Chemistry Coursework Rates of Reaction Investigation

The actual rate of many reactions is much less than that predicted by the collision rate by factors of about 1011 which means that only a few collisions actually result in a reaction.

Having enough energy and colliding with the right orientation are essential for a reaction to occur.

The transition state theory considers the details of the actual collision. When two molecules approach closely on a collision course, their electron clouds begin to repel. Unless they have a lot of kinetic energy, this repulsion will push them apart before they get close enough for new bonds to form. If they get sufficiently close, rearrangements of the electrons in the outer shells can take place so that new bonds can form and old ones break. The kinetic energy of the collisions is converted into potential energy.

The highly energetic and unstable species that exists briefly at the point of maximum potential energy is called the transition state or activated complex. In this species bonds are in the process of making and breaking. For example:

A B + C A + B C

The transition state could be represented as:

A----B---C

Where a dashed line represents bonds in the process of breaking and forming. One can represent the reaction on an enthalpy diagram.

The transition state is the highest point in the diagram. The energy gap between the reactants and the transition state is the activation energy EA. If two molecules collide with less energy than EA they cannot react. In an exothermic reaction, a small amount of energy has to be supplied to start the reaction even though energy is given out in the end . therefore by raising the temperature, more molecules in the system will have the same energy as EA or higher meaning that more molecules are likely to react.

This particular investigation aims to investigate the effect of temperature and concentration on the rate of reaction.

It's known that small changes in temperature can produce large changes in rate. As I mentioned before, for every 10K rise in temperature the rate of reaction doubles.

Temperature is a measure of the average speed of molecules.

Molecules in a solution do not all have the same speed. Their speeds and therefore energies are distributed according to the Maxwell-Boltzmann Distribution: a few having low speeds, a few having high speeds and most somewhere in the middle. This is shown in the graph.

As I mentioned before, reactions can only take place if the molecules have enough energy to start bond breaking ( activation energy). The activation energy is marked on the graph, the area under the graph to the right of the line represents the number of molecules with sufficient energy to react.

The shape of the graph changes with temperature. At higher temperatures there are more molecules with higher energies, so the peak of the graph moves to the right as does the number of molecules with very high energies. You can see from the graph that as the temperature increases a greater proportion of molecules have enough energy to react. This is the main reason for the increase in reaction rate with temperature. The total number of collisions also increases as the molecules move faster but this is much less significant than the number of effective collisions ( those with energy greater than EA ). Collisions also have to have the right orientation, called the steric factor.

This means that the rate of reaction depends on three factors (considering concentration isn't being varied):

* The total number of collisions

* The fraction of those which have the right orientation

* The fraction which have energy greater than the activation energy

And the actual rate of reaction is the product of all three.

The Maxwell-Boltzmann distribution is such that the fraction of molecules with energy greater than EA is given by e-E /RT , where R is the gas constant and t the temperature in Kelvin. This type of relationship is called an exponential relationship and means that a small rise in temperature results in a large increase in the fraction of molecules with energy greater than EA. The shaded area of the graph shows this.

This leads one to the Arrhenius equation.

Ln k = EA ( 1/T) where k is the rate constant for the reaction

R R is the gas constant, 8.31 K-1 mol-1

EA is the activation energy of the reaction, in J mol-1

T is the temperature in kelvins

This equation allows one to calculate the activation energy of a particular reaction by testing the reactivity of a solution at different temperatures.

The other variable I will be testing is concentration. By varying the concentration I hope to deduce an order of reaction with respect to [HCL].

The reaction of hydrochloric acid with magnesium metal is one that can be easily tested under the given laboratory conditions.

The acid reacts with the metal to give off hydrogen gas:

2HCL + Mg MgCl2 + H2

This reaction needs to be carried out in such a way that the results allow me to identify a possible order of reaction and a value for the activation energy.

There are two practical methods for carrying out this experiment that I would like to test:

* Gas production

* Mass change

Both of these methods allow me to work out a rate of reaction.

In the given reaction, it is possible to follow it, by measuring the volume of hydrogen gas evolved at any time.

I plan to test this method and compare the accuracy of results with those acquired using the mass change procedure.

Before starting the actual practical I need to decide which concentrations of HCL I intend to use. I am going to test five different concentrations of HCL: 0.2, 0.4, 0.6, 0.8, and 1.0 mol.