GCSE OCR Gateway Chemistry C5.2a-c and e: Rate and surface area

GCSE OCR Gateway Chemistry C5.2a-c and e

C5.2 Controlling reactions

Summary

Changing the physical conditions of a chemical reaction can change its rate and yield.

Common misconceptions

Learners often misinterpret rate graphs and think that catalysts take part in reactions and run out/get used up.

Underlying knowledge and understanding

C5.2a To be able to suggest practical methods for determining the rate of a given reaction

C5.2b To be able to interpret rate of reaction graphs

C5.2c To be able to describe the effect of changes in temperature, concentration, pressure, and surface area on rate of reaction

C5.2e To be able to explain the effects on rates of reaction of changes in the size of the pieces of a reacting solid in terms of surface area to volume ratio

Controlling reaction rates using the surface area of solid reactants

Let’s be honest the classic way in which the effect of surface area on reaction rate at this level has been discussed, illustrated and investigated has been using the reaction between marble chips (Calcium carbonate CaCO₃) and hydrochloric acid (HCl). 

So what happens when we change the size of the marble chips we are using?

What changing the size of marble chips does is to alter the surface area of the marble pieces.  As in the illustration below, the effect of reducing the size of pieces of marble but keeping the same mass and same volume, increases the extent of the surface area of marble that can come into contact with the hydrochloric acid. 

How do we carry out this kind of experiment?

First, to eliminate as many variables as possible and only leave the surface area to affect the reaction rate we need to use the same concentration of hydrochloric acid, the same mass of marble chips, at the same temperature, in the same container.

Second, you can see the apparatus you could use in the illustration below:

You can see that the mass of marble is the same in each experiment as is the volume and concentration of the hydrochloric acid.

Third, adding marble to the acid and starting the stop clock gives readings of the mass of the apparatus every half–minute.  These can be then converted into measurements of the actual mass of carbon dioxide lost per half–minute.

Fourth, a typical set of results looks like this showing the effect of both large and small chips:

Fifth, you can measure the initial rate of reaction by drawing a tangent to the curve at T=0 s.  However you can see that the gradient of the line for small chips is steeper and therefore the reaction with small chips is going faster. 

Sixth, reactions are faster with smaller chips or powder since there is a greater chance of successful collisions between the acid and the marble.

CaCO₃   +   2HCl   ⟶   CaCl₂   +   H₂O   +   CO₂

Perhaps now you can explain how it is that iron nails do not sparkle but iron sparklers do on Bonfire Night.

Or you can now explain how it is that factories where flour is ground take stringent precautions against sparks forming.

Or you can explain how it is that the sodium azide (NaN₃) in airbags is used in powdered form.