equilibria unit 4 chem kajana notes.pdf

8/11/2019 equilibria unit 4 chem kajana notes.pdf

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A2 Chemistry Unit 4 Dynamic Equilibria Tutorial 1

Kajana Sivarasa1

Dynamic Equilibria

Most reactions are reversible They can go both ways.

A + B C + D

When the reactants are mixed ,they eventually reach a position of dynamic

equilibrium.

When the system is in dynamic equilibrium both forward reaction and

reverse reaction are happening but at the same rate.

At dynamic equilibrium, the concentrations of the reactants and products do

not alter.

A dynamic equilibrium can only happen in a closed system at a constant

temperature.

When the dynamic equilibriumhas been reached, the reaction has not

stopped; it is simply moving in both directions at the same rate.

Important reversible industrial reactions,o The Contact Process to manufacture sulphuric acid for use in

fertilisers,dyes,medicines and batteries.

2SO2(g)+ O2(g) 2SO3(g)

o The Haber process to manufacture ammonia for making fertilizers

and other nitrogen containing compounds.

N 2(g)+ 3H2(g) 2NH3(g)


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Equilibrium Constants

1. Expressions for equilibrium constant (Kc)

Consider the reversible reaction aA + bB cC + dD

The ratio of product concentrations to reactant concentrations is known as the

Equilibrium Constantof the reaction.

Kc= [C]c[D]

d

[A]a[B]

b

Where a,b,c,d are the number of moles of each substance.

Example :

Consider the reaction

2SO2(g) + O2(g) 2SO3 (g)

If the equilibrium concentrations are SO2: 0.016mol/dm3

O2: 0.0083mol/dm3

SO3: 0.15mol/dm3

Calculate the value of the equilibrium constant Kc, stating

its units

Kc=!!"!!

!

!!"!!!

!!!!=

!!!!"!!

!!!!"#!!!!!!!"#= 1.1 x 10

4mol

-1dm

3

Even when the starting concentrations of the reactants and products are

different for different experiments, when equilibrium concentrations are

used to calculate the equilibrium constant, the Kcwill be constant for allexperiments at the same temperature.

Calculating the Units :

!"#$!

!"#$!!!"#$=

!

!"#$

which is

!

!"#

!"!!= mol-1dm3


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! Homogeneous and Heterogeneous equilibrium

The definition of Kc only applies to homogeneous equilibria.i.e once where

all the reactants and products are in the same phase.

If youve got more than one phase heterogeneous equilibrium.

If the mixture involves

o Solids and liquids Use Kc and leave out the concentration of

the solids.

o Solids and gases Use Kpand leave out the concentration of

solids.o

Gases and liquids Use Kpand leave out the concentration of

the liquids.


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Kajana Sivarasa4

Practice Questions

1.What is meant by dynamic equilibrium?

.

2.At 473K,the equilibrium constant, Kc, for the reaction below has a numerical

value of 125.

PCl3(g)+ Cl2(g) PCl5(g)

a) At 473K, a quantity of Cl2(g) was added to the mixture and equilibrium was

re-established. What effect would this have on the value of the equilibrium

constant? Explain your answer. [2 marks ]

b)

The temperature of the equilibrium mixture was allowed to drop to 423K.

What effect would this have on the value of the equilibrium constant?

Explain your answer. [2 marks]

c)

At 473K,what would be the numerical value of Kc for the reaction below?

PCl5(g) PCl3(g)+ Cl2(g) [2 marks]

..


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Kajana Sivarasa5

3. What are the units of the equilibrium constant Kc for the following reactions?

(a)N2(g)+ 3H2(g) 2NH3(g)

(b)H2(g)+ I2(g) 2HI(g)

4. The equilibrium constant Kc for the reaction 2SO2(g) + O2(g) 2SO3 (g)is 10 dm

3mol

-1at a certain temperature. The concentrations in the mixture were

found to be [SO3] = 5 mol dm-3

, [SO2] = 4 mol dm-3

, [O2] = 3 mol dm-3

.

Was the reaction at equilibrium?


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2. Working out the equilibrium concentrations to calculate Kc

Example Q :

0.20 moles of phosphorus(V) chloride decomposes at 600K in a vessel of

5.00dm3.The equilibrium mixture is found to contain 0.08 moles of chloride. Write

the expression for Kc and calculate its value, including units.

PCl5(g) PCl3(g)+ Cl2(g)

a. Find out how many moles of PCl5 and PCl3at equilibrium

b. Calculate the molar concentrations

c. Substitute in the expression for Kc

d. Work out the units.


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3. Kc can be used to find Concentrations in an Equilibrium mixture

Example Q:

When ethanoic acid was allowed to reach equilibrium with ethanol at 25oC, it was

found that the equilibrium mixture contained 2.0 mol dm-3 ethanoic acid and

3.5 mol dm-3

of ethanol. The Kc of the equilibrium is 4.0 at 25oC.What are the

concentrations of the other components?

CH3COOH(l)+ C2H5OH(l) CH3COOC2H5(l)+ H2O(l)


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4.The Equilibrium Constant can be calculated from Experimental Data

Example Q:

Fe2+

(aq)+ Ag+(aq) Fe

3+(aq) + Ag(s)

500cm3of 0.1 mol dm

-3silver nitrate solution was added to 500 cm

3of

0.1 mol dm-3

of iron(ll) sulphate solution.

The mixture was left in a stoppered flask at 298K, which eventually reached

equilibrium. When samples of equilibrium mixture were taken and titrated the

titration result for Fe2+

was 0.0439 mol dm-3

.

Work out the equilibrium concentrations of the other components.

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equilibria unit 4 chem kajana notes.pdf