Chemical Equilibria

What is meant when chemical reactions go to completion

The reaction will continue until one of the reagents has been used up. Magnesium burning in oxygen is a good example

Mg(s) +1/2 O₂(g) → MgO(s). Once MgO has been formed in a reaction you cannot reform Mg and ½ O₂ in a chemical reaction from MgO only

What is a reversible reaction

A reversible reaction is a chemical process where reactants form products, which can then revert back to the original reactants. E.g CH₃CH₂OH + CH₃COOH = CH₃COOCH₂CH₃ + H₂O

What exactly is equilibrium

Equilibrium is the state in a reversible reaction where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products.

What is dynamic equilibrium?

Dynamic equilibrium is the condition in a reversible reaction where the rates of forward and reverse reactions are equal and are happening at the same time keeping concentrations of reactants and products constant.

Yellow chromate ions reversible reaction example

2CrO₄^2- + 2H⁺ = Cr₂O₇^2- + H₂O

/ ← Add alkali

Add acid →

Yellow chromate = CrO₄^2-

Orange dichromate = Cr₂O₇^2-

Effect of change in concentration on equilibrium

If the concentrations of reactants are increased, equilibrium would move to the right ( 1 mark). To oppose the change (increase in the concentration of reactants) so more of the reactants react to reduce its concentration (1 mark). So yield of product increases (1 mark) and vice versa

Effect of partial pressure on equilibrium

If the partial pressure of the reactants are increase (effectively the concentration of reactants) Equilibrium move left to right (1 mark) to oppose the change (increase in partial pressure of reactants) and reduce the partial pressure of the reactants (1 mark). So the yield of product increases (1 mark)

What is the effect in total pressure on equilibrium

(Example: 2 moles of GAS on reactant side and 3 moles of GAS on product side)

If we decrease the pressure equilibrium noves to the side with more moles of gas to oppose the change an increase preussre (1 mark) There are 2 moles the left and 3 moles of gas on the right (1 mark) so equilibrium moves from Left to right to oppose the decreased pressure and the yield of product

Effect of changing temperature on equilibrium

The Endothermic/exothermic direction is determined by the overall energy change. If + ve it is endothermic (forward direction) and if -ve it is exothermic (forward direction). Say that the overall energy change is +ve. Increase in temperature moves equilibrium the endothermic direction so equilibrium moves from right to left to make more reactants and decrease the temperature

What happens when the volume of a container is reduced during a chemical reaction

The equilibrium will shift to the side with more moles of gas (lower pressure)

What happens when a catalyst is added to an equilibria reaction

Equlibrium does not move, a catalyst speeds up the rate of the forward reaction and reverse equally

Balanced symbol equation for the haber process

N2(g) + 3H2(g) = 2NH3 (g) Overall energy change = -92 KJmol^-1

Effect of increasing pressure on the haber process

If the pressure is increased, the system will oppose the ffect of this change by moving to the side with fewer moles of gas so reducing the pressure. 4 moles on the left and 2 moles on the right. Equilibrium moves left to right and yield of ammonia increases.

How is the yield of ammonia affected? (haber process)

Increases with increasing pressure, at 298 K, 100 KPa = 2% yield

3000 KPa = 32% yield

How does the rate of reaction change (haber process)

Increasing pressure increases rate due to more molecules in the same volume. Greater collision frequency, mopre succesful collisions E > or equal to Ea

Cost of high pressure equipment ((haber process)

Very expensive to produce, maintain and contain high pressure

Pressure used (haber process)

20000 KPa (200 atm)

Compromise pressure as increase in P increases both YIELD and RATE but Increases the cost and level of danger

What temperature is ued in the haber process and why?

450 degrees, it is a compromise between a reasonable yield at a reasonable rate

What catalyst is used in the haber process,

Fine divided iron catalyst (increases surface area)

How are nitrogen gas and hydrogen gas obtained for the haber process

Hydrogen is obtained from the hydration of methane with steam and nitrogen gas is obtained from the air

Why must the gases be purified? (With balanced symbol equation)

To remove impurities that may poison the catalyst, ensuring efficient reaction and higher yields. CH4(g) + H2O = CO(g) +3H2(g)

To purify it the mixture of ammonia is passed over the iron catalust at 450 degrees and 200KPa and then cooled to liquefy the ammonia.

Why is the ammonia cooled during the haber process

To liquefy the ammonia and separate it from the hydrogen and nitrogen gases which are recirculated

Example of another reversible reaction (The manufacture of sulphuric acid via the contact process)

This involves the manufacture of sulphur trioxide in an exothermic reaction 2SO2(g) + O2(g) = 2SO3(g)

What does carbon neutral mean?

An activity that has no net annual release of carbon (greenhouse gas) emissions to the atmosphere

Equation for the equilibrium constant

Kc = [products] / [reactants] Kc = [C]^c [D]^d / [A]^a [B]^b

units are mol/dm^-3 (c,d, = moles of products AND a,b, = moles of reactants)

(A,B = equilibrium concentration of A and B) (C, D = equilibrium concentration of C and D)

If there is a volume then its [equilibrium concentration/ volume]^moles

How does calculating equilibrium moles work?

Reactant side = initial moles - (worked out equilibrium mole)

product side = inital moles + (worked out equilibrium mole)