equilibria A2

Question 1

mole fraction equation

Answer 1

number of moles of A / total moles in gas mixture
mole fractions must add up to 1

Question 2

partial pressure equation

Answer 2

partial pressure, p(A) = mole fraction of A x total pressure (P)
the sum of the partial pressures equals the total pressure

Question 3

expressions of heterogeneous equilibria for Kc

Answer 3

solids and liquids are omitted from the Kc expression- it only includes species that are (g) or (aq)

Question 4

expressions of heterogeneous equilibria for Kp

Answer 4

Kp only contains species that are gaseous as only gases have partial pressures

Question 5

units of Kc

Answer 5

mol dm-3 (depends on the number of terms on the top and bottom of the fraction)

Question 6

units of Kp

Answer 6

atm, KPa or Pa (depends on the number of terms on the top and bottom of the fraction)

Question 7

Kp expression

Answer 7

H2(g) + I2(g) <—> 2HI(g)
p(HI)*2 / p(H2)p(I2)
where p is the equilibrium partial pressure
DO NOT USE SQUARE BRACKETS FOR KP AS IT DOES NOT INVOLVE CONC.

Question 8

what are the factors which affect Kc?

Answer 8

temperature only!

Question 9

when you increase the temperature of an exothermic reaction, what happens to the value of Kc/p?

Answer 9

it decreases

Question 10

when you increase the temperature of an endothermic reaction, what happens to the value of Kc/p?

Answer 10

it increases

Question 11

explain why the value of Kc decreases when an exothermic reaction increases in temperature, in terms of the p.o.e

Answer 11

A + B <—–> C + D
deltaH = -92kjmol-1
when temperature increases, the position of equilibrium shifts in the endothermic direction, which increases the concentration of reactants. As Kc is [products]/[reactants], the bottom half of the equation increases, so the overall value of Kc decreases.

Question 12

explain why the value of Kc increases when an endothermic reaction increases in temperature, in terms of the p.o.e

Answer 12

A + B <—–> C + D
deltaH = +92kjmol-1
when temperature increases, the position of equilibrium shifts in the endothermic direction, which increases the concentration of products. As Kc is [products]/[reactants], the top half of the equation increases, so the overall value of Kc increases.

Question 13

explain how the equilibrium constant is maintained when the concentration of a reactant is changed

Answer 13

A + B <—–> C
concentration of C increases, which increases the numerator in the Kc equation. to counteract this, the position of equilibrium shifts to the left, increasing the concentration of the reactants and therefore restoring Kc

Question 14

explain how the equilibrium constant is maintained when the pressure of a system is changed

Answer 14

A + 3B <—–> 2C
kp equation: p(C)2/p(A)p(B)3
pressure increases, which causes the total value of the denominator to increase more than the numerator, disrupting Kc (as the denominator is to the power of 4 whereas the numerator is to the power of 2). to counteract this, the position of equilibrium shifts to the right, increasing the partial pressure of the numerator and restoring Kc