CCEA ADVANCED SUBSIDIARY

CHEMISTRY

 

MODULE 2

2.8 Kinetics

Kinetics is the study of reaction rates and the factors which influence them. There is a wide diversity in the rates of chemical reactions. Many take place rapidly e.g. precipitation, explosions, and neutralisation. Others take place at moderate speeds e.g. zinc reacting with dilute acid and some are very slow e.g. iron rusting.

A number of factors can alter the rate of a chemical reaction. These  include :

 

Simple collision theory including how the factors affecting rates of reaction may be explained. Defnition of a catalyst. Definition of activation energy.

 

Collision theory

The collision theory assumes that before two chemicals can react together to form products the particles involved must collide. During this collision bonds are broken or made, resulting in the rearrangement of the atoms present and the formation of products. Collisions that result in the formation of products are said to be effective collisions. Only a fraction of the collisions that occur between particles result in the formation of products. There are two main reasons for this:

Activation energy

For reaction to take place the particles involved must collide with a certain minimum of energy called the activation energy. Collisions with energy in excess of the activation energy will result in bonds being broken and reaction occurring. If the energy is insufficient the particles will simply bounce apart without reacting. The activation energy varies from one reaction to another.

Steric factors

The particles must approach one another in the correct orientation for a reaction to take place on collision. This is called the steric factor.

 

·        The nature of the reactants

Reactions between ionic substances are usually very rapid as they have a small activation energy and are not inhibited by steric factors, as the ions are usually quite small. Reaction between organic molecules can have high activation energies and the orientation of the molecules is critical during collision.

 

If one of the reactants is powdered then a larger surface area is created. This means that there will be a larger number of collisions between reacting particles and a greater number of effective collisions giving a faster reaction rate.

 

Increasing the concentration of reactants means there will be more particles in a given volume. This will lead to a greater number of collisions overall and therefore a larger number of effective collisions, giving a faster rate of reaction.

 

Increasing the pressure in a gaseous reaction increases the number of particles in a given volume. Pressure, like concentration, leads to a greater number of effective collisions and a faster reaction rate.

 

At higher temperatures the reacting particles have more kinetic energy and are moving more quickly. This results in both a greater number of collisions and more energetic collisions. A greater proportion of the collisions will have energy in excess of the activation energy giving a bigger number of effective collisions and faster reaction rate.

 

A catalyst is a substance that increases the rate of a chemical reaction without itself being used up.

 

Some reactions which take place slowly in the dark are much more rapid when exposed to light. Light provides energy to speed up the chemical reaction.

Qualitative explanation of the effects of concentration, temperature and catalysis on rate of reaction in terms of the distribution of molecular kinetic energies and activation energy, where appropriate.

Maxwell-Boltzmann distribution curve

The molecules of a gas are continually colliding with each other and in the process speeding up and slowing down. At any particular instant the molecules would show a range of energies, from almost zero to very high energies. This range may be represented by a Maxwell- Boltzmann distribution curve, which illustrates the distribution of molecular energies in a gas.

Text Box: Fraction of molecules with energy E
 

 

 

 

 

 

 

 

 


                                      Energy E

Note that only a very small fraction of molecules have very high or very low energies.

The curve is not symmetrical; the average energy is to the right of the peak of the curve.

Concentration

At a given temperature only a fixed proportion of the molecules will have the necessary energy for reaction, given by the activation energy Ea.

Text Box: Fraction of molecules with energy E
 

 

 

 

 

 

 

 


                             Energy E                Ea

If the total number of molecules is changed by increasing the concentration, there will still be the same proportion of molecules with sufficient energy for reaction. However the total number of molecules has increased so there will be more molecules with the necessary activation energy Ea and the rate of reaction will be faster.

Temperature

The rates of most chemical reactions increase dramatically for only small increases in temperature.  A rule of thumb for many gaseous reactions is the reaction rate is approximately doubled by a rise in temperature of about 10 oC.  The observed increase in reaction rate with temperature rise is not simply due to an increase in the average velocity of the particles resulting in a greater number of collisions per second. (A 10 oC rise results in an increase in collision frequency of only about 1-2%). 

