Kinetics

Maxwell-Boltzmann distribution

A Maxwell-Boltzmann distribution is a plot of the number of gaseous molecules against the energy they have at a fixed temperature. It is also called a molecular energy distribution graph. A single plot on the graph shows the distribution of molecular energies at a constant temperature. • No molecules have no energy so the graph starts at the origin. • The area under a Maxwell-Boltzmann distribution curve is equal to the total number of molecules. • The actual curve gives the number of molecules with a certain energy value. Most molecules have moderate energy in the middle of the graph. • The peak of the curve represents the most probable energy of the molecules (Emp). • The mean (average) energy of all the molecules is a bit to the right of the peak. • Some molecules have more than the activation energy, Ea. These are the only ones that can react. • Very few molecules have very high energy but the graph does not touch the axis as there is no upper limit to the energy of the particles.

Presence of a catalyst

A catalyst is a substance that increases the rate of a chemical reaction without being changed in chemical composition or amount. Catalysts work by providing an alternative reaction route of lower activation energy. By lowering the activation energy, more collisions are successful in a given period of time and so the rate of reaction increases. Catalysts are important in many industrial processes by speeding up the process and so reducing costs. Activation energy on a Maxwell-Boltzmann distribution Adding a catalyst to a reaction lowers the value of the activation energy. The area bove the activation area for a reaction in the presence of a catalyst is greater, indicating that there are more molecules with sufficient energy to undergo a successful collision, increasing the rate of reaction. A catalyst does not affect the shape of the distribution as long as the temperature is constant and the number of molecules stay the same.

Effect of concentration and pressure

Effect of concentration: If concentration of a reactant is increased, more particles of that reactant are present, and on average the particles are closer together. This leads to more frequent successful collisions between reactant particles in a given period of time, causing the rate of reaction to increase. Maxwell Boltzmann distribution at different concentrations. If the concentration of the reactant molecules is increased, the shape of the Maxwell-Boltzmann distribution. will change. The curve retains the same basic shape. This means that the most probable energy of the molecules remains the same so the peak should be higher but at the same energy value on the horizontal axis. As there are more reactant molecules at the same temperature, the overall area under the curve increases. This increases the number of reactant molecules which have enough energy to undergo a successful reactions, leading to a higher rate of reaction. The rate of reaction increases at higher concentrations but the effect is much less significant than is achieved when increasing the temperature. Effect of pressure: If pressure on a gaseous reaction system is increased. The particles are forced closer together, which leads to more successful collisions between reactants particles in a given period causing the rate of reaction to increase.

Surface area of solid reactants

Increasing the surface area of solid reactants (by grinding them up) increases the exposed surface of the reactants. This increases the number of successful collisions in a given period of time which increases the rate of the reaction

Collision theory

Reactions can only occur when collisions take place between particles having sufficient energy. Most collisions are unsuccessful - only a very small proportion of collisions is successful and causes a reaction. The minimum energy required for the colliding particles to react is called the activation energy. Most collisions do not lead to a reaction as the particles must be facing each other in the right direction and they must collide with at least the activation energy. The particles must have at least the activation energy to break their bonds and start the reaction. Reactions with low activation energies often happen quite easily, but reactions with high activation energies do not. Extra energy is given to particles by heating them.

Effect of temperature on reaction rate

The rate of reaction is the change in concentration of a reactant or product in a given period. If temperature is increased, the particles gain energy and move faster. A greater proportion of molecules will have at least the activation energy and be able to react. This leads to more frequent and more successful collisions between the reactant particles in a given period of time. Maxwell-Boltzmann distribution at different temperatures • Lower temperature distributions are moved to the left and the peak is higher. • Higher temperature distributions are moved to the right and the peak is lower. The graphs should only cross once. • The curves should always start at the origin and should end up being asymptotic to the energy axis at higher values. If we compare the shaded areas above the activation energy under the distribution curves, it can be seen that at the higher temperature there are many more reactant molecules with enough energy to undergo a successful collision. This explains why there is a higher rate if reaction at a higher temperature. A small increase in temperature can cause a large increase in the number of molecules with enough energy to undergo a successful collision.

Measuring reaction rates

Timing how long a precipitate takes to form You can use this method when the product is a precipitate which clouds a solution. You can watch a mark through the solution and time how long it takes to be obscured. However, this method is subjective as different people might not agree in the exact moment the mark disappears. Measuring a decrease in mass When one or more of the products is a gas, you can measure the rate of formation using a mass balance. As gas is given off, the mass of the reaction mixture decreases. This method is accurate and easy to do. It is usually done in a fume cupboard. Measuring the volume of gas given off This involves using a gas syringe to measure the volume of gas being produced. This method is used when one or more of the products is a gas. Gas syringes usually give volumes to the nearest 0.1 cm3, so this method is accurate.

Required practical 3

To investigate how the rate of the reaction of sodium thiosulfate with hydrochloric acid changes as the temperature of the reaction is changed. Sodium thiosulfate reacts with hydrochloric acid according to the equation: Na2S2O3 (aq) + 2HCl (aq) → 2NaCl (aq) + SO2 (g) + S (s) The reaction produces a precipitate of sulfur. The rate of this reaction can be monitored by measuring the time taken for a fixed amount of sulfur to be produced. An easy method to do this is by timing how long it takes for a cross, marked under the bottom of the reaction vessel, to disappear as it is obscured by the sulfur precipitate. Dilute hydrochloric acid will be added to sodium thiosulfate solution at different temperatures in a series of experiments. The temperature must not exceed 55 °C. a) Add 25 cm3 of sodium thiosulfate into a conical flask. Place the conical flask on top of a piece of paper marked with a cross. b) Prepare 10 cm3 of hydrochloric acid in a test tube. c) Add the hydrochloric acid from the test tube into the conical flask and start the stop clock immediately. d) Record the time taken for the sulfur precipitate to obscure the cross on the paper below the flask.