6 Equilibria

Question 1

give two feauters of a reaction at equilibrium

Answer 1

the rate of the forward reaction is equal to the rate of the reverse reaction.This makes concentration of products and reactants appear constant

Question 2

state Le chateliers principle

Answer 2

if a system at equilibrium is disturbed the equilibrium moves in the direction that tends to reduce the disturbance

Question 3

give the meaning of the term dynamic equilibrium

Answer 3

the forward and backward recations proceed at equal rates.The concentration of reactants and products appear consatnt

Question 4

state the effect of a catalyst on the time taken to reach equilibrium

Answer 4

increases the rate at reaching equilibrium

Question 5

state the effect of a catalyst on the position of equilibrium

Answer 5

no affect

Question 6

for what reaction is dynamic equilibrium achieved

Answer 6

for a reversible reaction in a closed system so no reactants/products are able to escape

Question 7

state and explain what happens to the yield of methanol when the total pressure is increased
CO2(g) + 3H2(g) ⇌ CH3OH(g) + H20(g)

Answer 7

if you increase the pressure equilibrium will try and decrease the pressure so it moves to the side with fewer moles of gas which in this case is to the right and so increases the yield of methanol

Question 8

state and explain what happens to the yield of methanol when the total pressure is increased
CO2(g) + 3H2(g) ⇌ CH3OH(g) + H20(g)

Answer 8

if you increase the pressure equilibrium will try and decrease the pressure so it moves to the side with fewer molecules of gas which in this case is to the right and so increases the yield of methanol

Question 9

use le chateliers principle to explain why the mixture of gases becomes darker in colour when the mixture is heated at constant pressrue
N204(g) ⇌ 2NO2(g) ΔH=+58 kJ/mol

Answer 9

if temoerature increases equilibrium will try and decrease it so it shifts to the endothermic direction to absorb the heat which in this case is to the right and so the mixture of gases becomes darker

Question 10

use Le chateliers principle to explain why the amount of NO2 decreases when the pressure is increased at constant temperautre
N204(g) ⇌ 2NO2(g) ΔH=+58 kJ/mol

Answer 10

if you increase the pressure equilibrioum will try and decrease it and so it shifts to the side with fewer gas molecules which is to the left and so the yield of N2O4 increases and the yield of NO2 decreases

Question 11

use le chateliers principle to explain the effect of an increase in pressure at constant temperature on the yield of ammonia
N2(g) + 3H2(g) ⇌ 2NH3(g)

Answer 11

when you increase the pressure equilibrium will try and decrease it so it shifts to the side with fewer gas molecules which in this case is to the right and so increases the yield of ammonia

Question 12

use le chateliers principle to explain the effect of an increase in temperature at constant pressure on the yield of ammonia.(forward recation is exothermic)
N2(g) + 3H2(g) ⇌ 2NH3(g)

Answer 12

when you increase the temperautre equilibrium will try and decrease it so it moves to the endothermic dircetion to try and abosrb the heat which in this case is the left side and so decreases the yield of ammonia

Question 13

what happens when you increase the concentration of a reactant on the yield of products

Answer 13

when you increase the concenctration of the reactants equilibrium will try and make more product so the equilibrium shifts the right and so increases the yield of products

Question 14

what happens when you decrease the concentration of a reactant on the yield of products

Answer 14

when you decrease the concenctration of the reactant equilibrium will try and increase the concentration of the reactnat so the equilibrium shifts the left and so decreases the yield of products

Question 15

what happens to the position of equilibrium when you increase the pressure

Answer 15

equilibrium will try and dcerease the pressure and so shifts to the side fewer gas molecules

Question 16

what happens to the position of equilibrium when decrease the pressure

Answer 16

equilibrium will try and increase the pressure and so shifts to the side with more gas molecules

Question 17

what happens to the position of equilibrium when you increase the temperature

Answer 17

equilbriums will try and decrease the temperatutre and so shifts in the endothermic directions(positive ΔH) direction to abosrb the heat

Question 18

what happens to the position of equilibrium when you decrease the temperature

Answer 18

equilibrium will try and increase the temperature and so shifts in the exothermic (negative ΔH) direction to produce more heat

Question 19

what is the Kc value

Answer 19

Kc is a calculated value that tells us whether a reaction lies more towards the products side or the reactants side.If Kc is greater than zero equilibrium lies more on the products side.If Kc is less than zero equilibrium lies more towards the reactants side

Question 20

In industry, a fermentation reaction is carried out at 35 °C rather than 25 °C.
Suggest one advantage and one disadvantage for industry of carrying out the
fermentation at this higher temperature.

