GCSE OCR Gateway Chemistry C5.2a-d Concentration vs reaction rate

GCSE OCR Gateway Chemistry C5.2a-d

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

Learners should be familiar with the action of catalysts in terms of rate of reaction. They should know the term surface area and what it means.

Tiering
Statements shown in bold type will only be tested in the Higher Tier paper.
 All other statements will be assessed in both Foundation and Higher Tier papers.

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

C5.2d To be able to explain the effects on rates of reaction of changes in temperature, concentration and pressure in terms of frequency and energy of collision between particles

Controlling Reactions

This is all based on practical experimental work.

Let’s begin looking at a particular chemical change and see how we can control its rate.

The reaction we are going to examine is between magnesium and hydrochloric acid.

Equation:  Mg   +   2HCl    ⟶     MgCl₂     +    H₂

What we are going to do is change the concentration of the hydrochloric acid we are using and see how that affects the volume of hydrogen gas evolved over time.

The set up we’d use might look like this:

Magnesium ribbon reacts with hydrochloric acid to form hydrogen gas.  One way of following the reaction is to measure what volume of hydrogen has evolved every half minute until the reaction stops. 

In the experiment discussed below, excess magnesium reacts with 1M hydrochloric acid and then with 2M hydrochloric acid and in each case the results are collected. 

In practice you would carry out the experiment with each acid concentration say three times and find the average volume after each half minute.  In this way, you increase the reliability of your results.  I’m presenting results on the assumption that they are reliable and for the purpose of showing you how to compare the effects of increasing the concentration of the acid.

Results

The results look something like this:

First the data

Then the graph of the data:

These show you how to tabulate the data you collect and how to draw the graph.

For the graph, which is an X-Y scatter graph, the volume of hydrogen is the dependent variable so it is put on the vertical axis.

The time passing is the independent variable and that by convention is placed on the horizontal axis.

The rate of reaction changes as the reaction proceeds. 

It is the gradient of the line graph that tells you how fast the reaction is going at any one time.  

The reaction is fastest at the start time t=0 mins.

The reaction is slowest when it stops and the line is horizontal.

We are going to measure the initial rate of each reaction: first for the 1M acid then for the 2M acid. 

To measure the initial rate we take the tangent of the line graph at t=0.

These are drawn on the graph below for 1M in blue and 2M in red.

We measure the volume of gas evolved in 2minutes for the 1M acid and the 2M acid then we calculate the initial rate of each reaction.

The results are written on the graph below:

You will see that doubling the concentration of hydrochloric acid also doubles the initial rate. 

Explanation

The question we need to answer is how come the more concentrated acid reacts faster?

We can say that increasing the concentration of the acid increases the reaction rate.  Reaction rate is proportional to the concentration of the hydrochloric acid.

But to explain this conclusion more fully we have to think about what is happening to the particles of acid colliding with the magnesium ribbon. 

In this experiment we have been careful to keep the magnesium ribbon used in each experiment the same size and mass and surface area.

Only one factor has been changed: the concentration of hydrochloric acid.

Its concentration has doubled so that 2M has twice as many acid particles per cm3 as has 1M acid.

At any particular temperature say 25^oC, this means that the chances of hydrochloric acid particles with sufficient energy colliding with the magnesium ribbon and reacting has doubled.

There must be twice as many acid particles in the 2M with sufficient energy to react with the magnesium ribbon.  You see some particles won’t have enough energy to react they'll be moving too slowly.  But for the rate to double there has to be twice as many with what we call the activation energy that is enough energy to react with magnesium.