the same in both...at the end of the reaction!

Explanation!

On the left, we can see the diagram for experiment 1. Because there is a large lump of calcium carbonate chip, less of its particles are on show, which means that there will be fewer collisions between the green CaCO3 particles and the red HCl particles. This means there will be fewer successful collisions and so the rate is slower!

In experiment 2, the CaCO3 chips were all broken up in smaller pieces to start. So more of the CaCO3 particles were exposed and so there can be more collisions with HCl particles, hence more successful collisions and a faster rate!

INCREASING THE SURFACE AREA OF A SOLID INCREASES THE RATE OF A CHEMICAL REACTION.