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CHEMISTRY
2
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1 ENTHALPY CHANGES
ENTHALPY, H


Heat content that is stored in a chemical system

CHEMICAL SYSTEM


The reactants and products

SURROUNDINGS


Outside the chemical system

CONSERVATION OF ENERGY

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No energy is lost
Energy changes is measured as heat
Heat loss in a chemical system = heat gain to surroundings
Heat gain in a chemical system = heat loss from surroundings

ENTHALPY CHANGE, ΔH
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


The heat exchange with the surroundings during a chemical reaction, at constant pressure
ΔH = H products – H reactants
All chemical reactions either release or absorb heat

EXOTHERMIC REACTIONS
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ΔH, negative
Enthalpy of products is smaller than the
enthalpy of the reactants
Heat loss to the surroundings
E
...
chemicals reacting together in an inner
chamber of self-heating cans
CaO (s) + H2O (l)  Ca(OH)2 (aq) ΔH = -ve

ENDOTHERMIC REACTIONS
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ΔH, positive
Enthalpy of products is greater than the
enthalpy of the reactants
Heat gain from the surroundings
E
...
evaporation of water to absorb heat from
beer in self-cooling bear cans
H2O (l)  H2O (g) ΔH = +ve

EXOTHERMIC REACTIONS
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
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H products < H reactants (exothermic reactions)
H products > H reactants (endothermic reactions)

ACTIVATION ENERGY, Ea



Energy barrier that prevents many reactions from taking place spontaneously
Energy equired to break bonds in the reactants

Ea OF EXOTHERMIC REACTIONS





Energy is required to break the first bond
and start the reaction
Once the energy barrier has been
overcome, the net out output of energy
provides more energy that can be used to
overcome the activation energy for the
reaction to continue
Once an exothermic reaction begins, the
activation energy is regenerated and the
reaction becomes self-sustaining

Ea OF ENDOTHERMIC REACTIONS



The reaction has to overcome the energy
barrier
No excess energy to break more reactant
bonds, a sustained amount of energy
needs to be continually supplied to keep
the reaction going

STANDARDS



Chemists use standard enthalpy changes, measured under standard conditions
Ensures that all reactions and enthalpy changes are carried out under same conditions

STANDARD CONDITIONS
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
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A pressure of 100 kPa (1 atmosphere)
A stated temperature at 298 K (25 ⁰C) is usually used
A concentration of 1 mol dm-3 (for reactions with aqueous solutions)

STANDARD ENTHALPY CHANGE, ΔHƟ




H – enthalpy
Δ – change
Ɵ
– under standard conditions
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0 mol dm-3 (for reactions with aqueous solutions)

STANDARD STATES

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For a standard enthalpy change, any substance must be in its standard state
The physical state of a substance under standard conditions
E
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Mg (s), H2 (g), H2O (l)

STANDARD ENTHALPY CHANGES
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Standard enthalpy change of reaction
Standard enthalpy change of combustion
Standard enthalpy change of formation

STANDARD ENTHALPY CHANGE OF REACTION, ΔHrƟ




The standard enthalpy change of reaction, ΔHrƟ, is the enthalpy change that accompanies a
reaction in the molar quantities expressed in a chemical equation under standard
conditions, all reactants and products being in their standard states
1
E
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H2 (g) + 2 O2 (g)  H2O (l) ΔHrƟ = -286 kJ mol-1

STANDARD ENTHALPY CHANGE OF COMBUSTION, ΔHcƟ


The standard enthalpy change of combustion, ΔHcƟ, is the enthalpy change that takes place
when one mole of a substance reacts completely with oxygen under standard conditions, all
reactants and products being in their standard states

ΔHcƟ FOR ETHENE
1
2



C2H6 (g) + 3 O2 (g)  2CO2 (g) + 3H2O (l) ΔHcƟ = -1560 kJ mol-1



The complete combustion of 1 mol of C2H6 (g)

