1. EQUILIBRIA AND REDOX REACTIONS PART 1

as the reactants get used up

the forwards reaction rate slows down

as more product is formed

the reverse reaction rate speeds up

after a while the the forwards and backwards reactions will go at the exact same rate

this means their concentration (though not equal) will not change

there are two types of equilibrium

• dynamic

• static

the difference between dynamic and static equilibrium

• dynamic equilibrium doesn’t need external forces for the backwards reaction to occur

• static equilibrium requires external forces like heat for backwards reactions to occur

dynamic equilibrium is reached when

• the rate of the forwards reaction is equal to the rate of the backwards reaction

• conc of products / conc of reactants remains constant

• (whilst in a closed system)

factors that affect the position of equilibrium

• concentration

• pressure

• temperature

Le Chatelier’s principle

if a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change

catalysts have no effect on the position of equilibrium

they only allow the equilibrium to be reached fast and cannot increase the yield

increasing the concentration of reactant

equilibrium shifts to the right to oppose the increase in reactants, thus increasing the product yield

increasing the concentration of product

equilibrium shifts to the left to oppose the increase in products, thus decreasing the yield

increasing the pressure of a reaction

shifts the equilibrium to the side with fewer GAS molecules to reduce the pressure

increasing the pressure of the reaction: 2SO_2(g) + O_2(g) ⇄2SO_3(g)

there are 3 moles of the left and 2 moles on the right THEREFORE the equilibrium will shift to the right to oppose the increase in pressure thus increasing the product yield

increasing the temperature of a reaction means adding more heat

the equilibrium will shift in the endothermic(+ΔH) direction to absorbed heat

decreasing the temperature of a reaction means removing heat

the equilibrium will shift in the exothermic (-ΔH) direction to release heat

some reversible reactions have distinct colour changes that allow you to follow equilibrium reactions

[Cu(H2O)₆]²⁺ + HCl ⇄ [CuCl]^2-

for the reaction [Cu(H2O)₆]²⁺ + HCl ⇄ {CuCl}^2- if the solution is blue

the position of the equilibrium must lie on the left so there will be more reactants than products

for the reaction [Cu(H2O)₆]²⁺ + HCl ⇄ {CuCl}^2- if the solution is greeny-yellow

the position of the equilibrium must lie of the right so there will be more products than reactants

changing the concentration

changing the temperature

compromise conditions in industry

companies have to think about the

• cost to run a reaction,

• how much money they can make from it,

• the time taken for a suitable yield to be produced etc

C2H4(g) + H2O ⇄ C2H5OH(g), ΔH=-46kJ mol-1 industrial conditions to make alcohol from alkene:

• pressure: 60-70 atm

• temperature: 300℃

• phosphoric acid catalyst

300℃

forwards reaction is exothermic so a low temperature is will give a better yield HOWEVER it will also decrease the rate of reaction, 300℃ is a compromise between maximum yield and a faster reaction

60-70 atm

higher pressures favour the forwards reaction since the equilibrium will shift to the side with less moles HOWEVER an extremely high pressure is expensive to produce as stronger pipes and containers to withstand high pressure will be required so 60-70 atm is a compromise between maximum yield and minimum expense

because only a small proportion of ethene reacts each time the passes pass through the reactor

to save money and raw materials the unreacted ethene is separated from the ethanol and recycled back into the reactor so 95% of ethene is eventually converted to ethanol

equilibrium constant

the ratio worked out from the concentrations of the products and reactants after equilibrium is reached

the value of K_c

will only be true for that particular temperature, no other factors affect K_c