Unit 6 : Rates If Reaction

Front 1

Main Factors Effecting Rates of Reaction

Back 1

• the concentration of the reactants (or pressure if they are gases)
• the temperature of the reaction mixture
• the surface area of any solid reactants, or solid catalyst
• the presence of a catalyst.

Front 2

Collision Theory

Back 2

Molecules and ions need to collide before they can react.

Front 3

Increase in Concentration

Back 3

Concentration: this means the amount of substance dissolved in a given volume.
If we double the concentration of acid, there will be twice as many hydrogen ions in a given volume,
and so twice as many collisions with marble each second.
Although every collision does not lead to reaction, doubling the total number of collisions should double the rate of reaction.
Note that if we add a larger volume of acid of the same concentration, the rate will be the same at first, but reaction will carry on for longer, and more gas will be produced at the end. An experiment to show this might use one large chip of limestone, and the same volume of acid but of different concentrations.
A graph of rate against concentration of acid should be a straight line, going through the origin. For gaseous reactants, the pressure is the most convenient measure of concentration.

Front 4

Increase in Tempreture

Back 4

Temperature: when we heat a reaction mixture up, the rate of reaction rises sharply .
For many reactions the rate will double if the temperature rises by 10C, and then double again for the next 10o rise.
The minimum energy which molecules must gain before they can react is called the activation energy.
When a reaction mixture is heated up, the molecules move around more rapidly, and far more of them have enough energy to react (the activation energy). There is also a small increase in the number of collisions. Both factors mean that there are more successful collisions, in a given time, which lead to reaction, so the rate increases sharply

Front 5

Increase in Surface Area

Back 5

Surface Area: if a reaction involves a solid, it can only occur when molecules strike the surface.
Therefore if there is a larger surface area, more particles will be exposed and so there will be more
collisions, per second, and the rate will be faster.
In general: rate with large lumps is less than rate with small lumps is less than rate with powder.

If you are designing an experiment to show this, you must be careful to use the same mass of solid, the same temperature, and the same volume of acid. There should be an excess of solid, so most of it doesn’t get used up.

Front 6

Catalysts

Back 6

Catalysts: a catalyst is a substance which, when added in small amount, will speed up a chemical
reaction, without itself being used up.
Catalysts provide an alternative route for the reaction that has a lower activation energy. Therefore more particles have the minimum energy to react and so more of the collisions are successful.
There is no catalyst for the reaction between marble chips and hydrochloric acid. However, many other reactions are speeded up by catalysts. Different reactions need different catalysts.

Front 7

Examples of Catalysts

Back 7

• decomposition of hydrogen peroxide by manganese(IV) oxide
2H2O2 → 2H2O + O2 [MnO2 as catalyst]
• the Haber Process for manufacture of ammonia
N2 + 3H2 2NH3 [finely divided iron as catalyst]
• the Contact Process for manufacture of sulphuric acid
2SO2 + O2 2SO3 [Vanadium(V) oxide as catalyst]

Front 8

How a Catalyst Works

Back 8

A solid catalyst works by allowing reactants to bind temporarily to its surface. This can weaken their
chemical bonds (providing an alternative route with lower activation energy), and/or bring them together so that they can react. Once reaction is complete the products leave the surface, which can act as catalyst for more reactants.

Front 9

Biotechnology

Back 9

Enzymes used in washing powders. That help break down stains, sweat, grease and blood.

Other Uses: cheese making, yogurt, beer, bread, ethanol.

Front 10

Light

Back 10

Light radiation: Rates of certain reactions can be influenced by light radiation. These are called photochemical reaction.

Example:
Photosynthesis: green pigment called chlorophyll absorbs light energy from the sun and catalyze to form glucose for the plant.