3.6 chemical equilibrium

Question 1

reversible reactions

Answer 1

reactions that go forwards and backwards
A+B <–> C+D

Question 2

forward reaction

Answer 2

initially reactants are used up quickly but then slow as their concentration drops

Question 3

backwards reaction

Answer 3

initially reactants are reformed slowly but then speed up as the concentration of products increases

Question 4

DYNAMIC equilibrium

Answer 4

occurs in CLOSED systems
where rate of forward reaction equals the rate of backwards reaction, concentration of each substance remains constant, meaning this is NOT the same amount of reactant and product

Question 5

Le Chatelier’s principle

Answer 5

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

Question 6

equilibrium shift to the left vs right

Answer 6

shifting to the left would make more reactants
shifting to the right would create more products

Question 7

equilibrium: increase in concentration

Answer 7

increasing concentration of reactant or product will cause the equilibrium to shift to try and reduce the concentration (opposite direction to the side you increased)

Question 8

equilibrium: change in pressure

Answer 8

increase in pressure will cause the equilibrium to shift to reduce the pressure
increasing pressure: shift to side with fewer gaseous molecules
decreasing pressure: shift to side with most gaseous molecules

Question 9

equilibrium: change in temperature

Answer 9

increase in temp: equilibrium shifts to reduce temperature
- shift in endothermic direction
decrease in temp: equilibrium shifts to increase temperature
- shift in exothermic direction

Question 10

equilibrium: catalysts

Answer 10

catalysts have no effect on position of equilibrium
only effects rate of reaction by speeding up rate of forward and backwards reaction equally
effects the rate at which a reaction reaches equilibrium
NO effect on yield

Question 11

Le Chatelier’s principle making ethanol
C2H4(g)+H2O(g) <–> C2H5OH(g)
ΔH = -46 kJmol-1

Answer 11

conditions and reagents:
60 atm, 300°C, H3PO4
- temperature: forward reaction is exothermic so decreasing temperature will mean equilibrium shifts to the right, producing more ethanol. HOWEVER, lower temp means lower rate of reaction, 300°C is a compromise between yield and rate
- high pressure: equilibrium shifts to the right as there are fewer gaseous molecules on the right, producing more ethanol. high pressure also increases rate HOWEVER, high pressure is expensive to produce and high risk

Question 12

Kc equilibrium constant

Answer 12

can be worked out from the molar concentration in a reaction
Kc = products/reactants
big molar numbers go to the power

Question 13

temperature effect on Kc

Answer 13

-Kc is only valid for one temperature
- changing temperature will change equilibrium concentrations, hence Kc will change too
- shift to right (products) = Kc increase
shift to left (reactants) = Kc decrease

Question 14

concentration effect on Kc - investigation
Iron(III)Nitrate reacts with potassium thiocyanate to from Iron(III)thiocyanate in equilibrium

Answer 14

ionic equation:
Fe3+(aq) + 3SCN-(aq) <–> Fe(SCN)3 (aq)
yellow colourless blood red
test tube 1: control test tube with slight red colour
test tube 2: add more Fe3+ equilibrium shifts to right, more Fe(SCN)3 produced
MORE RED
test tube 3: add more SCN- equilibrium shifts to right, more Fe(SCN)3 produced
MORE RED
test tube 4: add more Fe(SCN)3 equilibrium shifts to left, more Fe+ and SCN- produced
MORE YELLOW

Question 15

investigating equilibrium, changing temperature
NO2 in equilibrium with N2O4 can be used to show how temperature effects equilibrium

Answer 15

2NO2(g) <–> N2O4(g) [exo forward]
brown colourless
- warm test tube: equilibrium shifts in endothermic direction (left) more brown NO2 produced
- cool test tube: equilibrium shifts in exothermic direction (right) more colourless N2O4 produced