3.1.6 Chemical equilibria, Le Chatelier’s principle and Kc

Question 1

when can dynamic equilibrium occur

Answer 1

in a closed system which is at a constant temperature

Question 2

what is Le Chatelier’s Principle

Answer 2

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

Question 3

in a reversible reaction at equilibrium…

Answer 3

-forward and reverse reactions proceed at equal rates
-the concentrations of reactants and products remain constant.

Question 4

define dynamic equilibrium

Answer 4

state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products over time

Question 5

what happens if you increase the concentration of a reactant

Answer 5

increase concentration of reactant = equilibrium tries to get rid of extra reactant = favours forwards reaction = equilibrium shifts right

Question 6

what happens if you increase the concentration of a product

Answer 6

increase concentration of product = equilibrium tries to get rid of extra product = favours backwards reaction = equilibrium shifts left

Question 7

what happens if you decrease the concentration of a reactant

Answer 7

decrease concentration of reactant = favour backwards reaction = equilibrium shifts left

Question 8

what happens if you decrease the concentration of a product

Answer 8

decrease concentration of product = favours forwards reaction = equilibrium shifts right

Question 9

what equilibria does changing pressure affect

Answer 9

equilibria involving gases

Question 10

what happens if you increase pressure

Answer 10

increase pressure = equilibrium shifts to side with fewer gas molecules to reduce pressure

Question 11

what happens if you decrease pressure

Answer 11

decrease pressure = equilibrium shifts to side with more gas molecules = increases pressure

Question 12

what happens if you increase temperature

Answer 12

increase temp adds heat = equilibrium shifts in the endothermic direction to absorb heat

Question 13

what happens if you decrease temperature

Answer 13

decrease temp removes heat = equilibrium shifts in the exothermic direction to try replace the heat

Question 14

conditions for reaction to make ethanol from ethene and steam

Answer 14

• pressure of 60 to 70 atmospheres
• temp of 300 degrees C
• phosphoric acid catalyst

Question 15

making ethanol from ethene and steam:
exothermic or endothermic

Answer 15

exothermic

Question 16

making ethanol from ethene and steam
temperature

Answer 16

low temp favours forwards reaction (so more product) but low temp means slow rate of reaction so 300 degrees C is a compromise

Question 17

making ethanol from ethene and steam
pressure

Answer 17

high pressure favours forwards reaction and increases rate of reaction. pressure isn’t higher as this is too expensive

Question 18

making ethanol from ethene and steam
equation

Answer 18

C2H4 (g) + H2O (g) = C2H5OH (g)
reversible

Question 19

making ethanol from ethene and steam
how much ethene reacts

Answer 19

small proportion of ethene reacts each time gases pass through reactor

Question 20

making ethanol from ethene and steam
what is done to increase how much ethene reacts

Answer 20

unreacted ethene is separated from the ethanol and recycled back into the reactor

Question 21

making ethanol from ethene and steam
how much ethene is converted to ethanol

Answer 21

95% of the ethene is converted into ethanol

Question 22

making methanol
equation

Answer 22

2H2 (g) + CO (g) = CH3OH (g)
reversible

Question 23

making methanol
pressure

Answer 23

50-100 atmospheres

Question 24

making methanol
temperature

Answer 24

250 degrees C

Question 25

making methanol catalyst

Answer 25

catalyst of a mixture of copper, zinc oxide and aluminium oxide

Question 26

equation for Kc for this general equation: aA + bB = dD + eE

Answer 26

[D]^d [E]^e / [A]^a[B]^b (square brackets representing concentrations)

Question 27

what factor affects the equilibrium constant

Answer 27

temperature

Question 28

what happens to Kc if the temperature change means more product is formed

Answer 28

more product formed = Kc increases

Question 29

what happens to Kc if the temperature change means less product is formed

Answer 29

less product formed = Kc decreases

Question 30

changing concentration and Kc

Answer 30

if concentration of 1 thing in the equilibrium mixture changes then the concentrations of the others must change to keep the value of Kc the same

Question 31

do catalysts have an affect on the equilibrium constant

Answer 31

no

Question 32

do catalysts have an affect on the position of equilibrium

Answer 32

no

Question 33

explanation for catalysts affect on position of equilibrium

Answer 33

catalysts increase rate of forward and backwards reaction by same amount so equilibrium position doesn't change but equilibrium will be reached faster

Question 34

Kc and temp

Answer 34

value of Kc will be true for that of a particular temperature

Question 35

increasing temp for exothermic reaction (effect on yield)

Answer 35

increasing temp for exothermic reaction = decreases yield

Question 36

decreasing temp for exothermic reaction (effect on yield)

Answer 36

decreasing temp for exothermic reaction = increases yield

Question 37

decreasing temp for endothermic reaction (effect on yield)

Answer 37

decreasing temp for endothermic reaction = decreases yield

Question 38

increasing temp for endothermic reaction (effect on yield)

Answer 38

increasing temp for endothermic reaction = increases yield

Question 39

making methanol exothermic or endothermic

Answer 39

exothermic

Question 40

making ammonia exothermic or endothermic

Answer 40

exothermic

Question 41

making ammonia equation

Answer 41

N2 (g) + 3H2 (g) = 2NH3 (g) REVERSIBLE

Question 42

ammonia uses

Answer 42

- fertilisers e.g. ammonium nitrate, ammonium sulfate, urea - explosives - dyes

Question 43

making ammonia pressure

Answer 43

200 atmospheres

Question 44

making ammonia temperature

Answer 44

670K

Question 45

making ammonia catalyst

Answer 45

iron catalyst