chemical kinetics

the increase in concentration of products or the decrease in concentration of reactants per unit time

The rate of a reaction is not constant during a reaction, but is greatest at the start and decreases as the reaction proceeds.

1. change in volume of gas produced

2. change in mass

3. change in light transmission: spectrophotometry/colorimetry

4. change in concentration measured by titration

5. change in concentration measured using conductivity

6. clock reactions and non-continuous methods

• useful when one product is a gas

• using a gas syringe

• using water displacement from an inverted burette with a gas that has low solubility in water

• one of the reactants or products is coloured and gives characteristic absorption in the visible region (wavelengths about 320–800 nm)

• an indicator can be added to generate a coloured compound that can then be followed in the reaction

• a colorimeter or spectrophotometer works by passing light of a selected wavelength through the solution and measures the intensity of the light transmitted by the reaction components

• as the concentration of the coloured compound increases, it absorbs proportionally more light, so less is transmitted.

• keeping the mixture on a scale

• not useful when the evolved gas is hydrogen, as it will not produce too much of a change in mass

• titrating against a known ‘standard’

• samples taken out from the mixture at regular time intervals

• electrical conductivity of a solution depends on the total concentration of its ions and on their charges

• using a conductivity meter which involves immersing inert electrodes in the solution

something observable which can be used as an arbitrary ‘end point’ by which to stop the clock

• energy of collision - activation energy is required to overcome the repulsion between the particles and often break bonds

• geometry of collision - proper orientation of particles that react together is a determining factor whether the reaction will take place

  • not all collisions are successful!

1. temperature

2. concentration

3. particle size

4. pressure

5. catalyst

• greater temperature = greater kinetic energy

• more particles will overcome the activation energy

• greater collision frequency

• increased rate of reaction

• for gases

• higher pressure = higher rate of reaction

• gas is compressed, which increases its concentration, so there is a greater chance that particles will collide

• decreasing the particle size increases the rate of reaction

• greater surface area

• greater chance for collisions

• increased concentration = more frequent successful collisions

• they are closer together so there is greater chance they will collide

• a substance that increases the rate of a chemical reaction without itself undergoing permanent chemical change

• lowers activation energy, so more particles have enough energy to collide

a particular reaction at a specified temperature

the power to which its concentration is raised in the rate equation

the sum of the individual orders for all reactants.

mol dm–3 s–1

s-1

mol–1 dm3 s–1

mol–2 dm6 s–1

• 0: no change

• 1: rate doubles

• 2: rate x4

• 0: no change

• 1: rate x3

• 2: rate x9

• 0: no change

• 1: rate x4

• 2: rate x16

the slowest step of the mechanism, determines the rate of the whole reaction, its transition state has the highest energy

the frequency of collisions and the probability that collisions have proper orientations.