Investigation of some of the factors affecting rates of reaction.

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Investigation of some of the factors affecting rates of reaction

Introduction

The rate of a chemical reaction is the speed at which that reaction proceeds. The speed at which reactions occur is of great importance to all living things, as their cells use chemical reactions to grow and reproduce. It is also very important in chemical industry to understand the speed of chemical reactions.

The rate of reaction can be measured by either following the speed of formation of the products or the speed of decrease of the reactants. We normally choose which is easier to follow, for example if a solid is formed as a product it will be easier to follow the formation of the product. As the rate is the speed of formation or depletion, it is calculated by measuring the time taken to form the product and:

Rate =      1___

                                                                      Time taken   

Therefore, the rate has units seconds-1.

Any chemical reaction involving reactants and products requires the particles of the reactants to collide with each other before reaction can take place. Therefore we can decide which factors are likely to affect the rate of the reaction based upon collision theory:

  1. Temperature. This will affect the rate of the reaction in that heat energy causes particles to move faster and have more kinetic energy. So, as temperature is increased, then the particles will be colliding more frequently and also with more force or energy. As the energy increases, the proportion of molecules with sufficient energy to react successfully will increase. This is due to the distribution of energies of all molecules being like that in the following sketch graph:

This graph shows that when the temperature increases the distribution shifts

upwards in terms of energy. The number of particles with the maximum energy is

less, but the number of molecules at any higher energy is greater. This is called

the Maxwell-Boltzman distribution.

  1. Concentration of the solution. This will affect the rate of the reaction as particles are more likely to collide if there are more of them in the same volume. It will not increase the energy of the collisions.
  2. Surface area. This will only affect the rate of reaction if one of the reactants is a solid as only those particles on the surface are available for collision.
  3. Pressure. This will only affect the rate of reaction if one or more of the reactants or products is a gas.
  4. Catalyst. This will speed up the rate of reaction without being used up. Some reactions have a catalyst and others do not.

The reaction which will be used in the investigation is that of hydrochloric acid with sodium thiosulphate. This is suitable as a suspension of sulphur, which is insoluble in water, is formed. The time taken for the suspension to build up (that is, rate of product formation) can be measured.

Na2S2O3(aq) + 2HCl(aq)         2NaCl(aq) + H2O(l) +  SO2(aq) + S(s) 

           

Therefore, as both of the reactants are solutions and there is no solid reactant, surface area cannot be used as a variable. Also, as no gases are involved either as products or reactants, then pressure cannot be used as a variable. No catalyst will be provided for this reaction, so it cannot be used as a variable. Hence, the two variables which will be used are temperature and concentration. These variables are good ones in that they are easy to control and also easy to measure.


A.

                         HEAT (from Bunsen Burner)

B.

        

Figure 1. The apparatus to be used.  In A, the flask is heated, if required. In B, the flask is placed on the red cross drawn on paper.

Hypothesis for the concentration experiment

It may be predicted that if the concentration of one or both of the reactants is increased, then the rate of reaction will increase. This is because the concentration of a substance in solution is the number of particles per unit volume. It is measured in moles per decimetre cubed. Therefore, the number of effective collisions between the particles will increase as there are more of them in the volume and the probability of a collision is greater as they are moving randomly. Collisions are necessary for reaction, and particles can only react once they have effectively collided. An increase in concentration at constant temperature will not increase the energy of the particles as no additional heat energy is being supplied.

Hypothesis for the temperature experiment

It may be predicted that if the temperature is increased, then both the frequency and the energy, or force, of the collisions will increase. Temperature increase means an increase in heat energy has occurred and this will make the particles move faster, so making them more likely to collide. Also as they move faster their kinetic energy will increase, so they will be hitting harder in each collision. Therefore the probability that any collision will be successful for reaction will be greater. This is due to the fact that particles colliding with insufficient energy fail to react, for example the molecules of oxygen and nitrogen in air are colliding but do not react as the temperature is not high enough. But if the temperature is raised, for example in a lightning bolt or in a car engine, then they can react. Hence the rate of reaction will increase when the temperature is raised, but its effect will be more pronounced than for concentration, as it affects both the frequency and the energy.

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Variables

The variables in this experiment, and they ways in which they may be controlled, are as follows:

  • Temperature. This will affect rate of reaction as stated above, and will be altered in this investigation.
  • Concentration of reactants. This will affect rate of reaction as stated above, and will be altered in this investigation. Since there are two reactants, only the concentration of one will be altered and the other kept constant. The acid will be constant at 2.00moldm-3.
  • Volume of reactants. This will be kept constant as far as possible. ...

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