Graph 4.1 shows the decrease in the amount of a solid reactant with time. The graph is curved, becoming less steep as the gradient decreases because the reactants are being used up, so the speed decreases. Here the gradient is a measure of the rate of the reaction. In the first few minutes the graph will (i) decline less steeply for larger 'lumps' and (ii) decline more steeply with a fine powder i.e. (i) less surface area gives slower reaction and (ii) more surface area a faster reaction.

Graph 4.2 shows the increase in the amount of a solid product with time. The graph tends towards a maximum amount possible when all the solid reactant is used up and the graph becomes horizontal. This means the speed has become zero as the reaction has stopped. Here the gradient is a measure of the rate of the reaction. However, I don't know of any practical method of following a reaction by measuring the amount of solid formed.

Graph 4.4 shows the increase in speed of a reaction with increase in temperature as the particles have more and more kinetic energy. The rate of reaction is proportional to 1/t where t = the reaction time. See the notes on rate in the Graph 4.7 paragraph below and the theory of temperature effect.

Graph 4.8 Some general interpretations of a set of results for the same reaction that produces a gaseous products from e.g. adding a reactant solid to a reactant solution.

(i) The graph lines W, X, original experiment E, Y and Z on the left diagram are typical of when a gaseous product is being collected. The middle graph might represent the original experiment 'recipe' i.e. in terms of initial concentration, initial amount of solid and its particle size and of course at a specific initial and constant temperature. Then the experiment repeated with variations of the controlling factors producing different graphs e.g. typically graphs W, X, Y and Z.

The average rate of reaction at 2 mins for each of the five experimental runs would be 31.0/2 = 15.5 cm³/min for W, 20.0/2 = 10 cm³/min for X, 16.0/2 = 8.0 cm³/min for E, 10.0/2 = 5.0 cm3/min for Y and 6.0/2 = 3.0 cm3/min for Z. BUT some of these values would be significantly different from the initial rate e.g. for run X the graph is very curved and you can see the initial rate is more like 6.0/0.2 = 30 cm³/min, very different from that calculated at 2 mins. Ideally you should try and measure the tangent gradient close to the start of the reaction where you should observe a short 'linear' portion on the graph line.

(ii) X could be the same 'recipe' as the original experiment but a catalyst added, forming the same amount of product, but faster - steeper initial gradient. You would get a similar result by increasing the temperature of the reactants, as long as you measure the gas volume itself at the same temperature and pressure.

(iii) Initially, the increasing order of rate of reaction represented on the graph by curves Z to X i.e. in terms of speed of reaction X > original E > Y > Z might represent one of the following situations ..

progressively increasing concentrations of reactant

progressively higher temperature of reaction

progressively smaller lumps-particle/increasing surface area of a solid reactant.

All three trends in changing a reactant/reaction condition variable produce a progressively faster reaction shown by the increasing INITIAL gradient in cm³/min which represents the rate/speed of the reaction.

(iv) Z could represent taking half the amount of solid reactants or half the original concentration.

The initial reaction rate is slower (halved) and only half as much product gas is formed.

(v) W might represent taking double the quantity of reactants, forming twice as much gas e.g. same volume of reactant solution but doubling the concentration, so producing twice as much gas, initially at double the speed (gradient twice as steep).

More details of laboratory investigations ('labs') involving 'rates of reaction' i.e. experimental methods for observing the speed of a reaction are given in the INTRODUCTION