C4 Rates of reaction and enthalpy

What is the rate of a reaction?

The change in mass or volume of a reactant or product per unit time

What is activation energy?

The minimum energy needed for a reaction to start

Name 5 things that affect reaction rate

Concentration, surface area, pressure, temperature, and catalysts

What does a catalyst do?

Speeds up a reaction without being used up, by providing an alternative route with a lower activation energy

Why does increasing concentration, surface area or pressure speed up a reaction?

Particles collide more frequently, so there are more successful collisions per second

What does the Boltzmann distribution show?

The spread of particle energies in a sample; the area beyond the activation energy is the proportion able to react

How does raising temperature change the Boltzmann distribution?

The curve shifts right and flattens, so a greater proportion of particles have energy above the activation energy

How does a catalyst affect the Boltzmann distribution?

The distribution stays the same but the activation energy moves left, so a greater proportion of particles exceed it

How can you measure reaction rate experimentally?

Track the change in mass or gas volume over time

How do you find the rate of reaction from a graph?

Find the gradient; on a curve, draw a tangent at that point and calculate its gradient

What is an exothermic reaction?

A reaction that releases energy, e.g. combustion or respiration

What is an endothermic reaction?

A reaction that absorbs energy, e.g. thermal decomposition or cracking

What is the sign of ΔH for an exothermic reaction?

Negative (energy is released to the surroundings)

What is the sign of ΔH for an endothermic reaction?

Positive (energy is absorbed from the surroundings)

What is the unit of ΔH?

kJ mol⁻¹

What is the enthalpy change of combustion, ΔcH?

The enthalpy change when one mole of a substance completely combusts in oxygen

What equation gives the thermal energy transferred to a substance in an experiment?

Thermal energy (J) = mass (kg) × specific heat capacity (J kg⁻¹ °C⁻¹) × temperature change (°C)

Why is an experimental enthalpy of combustion often smaller than the true value? (Reason 1)

Some heat is lost to the surroundings, so not all the energy released heats the water

Why is an experimental enthalpy of combustion often smaller than the true value? (Reason 2)

Combustion may be incomplete, so less energy is released than in complete combustion