The increase in reaction rate is due mainly to the increased number of particles that possess the activation energy. This is because the proportion of activated molecules increases rapidly.

 


                                          T1

Number of

Molecules                                                                                             T2>T1

 

  T2

 

                                     

 

 

 

 


                             K.E. (kJ mol-1)  or velocity                            Ea

                                                                                   

 

As shown by the graph as the temperature increases, the number of particles with energy ³ Ea (shown by the shaded areas) increases rapidly.

The lower the activation energy barrier the faster a reaction takes place.  Reactions with Ea ~80 kJmol-1 take place fairly rapidly at room temperature.  In general reactions between covalent molecules tend to have high activation energies and are slow.  Reactions between ions tend to have low activation energies and are rapid.

 

Catalyst

A catalyst speeds up a chemical reaction by providing an alternative route with a lower activation energy. Lowering the activation energy barrier to reaction means that a greater proportion of the particles will have the necessary energy leading to an increased rate of reaction.

Text Box: Fraction of molecules with energy E
 

 

 

 

 

 

 

 

 


                   Energy E             Ea(catalysed)   Ea(uncatalysed)

 

 

Use of reaction profiles to illustrate the role of catalysts in providing an alternative reaction pathway with a lower activation energy and to explain the difference between thermodynamic stability and kinetic stability.

Reaction profiles

The activation energy may be shown on diagrams called reaction profiles. These diagrams illustrate the role of activation energy as an energy barrier that must be overcome by reactants before they may form products.

 

Reaction profile of an exothermic reaction

 


                                                        Ea

Text Box: energy                             reactants

 

 


                                                                                           ∆H

                                                                         products

 


                                      reaction co-ordinate

 

Reaction profile for a catalysed reaction

                                                                                                                                     

 


    Ea (uncatalysed)

                                          

                                                

 


                   A + B                   Ea (catalysed)

 

 

 


     AB

 

 

 


The lower activation energy for the catalysed reaction results in a faster rate of reaction as a bigger fraction of the particles have sufficient energy to overcome the activation energy barrier.

 

 

Thermodynamic and kinetic stability

 

As a chemical reaction proceeds, the change in energy of the system can be described by an reaction profile.  For an exothermic reaction it would have the form shown below.

 

                                                                                                                                                                                                                                                                                                                                                                                  Ea                                                                     reactants                                 


enthalpy                                                                     DH                                                                                                                                                                                                                    products                                                                                                                                                                          Reaction co-ordinate                                                                                    
Compared to the products the reactants are energetically unstable (i.e. thermodynamically unstable) and reactants form products with a release of energy ( DH negative).

However the reaction will not occur unless the activation energy is supplied.  The reactants will be kinetically stable therefore if the activation energy is very high.  The reaction

 

          2NH3 (g)                         N2 (g)  +  3H2 (g)    Ea = +355 kJ mol-1

 

does not occur at room temperature, but reactions with a low activation energy will be kinetically unstable

 

          H2 (g)  +  Cl2 (g)                         2HCl (g)       Ea = +25 kJ mol-1

 

and occur readily.

 

Examples of homogeneous catalysis involving the formation of an intermediate, and heterogeneous catalysis involving chemisorption.

Catalysts can be classified as either homogeneous (same state as the reactants) or heterogeneous (different state from the reactants). There are two theories of catalysis.

 

Homogeneous catalysis

The intermediate compound theory explains most examples of homogeneous catalysis.  The catalyst reacts with one of the reactants to produce an intermediate compound.  This reacts further to form the required product and regenerate the catalyst.  The formation of the intermediate compound requires less energetic collisions between the particles than direct combination.  This lowers the activation energy barrier so more particles can react per unit time.                                                                            (AB)

 


    Ea(AB)

                                          

                                                 (AC + B)

 


                   A + B                   Ea(AC)

 

 

 


     AB

 

 

 

 


A  +  B                                      AB     High energy required for direct combination.

 

A  +  C                                      AC

      (catalyst)                             intermediate compound (unstable)      low energy required

AC  +  B                          AB  +  C

                                                catalyst regenerated

 

 

Heterogeneous catalysis

The catalyst used in this process acts by providing a surface on which the reactant molecules  can form sufficiently strong bonds to enable weakening of the bonds between the atoms in the reacting molecules. Gas molecules are adsorbed when they collide with and adhere to the catalyst surface. The adsorbed gas molecules are in close contact with each other in a state, which enables them to react together. The product molecules are desorbed from the catalyst surface, which is then free for further adsorbtion of reactant molecules. The transition metals and their compounds are good catalysts because of their ability to exist in a variety of oxidation states.

 

 

                          A            B                 

                        B          A                      reactant molecules

Text Box: Catalyst surface                              A                B

 

                                    A

                       

                                    B

                                                            molecules adsorbed

                                    B                      onto catalyst surface

 


                                    A

                                                AB        product desorbed

                                    AB

 

 

 

 

 

Enzymes regarded as biological catalysts. It is expected that candidates will be able to state that an enzyme is a protein (although detailed knowledge of proteins is confined to later modules) has an active site and provides a path of lower activation energy. Details of the mechanisms of enzyme catalysis are not expected in this module.

Enzymes

Enzymes are protein-based homogeneous catalysts found in living things. They consist of complex protein chains coiled into specific shapes. Part of the enzyme (often containing a transition metal ion), is the active site where reaction takes place. Reactant molecules must fit the shape of the reactive site (like a key fitting a lock). The shape of a protein molecule governs its catalytic activity. This shape is easily altered by relatively small changes in temperature or pH. When this occurs the protein’s efficiency as a catalyst is reduced and it is denatured. Enzymes thus have an optimum temperature, often around the body temperature of 37 oC. Up to this temperature the reaction rate increases as normal, above this temperature the reaction rate decreases.

 

 

A simple account of the role of catalytic converters in reducing the environmental damage due to vehicle emissions by facilitating the conversion of carbon monoxide to carbon dioxide, of unburnt hydrocarbons to carbon dioxide and water and of NOx to nitrogen; catalyst poisoning by lead. (Technical details of the construction of catalytic converters are not required.)

Catalytic converters

Motor vehicle exhausts are a major source of pollution and are responsible for significant amounts of carbon monoxide, hydrocarbons and nitrogen oxides (NO and NO2-often abbreviated as NOx). Catalytic converters fitted to the exhaust systems of cars remove pollutants by getting them to react with one another to form harmless products. e.g.

          2CO  +  2NO                                      2CO2  +  N2

          4-CH2-  +  6NO2                        4CO2  +  3N2  +  4H2O

The catalyst system consists of a ceramic honeycomb (to give a large surface area for reaction), coated with platinum, palladium and rhodium.

Catalytic converters have a number of drawbacks. They are expensive and the vehicle must run on lead-free fuel or the catalyst will be ‘poisoned’ by the lead and hence rendered ineffective. The catalyst system is only effective at temperatures over 400 oC and will be ineffective on short journeys as it does not reach the required operating temperature.

Catalytic effect of chlorine on ozone in the upper atmosphere.

 

Depletion of the ozone layer

About 25 km above the Earth’s surface ozone is formed by photochemical reactions in the atmosphere.

                   3O2 (g)                            2O3 (g)

the ‘ozone layer’ protects the Earth by filtering out most of the harmful ultraviolet light from the Sun. Normally the generation of ozone balances its destruction and the concentration of the gas remains steady. However in recent years chlorine atoms originating from chlorofluorocarbons (CFCs) used in aerosol propellants, refrigerators and plastics manufacture have led to an increased rate of destruction of ozone in the atmosphere. The chlorine atom catalyses the decomposition of ozone to oxygen.

                   O3  +  Cl•                         ClO  +  O2

                   O  +  ClO                         Cl•  +  O2

The result is that the rate of loss of ozone is greater than its rate of production. The UV filtering effect is reduced and more high-energy UV is reaching the Earth’s surface. This may result in increased incidence of skin cancer as well as affecting marine plankton which are vitally important as they are at the base of the marine food chain.

International agreements have restricted the manufacture and use of CFCs but it is predicted that it may take several decades before the CFCs already present in the atmosphere are used up.