Chemical Equilibrium - Chp 17

Reversible Reaction

is one in which the products react to give back the reactants, the reaction is going in both directions

Chemical Equilibrium

a state of dynamic balance in a reversible reaction where the rate of the forward reaction is the same as the rate of the backward reaction

Dynamic state

a state in which the reactants are continuously forming products and the products are continuously forming reactants

Dynamic Equilibrium

the rate of forward reaction is equal to the rate of the backward reaction

Le Chatelier’s Principle

states that is a stress is applied to a system at equilibrium, the system re-adjusts to relieve the stress applied

Name 2 industrial applications of Le Chatelier’s Principle

• Manufacture of Ammonia by the Haber Process

• Manufacture of Sulfuric Acid by the Contact Process

Write an equation for the manufacture of ammonia + state whether exothermic or endothermic going forward:

What does Le Chatelier’s Princple suggest for optimal ammonia production + what is the problem + what is the compromise?

Temperature 500 degrees C

Pressure of 200 atmospheres

Write an equation for the manufacture of sulfuric acid + state whether exothermic or endothermic going forward:

Exothermic

What does Le Chatelier’s Princple suggest for optimal sulfuric acid production + what is the problem + what is the compromise?

High Pressure - shifts equilibrium to the product side/right

Low temperature - shifts equilibrium to the product side/right

Problem

High pressure - explosion more likely + more expensive

Low temperature - rate of reaction too slow

Compromise

Pressure just below atmospheric pressure

Temperature of 450 degrees C

How do you write an expression for Kc?

[product]^{no. moles} [product]^{no. moles} / [reactant]^{no. moles} [reactant]^{no. moles}

square brackets represent concentration in moles per litre

What does Kc express?

the larger the value of Kc the more the equilibrium is shifted to the product side

What effects the value of Kc?

• Kc is always constant for the given chemical equation and temperature

• changes in temperature alone affect Kc

How do you find the value of Kc?

1. find the expression for Kc

2. find the concentration of reactants and products in moles per litre

3. sub into the Kc expression

How do you find the concentration of the products when given the value of Kc and the initial concentration of the reactants?

1. find the expression of Kc

2. sub known values into table

3. find the conc at equilibrium of the reactants and products in terms of x

4. sub values into Kc expression and form a quadratic equation

5. find x, choosing the most logical value between the two

6. sub values in to find the concentrations at equilibrium

Write an equation for the experiment to illustrate Le Chatelier’s principle

Iron (III) Chloride + Potassium Thiocynate ⇌ Ferrocyothyate + Chloride ions

Fe³⁺Cl₃^- + CNS^- ⇌ Fe(CNS)²⁺ + 3Cl^-

yellow colourless red

What 2 variable are investigated in the experiment?

• concentration

• temperature

Describe the steps in the investigation of the effect of concentration on the position of equilibrium

• some Iron (III) Chloride solution and one drop of Potassium Thiocynate solution are mixed in a test tube

• red colour is observed, due to the formation of Ferrocyothyate

• the equilibrium has shifted to the right side of the equation

• some dilute HCl acid is added until red disappears

• the equlibrium shifts left to use up added Cl^- ions

• some Potassium Thiocynate is added, and a red colour is observed

• the equilibrium has shifted to the right side of the equation to use up substance added

Describe the steps in the investigation of the effect of temperature on the position of equilibrium