Transcript Chemical Equilibrium - Garbally Chemistry

Chemical Equilibrium
Chapter 17
Chemical Equilibrium
• Chemical Equilibrium is a state of dynamic
balance where the rate of the forward
reaction is equal to the rate of the reverse
reaction.
Le Chatelier’s Principle
• When a system in equilibrium is subjected
to a stress, the system readjusts to relieve
the effect of the stress.
• Stresses include Pressure, Temperature,
Concentration of one or more reactants.
• Leaving Cert 2008 Q7, 2007Q10a, 2006 Q11b,2005 Q9 2004 Q9(C)
2003 Q11 2002 Q10(C)
Experiments to investigate Le Chatelier’s principle
using 3 named experiments.
Example 1
Cobalt (II) Chloride, conc HCL.
CoCl42- + 6H2O
Blue
Co(H2O)62+ + 4ClRed
Addition of HCl Red to Blue
Addition of Water - Goes Blue to Red
Addition of Heat
Exothermic
CoCl42- +
6H2O
Co(H2O)62+
+ 4Cl-
Increase in temperature- goes from red to blue therefore the reaction is ?
Exothermic
Example 2
Sodium Dichromate
Cr2O72– + H2O
Orange
2CrO42– + 2H+
Yellow
Addition of an alkali(NaOH)Orange changes to Yellow
Why?
Addition of an acid (HCL)Yellow to Orange
Why?
Example 3
Example 3:
Iron(III) Chloride Potassium Thiocyanate
Fe3+ + CNS- ↔ Fe(CNS)2+
Yellow
Red


(FeCl3 + CNS- ↔ Fe(CNS)2+ + 3Cl-)
Add conc. HCL-red colour changes to Yellow.
Add some Thiocyanate- the colour changes from yellow to red
Industrial Applications of Le
Chatellier
Manufacture of Ammonia(NH3) by the Haber Process
N2 + 3H2 --> 2NH3
According to Le Chatelier a high pressure and low
temperature are the best conditions to give a maximum
yield of ammonia.
In practice
the reaction is carried out at a pressure of 200 atmospheres
and a temperature of 500* C with an Iron catalyst.
HABER PROCESS
HABER PROCESS
N2(g) + 3H2(g)
Conditions
Pressure
Temperature
Catalyst
2NH3(g)
: DH = - 92 kJ mol-1
20000 kPa (200 atmospheres)
380-450°C
iron
Equilibrium theory favours
low temperature
exothermic reaction - higher yield at lower temperature
high pressure
decrease in number of gaseous molecules
Kinetic theory favours
high temperature
greater average energy + more frequent collisions
high pressure
more frequent collisions for gaseous molecules
catalyst
lower activation energy
Compromise conditions
Which is better?
A low yield in a shorter time
or
a high yield over a longer period.
The conditions used are a compromise with the catalyst
enabling the rate to be kept up, even at a lower temperature.
IMPORTANT USES OF AMMONIA AND ITS COMPOUNDS
MAKING
FERTILISERS
80% of the ammonia produced goes to make fertilisers such as
ammonium nitrate (NITRAM) and ammonium sulphate
NH3 + HNO3
——>
2NH3 + H2SO4 ——>
MAKING
NITRIC ACID
NH4NO3
(NH4)2SO4
ammonia can be oxidised to nitric acid
nitric acid is used to manufacture...
fertilisers (ammonium nitrate)
explosives (TNT)
polyamide polymers (NYLON)
Example 2
Manufacture of Sulfuric Acid by the Contact process.
Exothermic
2SO2 + O2
2SO3
What Conditions give the best Yield?
According to Le Chatelier a high pressure and low
temperature are the best conditions to give a maximum yield
of sulphur trioxide.
In practice the reaction is carried out at a pressure of 1
atmospheres and a temperature of 45O* C with a Vanadium
Pentoxide (V2O5)catalyst.
The Equilibrium constant(Kc)
Provided the Temperature remains constant there is a
relationship between the concentration of the reactants and
products in a equilibrium mixture. This constant is based on the
law of Chemical Equilibrium.
If
Therefore
Only a change in temperature changes the value of Kc
Example
Write the equilibrium constant expression (Kc) for the
following equation.
Answer:
Calculations involving the law of Chemical Equilibrium
(See BOOK pages 242-245)
Two types (a) Calculate KC (2004 Q 9)
(b) Given KC calculate the equilibriumconcentrations
(Revised sample paper Feb 2002 Q.9)
Leaving Cert 2005 Q9(C) 2004 Q9(C) 2003 Q11 2002 Q10(C)
Question 7 2010 paper Find KC
Given KC find the equilibrium concentrations.
2003 Q11(a)