The rate and extent of chemical change

Rates of reaction

• In industry, making the required amount of product as cheaply as possible maximises all-important profit.
• Faster reactions are often better as they yield more product in a given amount of time.
• It may be expensive to generate the conditions for very fast reactions.
• There can be safety concerns associated with very fast reactions.
• Therefore, reaction rates used in industry are the result of a trade-off between speed, cost and safety.

Plotting rates of reaction

Rates of reactions at specific times can be calculated using graphs:

• Draw a tangent to the curve (straight line that touches the curve at just 1 point).
• Work out the gradient of the tangent:
  • Rate of reaction = change in y / change in x

• Particles must collide for chemical reactions to happen.
• Importantly, these collisions must happen with enough energy. This amount of energy is called the activation energy.
• The activation energy is the minimum energy with which particles must collide in order to cause a chemical reaction.
• We can increase the rate of a reaction by increasing the frequency (number) of collisions and/or the energy of reactant particles.
• This can be achieved by changing lots of different factors.

• Increasing the concentration increases the frequency (number) of collisions. This increases the rate of reaction.
• Increasing pressure is like increasing the concentration.
• It increases the frequency (number) of collisions. This increases the rate of reaction (makes the reaction faster).
• Increasing the temperature increases the frequency (number) of collisions.
• Increasing the temperature increases the energy of reactant particles. This means that a greater proportion (more) of the particles will have more energy than the activation energy that is needed.
• Together, these lead to more successful collisions. This increases the rate of reaction.
• Increasing the surface area increases the frequency (number) of collisions. This increases the rate of reaction.
• This is often done by breaking up solids into smaller lumps (e.g. powders).
• This increase in surface area to volume ratio makes sure that more particles are exposed to attack.

• Catalysts are substances that increase the rate of a chemical reaction without being used up in the process.
• Catalysts are not used up during chemical reactions. This means that:
  • They can be reused indefinitely.
  • They are not found in chemical equations.
• However, catalysts will often need cleaning or regenerating, which has knock-on effects for energy expenditure and environmental impact.
• Different reactions need different catalysts:
  • Iron is used in the process that makes ammonia.
  • Platinum and palladium are used in catalytic converters in cars.
  • Enzymes catalyse reactions in biological systems.
• Catalysts often come as powders, pellets or fine gauzes because these types of substance have particularly high surface areas.

• Despite the fact that some catalysts are expensive precious metals (e.g. platinum and palladium), they are often cost-effective.
  • This is because a small quantity can speed up a reaction by a lot. It may be cheaper to pay for the catalyst at the start and increase the rate of reaction after that.
• Paying for a catalyst may be cheaper than paying for the energy needed to increase either temperature or pressure.
• By reducing the necessary temperatures and/or pressures, fewer fossil fuels need to be burned, thus reducing negative environmental impact.
• However, many catalysts are toxic (e.g. transition metals), and these can escape into the environment and contaminate ecosystems.

• A + B ⇌ C + D
  • The reactants (A + B) can combine to give the products (C + D).
  • The products (C + D) can combine to give the reactants (A + B).
• If the forwards reaction is exothermic (transfers energy from reactants to the environment), the backwards reaction will be endothermic (transfers energy from environment to the reactants).
• Exothermic and endothermic reactions can be monitored by measuring the temperature change of the surroundings.
• Energy is conserved during chemical reactions.
• The energy released/absorbed by the forward reaction will be exactly equal to the energy absorbed/released by the backward reaction.

Dynamic equilibrium

• A closed system is a system where reactants and products can't be added or removed
• At dynamic equilibrium, the rate of the forwards reaction equals the rate of the backward one
• The equilibrium is dynamic because both the forward and backward reactions are still taking place, but the concentrations of reactants and products remain constant.