Chemical Equilibrium Gr12 by Lizelle Swanepoel

INTRODUCTION

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Two key points:

A reversible reaction = a reaction that can proceed in both the forward and backward directions, whereby products can re-form reactants in a CLOSED SYSTEM.

Equilibrium = a reaction state where the rate of the forward reaction equals the rate of the reverse reaction. All reactant and product concentrations are constant at equilibrium.
DYNAMIC EQUILIBRIUM = A reaction in which the rate of the fwd reaction equals the rate of the reverse reaction in a closed system, where the fwd and rev reactions occur simultaneously.

Le Chatelier's Principle

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DEFINITION:

When the equilibrium in a closed system is disturbed, the system will re-instate a new equilibrium by favouring the reaction that will oppose the disturbance.

LE CHATELIER'S PRINCIPLE (LCP) EXAMPLE

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FACTORS AFFECTING CHEMICAL EQUILIBRIUM

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STRESSES (DISTURBANCES) CAN BE:

CONCENTRATION (OF REACTANTS OR PRODUCTS)
PRESSURE (ONLY IF REACTION SPECIES ARE GASES)
TEMPERATURE

HOW TEMPERATURE CHANGE STRESS AFFECT CHEM EQUILIBRIUM

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REMEMBER:
FOR EXOTHERMIC RXN (ΔH<0 or negative value for ΔH given):

If stress is T decreases, then response is FWD RXN FAVOURED.
If stress is T increases, then response is that REV RXN IS FAVOURED.

FOR ENDOTHERMIC RXN (ΔH>0 or positive value for ΔH given):

If stress is: T increases, then response is that FWD RXN IS FAVOURED.
If stress is: T decreases, then response is that REVERSE RXN IS FAVOURED.

Key: RXN = reaction, FWD = forward, REV = reverse

TEMPERATURE DISTURBANCE ON EQUILIBRIUM RXN

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EXAMPLE:

LCP AMOUNT GRAPHS

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EFFECT OF TEMPERATURE ON EQUILIBRIUM

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GRAPHS

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GRAPHS

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EFFECT OF CATALYST

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EXERCISE 1

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ICE TABLES

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To calculate Kc values:

ICE TABLE PROBLEM 1

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ICE TABLE PROBLEM 2

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EQUILIBRIUM GRAPHS

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These graphs help us to:

Calculate K_c values
Identify specific stresses made on a reaction at equilibrium

EXAM STYLE QUESTION

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MEMO TO EXAM STYLE QUESTION

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EXAMPLE CALCULATING Kc

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ANSWERS TO END-OF-CHAPTER EXERCISE

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EQUILIBRIUM APPLICATION IN INDICATORS

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They change colour to indicate whether a substance is acidic or basic.

Indicators are weak acids, which undergo ionisation in a REVERSIBLE REACTION, as follows:
HIn ⇌ H⁺+ In^-Colour1 Colour2

An indicator changes colour in an acid or in a base according to LE CHATELIER'S PRINCIPLE:

1) When an acid is added to an indicator, the equilibrium system experiences a stress in the form of an increase in [H⁺].

The system will respond to decrease the added H⁺, by consuming it.
The equilibrium shifts to favor the reverse reaction (equilibrium shifts to the left).
Concentration of the product decreases and concentration of reactant increases.
Indicator colour changes from colour2 to colour1.

2) When a base is added to the indicator, the base reacts with H⁺and removes H⁺.

The system responds to replace the removed H⁺ions, hence the equilibrium position changes to favor the forward reaction which produces H⁺ ions.
The equilibrium shifts to the left and product increase and reactant concentration decreases.
Indicator changes color from color1 to color2.

CHEMICAL EQUILIBRIUM

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What is chemical equilibrium:
A state of BALANCE in a reversible reaction where the forward and reverse reaction rate are equal.

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Equilibrium constant = K-value

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END-OF-CHAPTER EXERCISE

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Haber process: making ammonia

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Know for exams:

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