Coursework Guidance - Year 11

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It must not be copied without acknowledgement and
would not, by itself, be sufficient to gain you the
best marks for planning and analysis.
 

Coursework in Year 11 is an investigation into one factor that affects the rate of a chemical reaction. These factors include:

and of these factors, the first two are easier to investigate than the others.

By far the most commonly studied reaction for this coursework is the reaction of magnesium with hydrochloric acid:

Mg (s) + 2HCl (aq) ® MgCl2 (aq) + H2 (g)

How does concentration affect the rate of a reaction?

If the concentration is higher there will be more particles in the same volume of space.

Since the particles are rapidly moving around this means that there will be a greater number of collisions every second. With more collisions per second there will be more particles reacting every second i.e. an increase in the rate of reaction.

Particles in a material don't all have the same amount of energy, most have roughly an average amount, while some have less or more than the average. This is illustrated graphically below with what is called a Maxwell-Boltzmann Distribution:

So with a higher concentration of particles there will be more particles with enough energy to react when they collide (this amount of energy is called the Activation Energy). In the diagram above, the number of particles capable of reacting at low concentration is represented by the light brown area, while at a higher concentration the number of particles capable of reacting is represented by both the light and dark brown areas. If the number of particles capable of reacting is higher, the rate of reaction will be greater.

In summary: increasing the concentration means that in the same volume of space there are more particles. This makes collisions between particles more likely. More particles will have enough energy to react. These two factors make the rate of reaction greater.

Other websites with useful information:

(In Preparation)

How does temperature affect the rate of a reaction?

What is temperature? It is a measure of the amount of kinetic energy that particles possess. As the temperature increases, so does the kinetic energy of the particles. The particles don't all have the same amount of energy, most have roughly an average amount, while some have less or more than the average. This is illustrated graphically below with what is called a Maxwell-Boltzmann Distribution:

Particles move around faster when they have more kinetic energy. This makes collisions with other particles more frequent. When particles collide they may just bounce off each other (like two snooker balls), but if the particles have more than a certain amount of energy they will react when they collide (this amount of energy is called the Activation Energy). At a low temperature (T1) the number of particles with enough energy to react (represented by the light brown area on the graph) is small, so reaction is slow.

If the temperature is raised to a higher value (I have labelled the curve for the higher temperature T2), there are a lot more particles with enough energy to react (represented by both the light brown and red areas on the graph) so the reaction will go faster.

In summary: increasing the temperature gives all particles more kinetic energy. This means that:

  • there will be more frequent collisions between particles,
  • it becomes more likely that colliding particles will have enough energy to react,

so the rate of reaction will increase.

Other websites with useful information:

(In Preparation)

How does particle size affect the rate of a reaction?

A large piece of material will react a lot more slowly than the same amount of material that is ground up as a fine powder. It's all to do with surface area. The reaction happens at the surface of the material - the more surface there is, the faster the reaction can go. This principle can be demonstrated using the reaction of marble chips (calcium carbonate as a metamorphic rock) with hydrochloric acid:

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

Powdered marble reacts very fast indeed (the reaction is over in seconds with lots of fizzing and a froth that climbs out of the flask) while large lumps of marble produce CO2 quite slowly over a very long time.

Other websites with useful information:

(In Preparation)

How can rate of reaction be measured?

You need to measure either the amount of a material used during a reaction (called a 'reactant') or the amount of a material made during the reaction ('product'). The total amount of the material made/used by the end of the reaction is not important; what is important is to look at how fast the amount is changing - this is what is meant by the phrase "rate of reaction".

You need to find a method of measuring the amount of the chosen material, and measure how this changes during the reaction. For example, using the magnesium-hydrochloric acid reaction:

Mg (s) + 2HCl (aq) ® MgCl2 (aq) + H2 (g)

it is straightforward to measure the amount of hydrogen produced (using a gas syringe or an inverted measuring cylinder full of water) and record the total every 5 or 10 seconds. You might get results that give a graph like this:

This graph shows a slow experiment (Experiment 1) and a much faster one (Experiment 2). Notice that they both produced virtually the same amount of gas by the end of the reaction, so the total amount of gas does not tell you how fast the reaction goes.

How can rate of reaction be analysed?

The fastest reaction is the one with the fastest rate of gas production, shown as a steeper gradient at the start of the graph above.

The best analysis of the experimental results will be one that tries to show a relationship between the factor being investigated and the rate of reaction. This needs to be done graphically, using the initial rate from each experiment (e.g. the amount of gas produced during the first 5 or 10 seconds) plotted against the value of the factor being investigated. Examples of such graphs follow:

In this first graph, the rate increases as the temperature/concentration/etc increases. There is a linear relationship, perhaps the reaction rate is proportional to the factor being investigated.

Here the rate decreases as the temperature/concentration/etc increases. This may be evidence for an inversely proportional relationship.

In this final graph, there is no obvious pattern linking the rate with the temperature/concentration/etc. The conclusion in this case would be that no relationship exists.

The best analyses relate the results to the scientific theory and discuss precisely how the results lead them to their conclusions.

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