Collision theory states that particles must collide with the correct orientation and with sufficient energy to have a sucessful collision (to overcome the activation energy) and thus react. However, the frequency of collisions will also be important in determining the rate of reaction, since a greater number of collisions per unit time will result in a greater number of successful collisions per unit time. Thus '1,2 and 3' is the correct answer.

Rate of reaction can be measured in any unit of quantity per unit time. All the answers indicate a quantity changing with time (remember that mol dm^–3 means moles per cubic decimetre and is a unit of concentration and min^–1 means per minute) except 'g mol^–1' which means grams per mole (the unit of molar mass) and is thus the correct answer.

'1, 2 and 3' is the correct answer because all these quantities can be measured to follow the rate in an appropriate chemical reaction. e.g. colour of bromine disappearing; volume of hydrogen gas produced; mass decrease as calcium carbonate reacts with acid (and carbon dioxide is given off).

The correct (best) answer is 'The minimum energy that colliding particles need in order to react.' This is a definition that must be learned. The other answers are incorrect, although activation energy is required to 'break bonds' in reactants (but not necessarilly all bonds). The average kinetic energy of particles is often marked on a Maxwell-Boltzmann curve, and the enthalpy difference between reactants and products is part of the Energetics topic but neither are directly linked to activation energy.

A catalyst increases the rate of a chemical reaction by causing the reaction to follow a different reaction pathway of lower activation energy, and is not permanently chemically changed during the reaction. The catalyst does not otherwise affect the energy of the particles (or the position of equilibrium) - it does not increase the collision energy or collision frequency. Thus '1 and 2 only' is the correct answer.

All these answers are correct statements, but 'Particles move faster, collide more often and collide with greater energy' is the correct (best) answer since it covers all the important points: Increasing temperature causes particles to have greater kinetic energy (move faster), increases the collision frequency (collide more often) and also causes collisions to have greater energy (because the particles are striking each other at higher speeds). This results in a greater proportion of collisions that can overcome the activation energy per unit time, and thus a greater rate of reaction.

Increasing surface area does not increase the energy of the particles or of the collisions, nor does it alter the activation energy to allow particles of lower energy to react. So the correct answer is 'A greater number of particles are exposed so there are a greater number of collisions per unit time.' Increasing surface area increases the collision frequency (particles collide more often). This results in a greater number of successful collisions per unit time, and thus a greater rate of reaction.

The correct answer is 'concentration'. Increasing volume will lower concentration and may have the opposite effect (lower rate). Increasing temperature will increase the rate, but will not increase the number of particles in the same volume. Increasing activation energy would decrease the rate as fewer particles would have the minimum energy needed to react.

C is the activation energy of the reverse reaction and is the correct answer. A is the activation energy of the forward reaction; B is the difference in enthalpy between reactants and products; C minus B would be equal to A (activation energy in the forward direction).

If the temperature is increased particles will gain kinetic energy, but the number of particles will remain the same. Thus all three (1,2 and 3) correctly describe how the distribtuion should be redrawn as exemplified in red here: