Rates of Reaction

RATES OF REACTION

Understanding rates of reaction and the factors which affect them is an important part of chemistry in industry. It may be important to slow reactions, as in rusting or rotting, or it may be important to speed up reactions, as in the Contact process. Millions of dollars are spent each year looking for ways to make industrial processes faster and cheaper.

MEASURING RATES OF REACTION

Rates of reactions can be expressed in terms of the rate at which reactants are used up, or the rate at which products are formed.

In a reaction: Reactants  Products

A+B AB

Rates can therefore have a variety of units depending on how the rate is measured; moles (mol.s-1) Concentration: Mol.dm-3.s-1, Volume: cm3.s-1 etc.

The rate of reaction can be measured by observing the rate at which a particular property changes. This change can then be linked to the concentration of a reactant being used up or product formed.

Examples of properties measured include: pH, colour intensity, conductivity, volume of gas, mass of solid formed/lost, turbidity (precipitate formation)….

Equipment used in measuring rate:

1. Electronic scale – mass change
2. Gas syringe – volume of gas
3. Spectrophotometer – colour change, increasing opaqueness of solution
4. pH meter – acid/base reactions
5. electrodes and ammeter – conductivity (change in concentration of ions)

The units used may vary but the most common are mol.s-1. However for matric purposes we often do not use moles but work in more straightforward units (e.g. cm3gas formed per minute or mass of gas lost per minute etc)

ENERGY AND RATES OF REACTIONS

In order for a reaction to occur a collision between particles has to occur. This collision must:

1. be with the correct orientation. (esp in organic chemistry)

1. have sufficient energy to form an activated complex (overcome the ACTIVATION ENERGY BARRIER) (more later).

It is thought that when two reactants collide with sufficient energy and the correct orientation an ACTIVATED COMPLEX is formed. This is a temporary, high energy, unstable transitional complex. It may then decompose to form either the reactants again or form the products.

AB + C ⇌ [ABC] ⇌ A + BC

complex

Exothermic reaction (ΔH negative):

Endothermic reaction:(ΔH positive):

FACTORS WHICH AFFECT THE RATE OF A REACTION

1.CONCENTRATION OF REACTANTS (Pressure for gases)

The higher the concentration of a solution – the higher the reaction rate.

This is because the rate of successful collisions increases. If gases are at higher pressure there is an increase in the number of collisions so it has the same effect.

In simple situations, the doubling of a concentration doubles the rate. However this can only be confirmed by experimentation.

Reaction curves

When substances react, the reaction usually starts off rapidly and then slows down as the concentration of one or more of the reactants decreases. Eventually it stops as one (or more) of the reactants runs out.

e.g.If hydrochloric acid is added to marble chips the mass of the reaction mixture decreases as carbon dioxide gas is given off.

CaCO3 + 2HCl  CaCl2 + H2O + CO2

If the mass is plotted against time a curve similar to the one below is plotted.

2.TEMPERATURE

The higher the temperature – the higher the reaction rate.

The higher the temperature, the faster the average speed of the molecules. This results in two things:

i)the molecules collide more often and

ii) more of the collisions are with sufficient energy for a reaction to occur. (This is the main cause of the increased rate.)

A 10oC rise in temperature approximatelydoubles the rate of a reaction.

A pressure cooker cooks food more quickly because the food is at a higher temperature.

3.SURFACE AREA OF A SOLID OR LIQUID(state of division)

A powder will react faster than large pieces of a solid.

This is because there is a greater surface area for the same mass of solid resulting in a greater rate of collisions.

This can be a problem in coal mines and flour mills where the dust is flammable.

Sprays of flammable liquids in air also have a faster rate of reaction for the same reason. Use is made of this in a petrol or diesel engine.

4.CATALYST

Definition: A catalyst is a substance which, when added to a reaction mixture, will alter the rate of the reaction and can be collected unchanged chemically at the end.

A catalyst works by providing a lower energy pathway for the reaction. It does this in a variety of ways but the result is that more collisions will be with sufficient energy for a reaction to occur. (see graphs/notes at end)

There are numerous examples of catalysts but they can be lumped into two groups:

Homogeneous – in the same phase as the reactants e.g. enzymes.

Heterogeneous – in different phase to the reactants e.g. Pt in Ostwald’s

(see diagrams at end of notes.)

1. NATURE OF SUBSTANCES Simple or Complex

Complex molecules take longer to react than simple ones. This is due to the fact that that the complex molecules have many bonds to break whereas simple ones don’t.

eg. COVALENT MOLECULES vs IONS IN SOLUTION

Ions in solution tend to react faster than covalent substances in solution. This is because the ions simply collide and react whereas the covalent bonds must first be broken for a reaction to occur. The activation energy barrier of the former is lower than the latter.

e.g. Both FeCl2 and oxalic acid ((COOH)2) can reduce purple KMnO4 to colourless Mn2+ ions. With FeCl2, which forms ions in solution, the reaction is instantaneous. With oxalic acid, which is predominantly covalent in solution, the reaction is very slow and takes several minutes to become colourless.

KMnO4(aq) + 5FeCl2(aq) + 8HCl(aq)MnCl2(aq) + 5FeCl3(aq) + KCl(aq) + 4H2O(l) FAST

5H2C2O4 + 2KMnO4 + 3H2SO4 K2SO4 + 2MnSO4 + 10CO2 +8H2O SLOW

7.RADIATION (Not Examinable)

Some reactions are light sensitive and radiation will increase the rate of reaction.

The radiation contains photons with the right energy to break bonds.

e.g 1. UV light will break Cl2 bonds. So when a mixture of H2 and Cl2 is irradiated the reaction occurs rapidly.

H2 + Cl2 2HCl

e.g.2. Alkanes react very slowly with bromine in the dark but more quickly in sunlight.

CH4 + Br2 CH3Br + HBr

e.g. 3. Photography makes use of the fact that light breaks down certain chemicals. In the case of black and white photography the light breaks down silver bromide to silver metal.

2AgBr  2Ag + Br2

e.g 4. Photosynthesis

6H2O + 6CO2 C6H12O6 + 6O2

CATALYSTS AND ACTIVATION ENERGY

Collisions only result in a reaction if the particles collide with enough energy to get the reaction started. This minimum energy required is called the activation energy for the reaction. (Below is a Maxwell-Boltzmann distribution curve.)

To increase the rate of a reaction you need to increase the number of successful collisions. One possible way of doing this is to provide an alternative way for the reaction to happen which has a lower activation energy. This can be done by adding a catalyst. A catalyst provides an alternative route for the reaction with a lower activation energy.

Spontaneous Reaction:-these happen without supplying energy from an external source because the activation energy is low. E.g. acids reacting with alkalis.

Non-spontaneous Reaction:-the activation energy is so high that no collisions at room temperature are with sufficient energy for a reaction to take place. E.g. butane and oxygen.