Nat 5 Chemistry Notes Preview

Rates of Reaction

Introduction

In any given chemical reaction, the speed at which the reaction takes place can be measured as its "rate". Some reactions can happen so quickly that they can't even be observed, others can take so long that you need to provide more energy (usually in the form of heat) for the reaction to take place at all.

Measuring the Rate of Reaction

We can measure the rate of a chemical reaction using a number of methods. In any case, we observe the rate by measuring the change in mass or the change in volume of either the reactant or the product. We make use of the following equation to calculate the average rate:

rate = change in quantity / change in time

So experimentally, we can work out the change in time by using a stop clock and timing how long the reaction takes to reach completion. This is the point at which the quantity of reactant or product no longer changes. We can calculate change in quantity through a number of methods depending on the experiment. For example, if we are producing a gas in the experiment (for example if we dissolve a metal carbonate in an acid then we produce a salt, water and carbon dioxide - see later), then we can measure the volume of gas produced in the reaction.

(An upside down measuring cylinder in a beaker of water can also be used in place of the gas syringe).

In an experiment, we would measure the gas produced over regular time intervals, for example every 10 seconds. We can then draw a graph of this data which would look like this.

The average rate over the first 2.5 seconds can simply be found by reading off the graph and using the above equation. So we have 8cm3/2.5 seconds = 3.2 cm3/s (note the units - we divided cm3 by seconds and so we end up with cm3/s). This average rate can be seen to be decreasing over the course of the reaction by the fact that the gradient of the graph is decreasing. This can be confirmed by calculating the average rate over the next 2.5 seconds: 12cm3-8cm3/2.5s = 1.6cm3/s.

The graph flattens out at about 15 seconds which means that this is the time that it took for the reaction to reach completion (so one or both of the reactants must have been fully reacted). This is the end point of the reaction. It can also be seen that roughly 14.5cm3 is the quantity of product.

We cannot see it in this reaction as we did not measure it, but we can also work out the quantity of reactant used. If we instead measured the mass of solid lost as time changes, for example by placing a conical flask containing acid on a set of scales, and adding a known mass of solid. We could then measure how the mass reduces as time increases, by measuring the mass at regular time intervals.

Factors Affecting Rate

Whenever rate is assessed in exams, it is very likely that a student will be asked to name and explain how different factors affect the rate of reaction. The reasoning should always be linked back to collision theory.

These are the factors which increase rate:

• increasing temperature

• increasing the concentration of reactants

• increasing the surface area of the reactant particles (by decreasing particle size)

• by using a catalyst

Collision theory states that in order for a reaction to occur, the particles from the reactants need to collide with the correct orientation and they also need to have enough energy to react on collision. Each of these factors increases the rate by basically increasing the likelihood of a successful collision.

Each of the following paragraphs will provide a perfect exam answer as to why each factor increases the rate of reaction.

Increasing the temperature increases the rate of reaction for two reasons. Firstly, it increases the kinetic energy of the reactant particles which means that there is a greater number of collisions per unit time (as the particles are moving more quickly) and consequently there will be a greater number of successful collisions per unit time. Also, the particles have a greater internal energy and so they are more likely to react on collision and so the proportion of successful collisions is also increased.

Increasing the concentration of reactants increases the number of reactant particles in a given volume and so there is a greater number of collisions per unit time and thus there are more successful collisions per unit time.

Decreasing the size of the reactant particles (e.g by switching from metal ribbons to metal powder (smaller) or metal nanoparticles (smallest)) causes an increase in the surface area to volume ratio (see BBC bitesize). This means that there is a greater surface area for interaction and so there are more collisions per unit time and therefore more successful collisions per unit time.

In an exam, I would advise following this structure: what is changing? (e.g temperature is increasing), what does this change do? (the particles move faster), how does this change link to collision theory? (there is a faster rate of collisions).

For a catalyst, just the definition is required in exams: Catalysts are substances that speed up the rate of reaction but can be recovered chemically unchanged at the end of the reaction. In other words: Catalysts speed up the rate of reaction without being used up in the reaction.

Rate graphs

If the rate of reaction is increased by an increase in temperature, an increase in surface area or by the use of a catalyst then the gradient of the above graph would be increased but the graph would flatten out at the same total volume of gas produced. This is because the total amount of reactant is still the same so the quantity of product must be the same. However, if the concentration of reactant is increased then the gradient will be greater and the total volume of gas will be produced as there is a greater number of moles (amount) of reactant overall and so there will be more product produced.

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