Reversible reactions and equilibria

• a chemical reaction where the reactants form products, and the products can react to reform the reactants • this means that the reaction can happen both forwards and backwards

the reaction that goes from reactants to products

the reaction that goes from products to reactants

A + B ⇌ C + D

⇌

• hydrated copper (II) sulfate (CuSO4·5H2O) is blue • heating causes dehydration - water molecules leave, forming anhydrous CuSO4 powder (white/pale) • dehydration involves bond-breaking and absorbs heat energy from surroundings = endothermic • adding water causes hydration - water molecules bond back, turning it blue again • hydration involves bond-making and releases heat energy into surroundings = exothermic • both dehydration and rehydration are reversible reactions

• add water = colour change from white powder to blue crystals • add water = temperature increases (refer to explanation card) • heating = colour change from blue crystals to white powder • heating = condensation

CuSO4 (s) + 5H2O (l) ⇌ CuSO4·5H2O (s)

• heating white solid ammonium chloride in a test tube at an angle with a bunsen burner makes it sublime from the bottom and deposit as white crystals at the cooler top • when heated NH4Cl breaks down into colourless gases ammonia (NH3) and hydrogen chloride (HCl) • the forward reaction is endothermic because it absorbs heat to break bonds in NH4Cl and make NH3 and HCl • the hot gases rise up the test tube and cool at the top • they recombine to form solid ammonium chloride again • the backward reaction is exothermic because new bonds form • this means the reaction is reversible

NH4Cl (s) ⇌ NH3 (g) + HCl (g)

• decomposition (solid NH4Cl breaks down into colourless gasses NH3 and HCl) • neutralisation (NH3 is alkaline and HCl is acidic)

• when the rate of the forward and backward reactions are equal, but both reactions continue to occur simultaneously • when the concentrations of the reactants and products remain constant in a closed system

• in order for dynamic equilibrium to occur, the reaction must occur in a closed system • this is a container which has been sealed so that the reactants and/or products cannot escape the container, and no other chemical species can enter

• x-axis = time • y-axis = rate • the line for the forward reaction (reactants to products) starts at a high rate and decreases over time as the reactants get used up to make products • the line for the reverse reaction (products to reactants) starts at a rate of 0 and increases over time as the products begin to form and can be used in the reverse reaction • eventually, both lines level off at the same rate, showing that the rate of the forward and backward reactions is equal • at this point, dynamic equilibrium is reached, as the concentrations of reactants and products remain constant • as the reaction is reversible, both the forward and backwards reactions continue to occur simultaneously

• x-axis = time • y-axis = concentration (mol/dm3) • the line for the forward reaction (reactants to products) starts at a high concentration and decreases over time as the reactants get used up to make products • the line for the reverse reaction (products to reactants) starts at a concentration of 0 and increases over time as the products begin to form and can be used in the reverse reaction • eventually both lines flatten out, showing the concentrations of reactants and products remain constant = this is dynamic equilibrium • as the reaction is reversible, both the forward and backwards reactions continue to occur simultaneously

• the position of equilibrium describes the relative amounts of reactants and products in a reversible reaction at equilibrium • it shows whether the forward or reverse reaction is more favoured under certain conditions

• if the position of equilibrium lies to the right, it means that the forward reaction is favoured • so more there is a higher product conc than reactant conc at equilibrium

• if the position of equilibrium lies to the left, it means that the reverse reaction is favoured • so there is a higher reactant conc than product conc at equilibrium

if the forward reaction is EXOTHERMIC by -100kJ/mol, the reverse reaction would be ENDOTHERMIC by 100kJ/mol

pressure is caused by gas molecules exerting a force on its container walls by hitting them

• by INCREASING pressure = the position of equilibrium shifts to favour side/reaction that produces fewer moles of gas to oppose the change and reduce the pressure, restoring a new position of equilibrium • by DECREASING the pressure = the position of equilibrium shifts to favour the side/reaction that produces moles of gas to oppose the change and increase the pressure, restoring a new position of equilibrium

• by INCREASING the temperature, the position of equilibrium shifts to the endothermic side, which absorbs heat from the surroundings and ∴ opposes the change, reducing the effect of the temperature increase and restoring equilibrium • by DECREASING the temperature, the position of equilibrium shifts to the exothermic side, which releases heat and ∴ opposes the change, increasing the temperature and restoring equilibrium

• INCREASING the concentration of a reactant causes the position of equilibrium to shift to the right to make more products and use up the extra reactant, while increasing the concentration of a product shifts it to the left to reduce the excess product • DECREASING the concentration of a reactant shifts equilibrium to the left to make more of it, and decreasing the concentration of a product shifts it to the right to replace it • these shifts happen to oppose the change and restore a new position of equilibrium

• the presence of a catalyst doesn’t affect the position of equilibrium, but does affect the rate at which equilibrium is reached • this is because the catalyst increases the rate of the forward and reverse reactions by the same amount, as it reduces the activation energies for both reactions • this means that the concentrations of reactants and products become constant earlier on in the reaction than they would without a catalyst, but the concentrations themselves stay the same as they would be without a catalyst