Monitoring and controlling chemical reactions Revision Notes

Temperature and Reaction Rate

Increasing temperature increases reaction rate. Particles gain more kinetic energy, move faster, and collide more frequently. These collisions are also more energetic, meaning a greater proportion of them meet or exceed the activation energy required for a successful reaction. This results in more effective collisions per unit time, speeding up the reaction.

Concentration and Pressure for Reaction Rate

For solutions, increasing the concentration means more reactant particles are present in the same volume, leading to more frequent collisions and a faster reaction. For gases, increasing pressure reduces the volume occupied by particles, bringing them closer together. This also increases collision frequency, leading to a faster reaction rate.

Surface Area and Reaction Rate

For reactions involving solid reactants, increasing the surface area speeds up the reaction. By breaking a solid into smaller pieces or powder, more of its particles are exposed to the other reactants. This increases the number of sites where collisions can occur, leading to a higher frequency of effective collisions and a faster reaction rate.

Catalysts and Reaction Rate

Catalysts are substances that increase the rate of a chemical reaction without being chemically changed or used up in the process. They achieve this by providing an alternative reaction pathway with a lower activation energy. This allows a greater proportion of reactant particles to have the necessary energy for successful collisions, thus speeding up the reaction.

Le Chatelier's Principle (Higher)

Le Chatelier's Principle is a rule used to predict how a system at equilibrium will respond to changes in conditions. It states that if you change the conditions of a reversible reaction at equilibrium, the system will shift in a direction that counteracts the change to re-establish equilibrium. This principle is vital for optimising industrial processes to maximise product yield.

Effect of Concentration Changes (Higher)

If the concentration of a reactant is increased, the equilibrium will shift to the right (towards the products) to use up the added reactant. If the concentration of a product is increased, the equilibrium will shift to the left (towards the reactants) to use up the added product. If a reactant is removed, the equilibrium shifts left. If a product is removed (e.g., by distillation), the equilibrium shifts right to replace it, increasing the overall yield of that product.

Effect of Temperature Changes (Higher)

To predict the effect of temperature, you need to know whether the forward reaction is exothermic (releases heat, ΔH is negative) or endothermic (absorbs heat, ΔH is positive). If the forward reaction is exothermic, increasing the temperature will shift the equilibrium to the left (towards reactants) to absorb the excess heat. Decreasing the temperature will shift it to the right (towards products) to release more heat. If the forward reaction is endothermic, increasing the temperature will shift the equilibrium to the right (towards products) to absorb the extra heat. Decreasing the temperature will shift it to the left (towards reactants) to absorb heat.

Effect of Pressure Changes (Higher)

Pressure changes only affect reactions involving gases and only when there is a change in the total number of moles of gas from reactants to products. An increase in pressure will shift the equilibrium towards the side with fewer moles of gas to reduce the pressure. A decrease in pressure will shift the equilibrium towards the side with more moles of gas to increase the pressure. If the number of moles of gas is the same on both sides of the equation, pressure changes have no effect on the equilibrium position. Catalysts do not affect equilibrium position, only the rate at which it is reached.

Factors Influencing Reaction Pathway Choice (Higher)

When an industrial chemist decides which reaction pathway to use for manufacturing a specific chemical product, they must consider several critical factors. These factors are not independent and often involve trade-offs. The ultimate goal is usually to achieve the most efficient, cost-effective, and sustainable production method while ensuring safety. Evaluation of relevant data for each potential pathway is essential to make an informed decision.

Economic Considerations (Higher)

Economic factors are often paramount. The cost of raw materials is a primary concern; cheaper and more abundant starting materials are preferred. Energy costs associated with maintaining high temperatures or pressures, or with operating separation processes, can significantly impact overall production expenses. The percentage yield of the reaction directly affects how much product is obtained from the reactants, influencing profitability. The time taken for the reaction (reaction rate) also has economic implications, as faster reactions allow for higher throughput and productivity. Costs associated with catalysts, waste disposal, and equipment maintenance are also included.

Environmental and Safety Considerations (Higher)

Beyond economics, environmental impact and safety are increasingly important. Pathways that produce fewer hazardous by-products or consume less energy are more sustainable. The toxicity, flammability, and corrosiveness of reactants, intermediates, and products pose significant safety risks to workers and require costly safety measures. Waste management and disposal costs, alongside potential environmental fines for pollution, contribute to the overall environmental footprint and financial viability of a process. Regulatory compliance and public perception also influence pathway selection.

Technical and Efficiency Considerations (Higher)

Technical aspects include the required reaction conditions (temperature, pressure, catalyst requirements), which influence equipment complexity and operating costs. The purity of the desired product and the ease of separating it from by-products or unreacted starting materials are vital. Highly selective reactions, which produce minimal unwanted by-products, are preferred as they reduce purification costs and waste. The overall reaction rate also impacts production efficiency; a faster rate means more product can be manufactured in a given period.

What is a Catalyst?

A catalyst is a substance that speeds up the rate of a chemical reaction without being chemically changed or used up in the process. This means that a catalyst can be recovered at the end of the reaction and reused. Catalysts are crucial in many industrial processes because they allow reactions to occur faster or at lower temperatures, saving energy and cost. They are also vital in biological systems, where enzymes act as biological catalysts.

How Catalysts Work

Catalysts work by providing an alternative reaction pathway that has a lower activation energy. Activation energy is the minimum energy required for reactant particles to collide successfully and form products. By lowering this barrier, a greater proportion of reactant particles possess the necessary energy for effective collisions, leading to an increased rate of reaction. It's important to remember that catalysts do not increase the energy of the reactant particles themselves; they simply make it easier for the reaction to happen.

Recognising Catalysts in Reactions

To identify a catalyst, look for a substance that is added to the reaction mixture but does not appear in the overall balanced chemical equation as a reactant or product. It's present at the beginning and remains chemically the same at the end. For example, in the decomposition of hydrogen peroxide, manganese(IV) oxide acts as a catalyst; it is added to make the reaction go faster, but it is still manganese(IV) oxide at the end. Catalysts do not alter the products formed or the overall yield of the reaction; they only affect how quickly the products are made.