Chapter 4- Equilibrium

what is a closed system?

• a system where reactants and products can neither be added or removed

what occurs at dynamic equilibrium?

• the rate of the forward reaction equals the rate of the backward reaction

• both the forward reaction and backward reactions are still taking place

• the concentrations of the reactants and products stays constant and the reaction is continuous

what is le chatelier’s principle?

• when conditions on a system in equilibrium are changed, the equilibrium moves in the direction to minimise effects of the change

effect of concentration on equilibrium?

• increasing concentration of OH- ions causes the equilibrium to shift to oppose this and move in the forward direction to remove OH- ions

• the position of equilibrium will shift to the right, giving a higher yield of I- and IO-

  • the colour would change from brown to colourless

rules for the effect of concentration on equilibrium?

• increasing conc of reactants ——> shifts PoE to the right to use up excess reactant

• decreasing conc of reactants ——> shifts PoE to the left to produce more reactant

• increasing conc of the products ——→ shifts PoE to the left to consume added product

• decreasing conc of the products ——> shifts PoE to the right to produce more product (use up reactants)

effect of temperature on equilibrium?

• if temp is increased, the equilibrium will shift to oppose this and move in the endothermic direction to try to increase the temp by absorbing heat

• if temp is decreased, the equilibrium will shift to oppose this and move in the exothermic direction to try and increase the temp by giving out heat

effect of pressure on equilibrium?

• increasing pressure causes equilibrium to shift to the side with fewer moles of gas to oppose the change and therefore reduce the pressure

• decreasing the pressure will cause the equilibrium to shift to the side with more moles of gas to oppose the change and therefore increase the pressure

• if the number of moles of gas is the same on both sides of the equation, then changing pressure will have no effect on position of equilibrium

explain why actual conditions used in the chemical industry might be different from what may be stated in questions?

• too expensive to use a high pressure

• too expensive to use a low temperature

what is a catalyst?

• a substance that alters the rate of a reaction without being used up

• substance that lowers the activation energy of a reaction by providing an alternative route

effect of catalysts on equilibrium?

• no effect on position of equilibrium, but speeds up the rate at which the equilibrium is achieved

• it speeds up/ alters both the forward and backward rate equally/ by the same amount

why are catalysts used in the industry?

• because they lower the costs of the reaction process

• they allow lower temps and pressures to be used, whilst achieving the same rate of reaction

• they can also give a higher atom economy

how do catalysts increase the sustainability of a reaction?

• reactions can be carried out at lower temperatures

• reducing energy demand from the combustion of fossil fuels

  • this results in a reduction of CO2 emissions/ less fossil fuels burnt

what are homogeneous catalysts?

catalysts that are in the same phase as the reactants

what are heterogeneous catalysts?

• catalysts that are in a different phase to the species in the reaction

  • example is in the haber process: solid iron catalyst used to speed up the reaction between hydrogen and nitrogen gases

transition metal catalysts?

• electrons are transferred to produce a reactive intermediate and speed up the reaction rate

  • they make good catalysts because they have variable oxidation states

• example is vanadium oxide, used in the contact process

what is ammonia used for?

• making fertilisers

• making nitric acid

pressure and temperature needed for the haber process?

• pressure: 50 - 1000 atm

• temperature: 200 - 600 C

describe and explain why the conditions for the haber process are a compromise between rate and equilibrium?

rate

• increased pressure increases rate because molecules are closer together

• increased temp increases rate because molecules have more kinetic energy

equilibrium

• increased pressure pushes equilibrium to the RHS because there are fewer moles on the RHS

• increased temp pushes equilibrium to LHS because forward reaction is exothermic

compromise

• if temp is too high, low yield

• if temp is too low, slow rate

• if pressure is too high, increased costs/ safety issues

what is the contact process?

the process used to make sulfuric acid

stage 1 of the contact process?

• sulfur is burnt in air to produce sulfur dioxide

stage 2 of the contact process?

• sulfur dioxide is reacted with oxygen to produce sulfur trioxide in a reversible reaction

• conditions: 450C, 10atm, catalyst- V2O5

stage 3 of the contact process?

• sulfur trioxide is dissolved in 98% sulfuric acid to produce fuming sulfuric acid

• this is then diluted to produce concentrated sulfuric acid

compromise between rate of reaction and product yield?

• e.g- decreasing the temp of a reaction with forward endothermic reaction would increase the product yield, but decrease the rate of reaction

• therefore, a compromise is made

• reaction conditions are selected to give both a relatively good product yield and a relatively fast rate of reaction.

what is the expression for the equilibrium constant Kc?

how to work out the unit of Kc?

*the unit of Kc changes and depends on the equation

what does the magnitude of Kc tell us?

• the relative proportions of reactants and products in the equilibrium system?

magnitude of Kc?

• a Kc value of 1 shows that the PoE is halfway between the reactants and productsw

• a Kc value of greater than 1 shows that the PoE is leaning towards the products

• a Kc value of less than 1 shows that the PoE is leaning towards the reactants

  • therefore, the larger the value of Kc, the further the position of equilibrium lies to the right hand side and the greater the concentrations of the products compared to the reactants