1. Fast and slow reactions
Key knowledge
Collision theory and factors that affect the rate of a reaction including activation energy
Energy profile diagrams and the use of H notation

2. Chemical energy
The chemical energy of a substance is the sum of its potential energy (stored energy) and kinetic energy (energy of movement)
These energies result from such things as:
Attractions between electrons and protons in atoms
Repulsions between nuclei
Repulsions between electrons
Movement of electrons
Vibrations of and rotations around bonds
Chemical energy of a substance is sometimes called its heat content or enthalpy and is given the symbol H

3. Energy changes during chemical reactions
Exothermic reactions
Total chemical energy of the products less than the energy of reactants
Since energy is never lost, the products release energy into the environment – often as heat
E.g. combustion of petrol
Endothermic reactions
Chemical energy of the products is greater than the energy of reactants
Energy must be absorbed from the environment around the reactants in order for the reaction to occur

4. H = H(products) – H(reactants)
Heat of reaction
The energy released or absorbed during a chemical reaction is called heat of reaction
The heat of reaction is equal to the difference in enthalpy between products and reactants
The heat of reaction is given H (delta H)
H = H(products) – H(reactants)
Exothermic reactions H(products) is less than H(reactants) H will be negative, H0
Endothermic reactions H(products) is greater than H(reactants) H will be positive, H0

5. Enthalpy

7. Energy profiles
You need to recall what happens to chemical bonds during the course of a reaction
The bonds between atoms in the reactants must first be broken. For this to occur, energy must be absorbed
Then new bonds form as the products are created
Energy required to break the bonds of reactants so that a reaction can proceed is called activation energy

9. Profile of an exothermic reaction

11. Profile of an endothermic reaction

12. Thermochemical equations
Thermochemical equations show the energy released or absorbed during a chemical reaction
Energy is measured in joules (J) or kilojoules (kJ).
The H, has units J mol-1 or kJ mol-1 \
Energy corresponds to the mole amounts specified by the equation

13. H calculations

16. Energy calculations

19. The collision theory
For chemical reaction to occur, particles involved must collide with each other with sufficient energy to overcome the activation energy ‘barrier’ to the reaction
It makes sense the greater the number of collisions, the greater the rate of reaction

20. Factors that affect rates
Four main ways
Increasing the surface area of solids
Increasing the concentration of reactants in solution (or pressure of gaseous reactants)
Increasing the temperature
Adding a catalyst

21. Increasing surface area
In a solid, only those particles that are at the surface can be involved in the reaction.
Crushing a solid into smaller parts means that more particles are present at the surface.
As a consequence of the greater number of exposed particles, the frequency of collision between the particles and reactants particle increase, and so the reaction occurs more rapidly
Example Sherbet

22. Increasing concentration
More molecules or ions dissolved in solution occur faster if the concentration of the dissolved particles is increased.
With more particles moving randomly in a given volume of solution, the frequency of collisions is increased.

23. Increasing the temperature
As temperature increases, the average speed and average kinetic energy of the particles increases as well
Hence, as temperature increases so does the rate of reaction
The effect of temperature on reaction rate cannot be simply explained by the increased frequency of collisions
A temperature increase of just 10C causes the rate of many reactions to double, but collisions have increased about one-fiftieth of the amount

24. Distribution of energies of particles at particular temperatures

25. The distribution of energies of particles at two temperatures
The distribution of energies of particles at two temperatures. Raising the temperature increases the proportion of particles with higher energies.

26. The energies of particles at two temperatures
The energies of particles at two temperatures. EA represents the activation energy for the reaction. The shaded area under the graph represents the proportion of particles with sufficient energy to react.

27. Catalyst
Many reactions occur more rapidly in the presence of particular elements or compounds
These substances are catalyst, they are not consumed during the reactions and therefore do not appear in reaction equations

28. Industrial catalyst uses

29. Types of catalysts
Homogeneous catalysts in the same state as the reactants and products e.g. outer atmosphere, CFCs (Chlorine) turns ozone into oxygen gas
Heterogeneous catalysts are in different states from the reactants and products e.g. solid catalyst in car exhaust systems

30. How a catalyst works Iron is a heterogeneous catalyst
Nitrogen and hydrogen molecules come into contact with the catalyst
The bonds in nitrogen and hydrogen are broken
New bonds are created

31. Energy changes in the uncatalysed and the catalysed reaction of nitrogen and hydrogen to form ammonia.

32. A higher proportion of particles have sufficient energy to react when a catalyst is added b, compared with no catalyst a.

33. Questions

39. Answers