C5 - Reaction Rates

Front 1

Describethe meaning of exothermic and endothermic reactions

Back 1

Chemical reactions that release heat energy to the surroundings are called exothermic reactions.

Chemical reactions which take in heat from the surroundings are called endothermic reactions.

Front 2

Interpret energy level diagrams showing exothermic reactions

Back 2

The energy in the reactants is greater than the energy in the products.

The energy taken in to break bonds in the reactants is less than the energy given out when new bonds are made.

Front 3

Interpret energy level diagrams showing endothermic reactions

Back 3

The energy in the reactants is lower than the energy in the products.

The energy taken in to break bonds in the reactants is greater than the energy given out when new bonds are made.

Front 4

Describebond breaking as an endothermic process and bond forming as an exothermicprocess

Back 4

Breaking a bond requires energy put in, so bond breaking is endothermic. When new bonds are formed, the opposite happens - energy is given out to the surroundings, so bond making is exothermic.

Front 5

Drawand label energy level diagrams for exothermic and endothermic reactions usingdata provided

Back 5

The difference between the energy of the reactants and products is shown by the symbol ΔH (delta H). If heat energy is given out, ΔH is given a negative sign. If heat energy is absorbed it is given a positive sign.

Front 6

Calculatethe energy of a reaction using bond energies

Back 6

Each type of bond has a particular amount of energy needed to break it. This is called the bond energy. Bond energy is the amount of energy needed to break one mole of a particular bond in one mole of gaseous atoms. The symbol for bond energy is E.

ex) It needs 498kJ to break bonds in one mole of hydrogen molecules. E(H-H) = +436 kJ/mol

Front 7

Describeand explain the effect of concentration, particle size, catalysts (includingenzymes) and temperature on the rate of reactions

Back 7

• The higher the concentration of a substance, the faster the rate of reaction, as a substance has more particles of it per square metre the more concentrated it is and the more particles it has that can react, the higher the collision rate.
• The smaller the particle size, the faster the rate of reaction, as the more surface area there is that can undergo the reaction.
• A catalyst, for example, an enzyme, is a substance that speeds up a chemical reaction. Catalysts allow reactions to happen faster and at lower temperatures, they remain the same as they were at the start of the reaction, and are reusable.
• The rate of reaction increases with an increase in temperature because more of the colliding molecules have energy equal to, or above the activation energy, the minimum amount of energy particles must collide with in order to react, and there is an increase in collision rate.

Front 8

Describethe application of the above factors to the danger of explosive combustion withfine powders (e.g. flour mills) and gases (e.g. methane in mines)

Back 8

Many industrial processes cause fine powders to get into the air. These powders are highly combustible. They burn very readily in the air because of their very large surface area. Examples are flour in flour mills and wood dust in sawmills. In coal mines, there is another hazard - the methane gas which is often present can form an explosive mixture with air.

Front 9

Demonstrateknowledge and understanding of a practical method for investigating the rate ofa reaction involving gas evolution

Back 9

If a gas is given off in a reaction, you can collect it in a gas syringe. The volume of gas is recorded at intervals of time. Then, you can draw a graph of volume of gas against time.

You could also record changed in pressure for reactions involving gases and that variable.

Front 10

Interpretdata obtained from experiments concerned with rate of reaction

Back 10

Rate of reaction =

change in concentration of reactant or product

time

We often need to know how the reaction rate changes as the reaction proceeds. On a graph, the gradient gives us the reaction rate at any particular time.

Front 11

Devise and evaluate a suitable method for investigating the effect of a given variable on the rate of a reaction

Back 11

There are many methods for measuring the rate of reaction. You can use any property that changes during a reaction, for example, a pH meter or electrical conductivity meter if the hydrogen ions are used up in the reaction.

Front 12

Describeand explain the role of light in photochemical reactions and the effect oflight on the rate of these reactions

Back 12

A few chemical reactions are started by ultraviolet or visible light.These are called photochemical reactions. The rate of photosynthesis increases with an increase in light intensity, but it levels off due to carbon dioxide being a limiting factor.