the rate & extent of chemical change

• the rate of a chemical reaction tells us how quickly reactants are converted into products
• in industry, reaction rates are very important, however faster reaction rates may not always be better
• 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
• collision theory explains how reactions occur and why they occur at different rates
• particles must collide for chemical reactions to happen, and these collisions must happen with enough energy, known as 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 reaction by increasing the frequency of collisions and the energy of reactant particles, which can be achieved by changing lots of different factors
• one of the main factors that affect the rate of chemical reactions is concentration of dissolved reactants. increasing the concentration increases the frequency of collisions, increasing rate of reaction
• one of the main factors that affect the rate of chemical reactions is pressure of gas reactants. increasing pressure increases the frequency of collisions, increasing rate of reaction
• one of the main factors that affect the rate of chemical reactions is temperature. increasing the temperature increases the frequency of collisions, and the energy of reactant particles, meaning that a greater proportion of particles will have more energy than the activation energy, increasing rate of reaction
• one of the main factors that affect the rate of chemical reactions is surface area of solid reactants. increasing the surface area increases the rate of reaction, which is done by breaking up solids into smaller lumps, increasing surface area to volume ratio and exposing more particles to attack
• one of the main factors that affect the rate of chemical reactions is catalysts, which are substances that increase the rate of a chemical reaction without being used up in the process
• catalysts increase the rate of chemical reactions by lowering the activation energy, by providing a different reaction pathway
• catalysts can be used indefinitely, but are not found in chemical equations
• 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 substances have particularly high surface areas
• catalysts are popular in industry for many reasons, however they do have their disadvantages
• an advantage of catalysts is that despite the fact that some are expensive precious metals, they are often cost-effective, as a small quantity can speed up a reaction by a lot
• an advantage of catalysts is that paying for a catalyst may be cheaper than paying for the energy needed to increase either temperature or pressure
• an advantage of catalysts is that by reducing the necessary temperatures and pressures, fewer fossil fuels need to be burned, thus reducing negative environmental impacts
• a disadvantage of catalysts is that many are toxic, and can escape into the environment and contaminate ecosystems
• some chemical reactions are reversible, meaning they can proceed in both forwards and backwards directions
• if the forwards reaction is exothermic, the backwards reaction will be endothermic
• energy is conserved during chemical reactions, and the energy released/absorbed by the forward reaction will be exactly equal to the energy absorbed/released by the backward reaction
• if a reversible reaction happens in a closed system, a dynamic equilibrium will eventually be reached
• a closed system is a system where reactants and products can neither be added nor removed
• at dynamic equilibrium, the rate of the forwards reaction equals the rate of the backward reaction, and the equilibrium is dynamic because both the forward and backward reactions are still taking place
• changing the conditions in a reversible reaction affects the position of equalibrium
• the relative amounts of the substances in a reaction at equilibrium are determined by the conditions
• le chatelier's principle says that if any of the conditions of a reversible reaction at equilibrium are changed, the system will change to counteract this change
• le chatelier's principle is used to predict the outcome of any changes imposed on a system at equalibrium
• changing the temperature for a reaction at equilibrium takes the system out of equilibrium, and the system will react to try and restore the equilibrium
• if the temperature is decreased, the position of equilibrium will shift in the exothermic direction, resulting in an increase in the products of the exothermic reaction and a decrease in the products of the endothermic reaction