Answer 20

advantage: ethanol is produced at a faster rate
disadvantage: more eneergy is required in the reaction

Question 21

explain why, for a reversible reaction used in an industrial process, a compromise temperature 300*C may be used.
C2H4 + H20 ⇌ C2H5OH ΔH = -46 kj/mol

Answer 21

300C is a compromise because this is an exothermic reaction lower temperatures favour the forward reaction so you get a better yield of ethanol but lower temperatures means a slower rate of reaction so a 300C is a compromise between a reasonable yield and a faster reaction

Question 22

explain why, for a reversible reaction used in an industrial process, a compromise pressure of 60-70 atmopsheres may be used.
C2H4(g)+ H20(g) ⇌ C2H5OH(g) ΔH = -46 kj/mol

Answer 22

high pressure shifts the equilibrium to the side with fewer gas moles which is to the right increasing the yield of ethanol also a high pressure increases the rate of reaction however a very high pressure is really expensive to produce as you need strong equipment to withstand high pressures.So a 60-70 atmospheres is a compromise it gives a reasonable yield for the lowest possible price.

Question 23

how do you set up your workings for a question based on calculating Kc

Answer 23

elements  ⇌ elements 
ratio:
initial no.moles:
change:
final/equilibrium no. moles:

the change will vary on the sign(+ or -) and the ratio of moles whatever the change is on the right hand side is opposite sign to the change on the left hand side

Question 24

what is the Kc expression for this reaction 2HI ⇌ H2 + I2

Answer 24

Kc =[H2][I2] / [HI]^2

Question 25

what is the Kc units for this reaction 2HI ⇌ H2 + I2

Answer 25

make moldm^-3 x
(x)(x)/(x)2
= x^2/ x^2
=1 therfore no units

Question 26

what is the Kc expression for this reaction

2CHCLF2 ⇌ C2F4 + 2HCL

Answer 26

Kc=[C2F4][HCL]^2 / [CHCLF2]^2

Question 27

what is the Kc units for this reaction

2CHCLF2 ⇌ C2F4 + 2HCL

Answer 27

make moldm^-3 x
(x)(x)^2 /(x)^2 
=x^3/x^2
=x 
moldm^-3

Question 28

state and explain the effect,if any, of adding a catalyst to the value of Kc

Answer 28

no affect because a catalyst increases the rate of the forward reaction and the reverse recation equally

Question 29

state t and explain the effect,if any, of changing the concentration to the value of Kc

Answer 29

no affect because concentraion appears constant

Question 30

state and explain the effect,if any, of increasing the temperature to the value of Kc for an exothermic reaction

Answer 30

decreases the value of Kc

Question 31

state and explain the effect,if any, of increasing the temperature to the value of Kc for an endothermic reaction

Answer 31

increases the value of Kc

Question 32

for decreasing the temperature for an endothermic reaction state:
the temperature change
effect on Kc
effect on products 
effect on reactants 
direction of change of equilbrium

Answer 32

tempertaure change:decrease
effect on Kc:decrease
effect on products:decrease
effect on reactants:increase
direction of change of equilibrium:moves left

Question 33

for increasing the temperature for an endothermic reaction state:
the temperature change
effect on Kc
effect on products 
effect on reactants 
direction of change of equilbrium

Answer 33

tempertaure change:increase
effect on Kc:increase
effect on products:increase
effect on reactants:decrease
direction of change of equilibrium:moves right

Question 34

for increasing the temperature for an exothermic reaction state:
the temperature change
effect on Kc
effect on products 
effect on reactants 
direction of change of equilbrium

Answer 34

temperature change:increase
effect on Kc:decrease
effect on products:decrease
effect on reactants :increase
direction of change of equilbrium:moves left

Question 35

for decreasing the temperature for an exothermic reaction state:
the temperature change
effect on Kc
effect on products 
effect on reactants 
direction of change of equilbrium

Answer 35

temperature change:decrease
effect on Kc:increase
effect on products:increase
effect on reactants :decrease
direction of change of equilbrium:moves right

Question 36

state the general equation for Kc

Answer 36

[products] / [reactants]