AQA A level Chemistry - Le Chatelier's Principle, Equilibrium, Kc

what are the characteristics of dynamic equilibrium?

• occurs in a closed system

• forwards and reverse reactions occur at the same rate

• concentrations of reactants and products remain constant

what is a homogenous reaction?

reaction where all reactants and products are in the same phase (state)

define Le Chatelier’s Principle

when a system in equilibrium is disturbed, the position of equilibrium will move in a direction to reduce the effect of a disturbance

how do you include Le Chatelier’s Principle in exam questions?

• to oppose the change in x

• to reduce the x or remove x etc.

what is the effect of increased pressure on the position of equilibrium?

• shifts position of equilibrium to side with fewer gas moles

• to reduce pressure

• to oppose the change

• opposite for decreased

what is the effect of increased temperature on the position of equilibrium?

• shifts position of equilibrium in direction of endothermic reaction

• to reduce temperature/oppose the change

• opposite for decrease

what is the effect of increased concentration of a reagent/product on the position of equilibrium?

• position of equilibrium shifts to side with lower concentration of stuff (side conc wasn’t increased on)

• in order to oppose the change

what is the effect of a catalyst on the position of equilibrium?

• no effect on position of equilibrium

• as it increases the rate of the forwards and reverse reactions equally

what is the equation for the Haber Process?

• N2 + 3H2 ←> 2NH3

• delta H = -92

what is the temperature used for the Haber process and why?

• 450C

• compromise temperature as:

  • higher temperature increases rate of reaction

  • as well as shifts position of equilibrium to left :(

what is the pressure used for the Haber process and why?

• 20MPa

• compromise pressure as:

  • higher pressure shifts equilibrium to the right and increases the yield

  • but is also very expensive

what is the catalyst used for the Haber process?

finely divided Fe (s)

what is the equation for the manufacture of ethanol from ethene?

• C2H4 + H2O ←> C2H5OH

• delta H = -42

what is the temperature used for the manufacture of ethanol and why?

• 300-600C

• compromise temperature as:

  • higher temperature shifts position of equilibrium to left as reaction is exothermic

  • but high temperature = high rate

what is the pressure used for the manufacture of ethanol and why?

• 5-10 MPa

• compromise pressure as:

  • high pressure shifts equilibrium to the right

  • but high pressure is ezpeensive

what is the catalyst used for the manufacture of ethanol?

concentrated phosphoric acid

why are compromise pressures and temperatures used?

• compromise temperature used to strike a balance between eq position and rate

• compromise pressures used to balance between eq position and cost

what is Kc?

ratio of products to reactants at equilibrium at a set temperature

how would you construct an expression for Kc?

• find the units by subbing in the units for each concentration of A, B, C, D into expression and simplify

what does it mean if Kc > 1?

• concentration of products is greater than concentration of reactants

• equilibrium lies to the RHS

what does it mean if Kc < 1?

• concentration of reactants is greater that the concentration of products

• equilibrium lies to the LHS

which factors is Kc not affected by?

• changes in concentration

• volume

• pressure

• catalyst

• due to le chateliers principle

which factor is Kc affected by?

changes in temperature

how does an increased temperature affect Kc?

• position of equilibrium shifts in the endothermic direction

• so Kc increases for forwards endothermic

how does a decreased temperature affect Kc?

• position of equilibrium shifts in the exothermic direction

• so Kc increases for forwards exothermic

how do you calculate Kc given mole values?

• construct I C E table

• sub in values given

• use values + ratios to work out change in moles from initial to equilibrium - moles always change in same ratio

• work out concentrations if necessary

• sub values in to expression

• might have to do in reverse (find moles) or might have to work it out through titration