A level OCR A Chemistry How far? (A2)

what is meant by the rate of reaction

what is meant by the rate of reaction

change in volume with time

what are the features of an equilibrium

~concentration of reactants and products are constant but are constantly interchanging

~ rate of forward reaction= rate of reverse reaction

how would you write an expression for Kc

Kc = products/ reactants

how do you calculate the mole fraction of a gas in a mixture

number of moles of a gas/total number of moles of a gas in the mixture

how do calculate the partial pressure of a gas

mole fraction x total pressure of the mixture

CO(g) + 2H2(g) ⇌ CH3OH(g)

an increase in pressure shifts equilibrium to the right, why?

fewer moles of gas on the right

CO(g) + 2H2(g) ⇌ CH3OH(g)

what effect does an increase in pressure have on the Kc value

no effect

CO(g) + 2H2(g) ⇌ CH3OH(g)

temperature was increased, Kc value decreased

what happened to the equilibrium position and what would the sign of ∆H for the forward reaction in the equilibrium be

~ shifts to the left

~ negative because the high temperature favours the endothermic direction

PCl5(g) ⇌ PCl3(g) + Cl2(g)

whats the expression for Kc for this reaction

Kc = [PCl3] [Cl2]/ [PCl5]

PCl5(g) ⇌ PCl3(g) + Cl2(g)

Kc = 0.245

0.30 moldm-3 of each gas is added to a container

as the mixture approaches equilibrium what happens to the concentration of each gas and why

PCl5 > 0.30

PCl3 < 0.30

Cl2 < 0.30

at the start, the system is out of equilibrium with too much PCl3 and Cl2 and not enough PCl5

when the temperature is constant and the equilibrium mixture is compressed what happens to the value of Kc

no effect on Kc

temperature is constant

PCl5(g) ⇌ PCl3(g) + Cl2(g)

temperature is constant an pressure is increased

what is the composition of the equilibrium mixture

~ amount of products decreased

~ more moles on RHS

~ amount of reactant increased

~ fewer moles on LHS

PCl5(g) ⇌ PCl3(g) + Cl2(g)

the equilibrium mixture is heated

what happens to the value of Kc

~ decreases

~ as there more reactants than products

PCl5(g) ⇌ PCl3(g) + Cl2(g)

Kc value decreases as the temperature is heated

what else does this tell you about the reaction

~ forward reaction is exothermic

~ equilibrium shifts to LHS to oppose the increase in energy

N2(g) + 3H2(g) ⇌ 2NH3(g) ∆H = -92 kJ mol-1

what are the advantages and disadvantages of running this reaction:

~ at a pressure of 200-300 atm;

~ at a temperature of 500°C;

~ with an iron catalyst.

High pressure:

advantages:

~ Fewer moles on r.h.s. → equilibrium moves to right

~ Greater pressure → faster rate/more frequent collisions

disadvantages:

~ Safety issues from high pressure

~ Expense of high pressure

High temperature:

advantages:

~ more collisions exceed activation energy/more successful collisions/more energetic collisions/molecules have more energy

disadvantages:

Equilibrium moves to left because forward reaction is exothermic

Catalyst:

~ lowers activation energy/allows reaction to take place at a lower temperature

H2(g) + I2(g) ⇌ 2HI(g)

the equilibrium mixture is compressed and temperature is kept constant

what happens to the Kc value and the composition of the equilibrium mixture

~ no effect on Kc as temperature is constant

~ composition of the mixture is the same as there are the same moles of gas on both sides

how would you write an expression for Kp

Kp= p(products)/p(reactants)

2SO2(g) + O2(g) ⇌ 2SO3(g)

~ the conversion of sulphur dioxide and oxygen into sulphur trioxide is carried out at slightly above atmospheric pressure. Comment on this statement.

~ explain what happens to the equilibrium amounts of SO2, O2 and SO3 as temperature increases at constant pressure.

~an increase in pressure moves equilibrium to the right because there are less gaseous moles on the right hand side

~ increased pressures are expensive to generate

~ Kp gets less with increasing temperature ~ SO2 and O2 increase/SO3 decreases

2SO2(g) + O2(g) ⇌ 2SO3(g)

Temp °C Kp

25 4.0 × 1022

200 2.5 × 108

800 1.3 × 10-3

Deduce the sign of ∆H for the forward reaction in the equilibrium

~ equilibrium → left to oppose increase in temperature

~ forward reaction is exothermic so ∆H is negative

how could the initial rate of of a substances formation be measured from a concentration/time graph?

~ draw a tangent

~ at the start of the reaction/ t=0

O3(g) + C2H4(g) → 2 CH2O(g) + 1/2 O2(g)

the initial rate of methanal formation is different from that of oxygen formation, why

stoichiometry of CH2O : O2 is not 1:1

CO(g) + 2H2(g) ⇌ CH3OH(g) ∆H = -91 kJ mol-1

what are the advantages and disadvantages of running this reaction at:

~ a high pressure,

~ a high temperature

High Pressure

~ equilibrium shifts to the right as there are fewer moles on right hand side and the shift reduces number of molecules

~ rate increases as there are more frequent collisions

High temperature

~ equilibrium shifts to the left to compensate for increased temperature

~ rate increases as there are more successful collisions

Other effect

~ High pressures/temperatures are expensive

~ high pressures cause safety problems