STANDARD ENTHALPY CHANGE OF FORMATION


The standard enthalpy change of formation, ΔHfƟ, of a compound is the enthalpy change
that takes place when one mole of a compound is formed from its constituent elements in
their standard states under standard conditions

ΔHfƟ FOR WATER



1

H2 (g) + 2 O2 (g)  H2O (l) ΔHfƟ = -286 kJ mol-1
1 mol of H2O (l) being formed from its elements

ΔHfƟ OF AN ELEMENT
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

No chemical change
All elements have a standard enthalpy change of formation of 0 kJ mol-1

DETERMINATION OF ENTHALPY CHANGES


Q = mcΔT Joules
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00 mol dm-3 CuSO4 (aq)
...
0 ⁰C to 65
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Find the enthalpy change of reaction for the following
equation: Mg (s) + CuSO4 (aq)  MgSO4 (aq) + Cu (s)
Specific heat capacity of solution, c = 4
...
00 g cm-3
Q = mcΔT = 100 x 4
...
0 – 20
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05 kJ mol-1

2
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200 mol

-18810
=
0
...
g
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EXO OR ENDO?

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

Depends of bond enthalpies (relative strengths of bonds being broken or made)
Exothermic reaction – bonds formed are stronger than the bonds that are broken
Endothermic reaction – bonds broken are stronger than the bonds that are formed

BOND ENTHALPIES TO DETERMINE ENTHALPY CHANGES
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For reactions involving simple gaseous molecules
ΔH(bond enthalpies of bonds broken) – (bond enthalpies of bonds made)
(bond enthalpies of bonds broken) = energy required to break bonds
(bond enthalpies of bonds made) = energy released when bonds are made

BOND ENTHALPIES TO DETERMINE ENTHALPY CHANGES (EXAMPLE)
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Average bond enthalpies is used to
work out the enthalpy change of
reaction for reactions involving gases
CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O
(g)
Average bond enthalpies:
 C–H: +413 kJ mol-1; O=O:
+497 kJ mol-1; C=O: +805 kJ
mol-1; O–H: +463 kJ mol-1
Bonds broken = 4 (C–H) + 2 (O=O)
Bonds made = 2 (C=O) + 4 (O–H)
ΔH(bond enthalpies of bonds broken) – (bond enthalpies of bonds made)
ΔH = [(4 x 413) + (2 x 497)] – [(2 x 805) + (4 x 463)]
ΔH = -816 kJ mol-1

HESS’ LAW
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Hess’ law states that, if a reaction can take place by more than one route and the initial and
final conditions are the same, the total enthalpy change is the same for each route
Method for finding an enthalpy change indirectly
Route 1: A (reactants  products)
Route 2: B + C (reactants  intermediate product)
Total enthalpy change is the same for each route (by Hess’ law)
A=B+C

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3
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COLLISION THEORY


A chemical reaction can take place only
when the reacting molecules collide

CONDITIONS FOR REACTIONS TO TAKE PLACE



The molecules must have enough energy
to overcome the activation energy
The molecules must collide in the correct
orientation
 Correct: A + BC  AB + C
 Incorrect: A + BC  AC + B

EFFECT OF CONCENTRATION ON REACTION RATE
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If concentrations of reactants are increased, the rate of reaction also increases
More molecules in the same volume
Molecules are closer together
Greater chance of the molecules colliding
Collisions will be more frequent
More collisions with energy greater than the Ea
More collisions in a certain length of time

EFFECT OF PRESSURE ON REACTION RATE
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If pressure of a gas is increased, the molecules are pushed closer together
Same number of molecules occupy a smaller volume
Same as increasing the concentration for a gaseous reaction
More collisions will take place
More collisions with energy greater than the Ea
More frequent collisions
Rate of reaction will increase

CATALYST
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Increases the rate of a chemical
reaction without being used up in
the process
A catalyst may react to form an
intermediate
It is later regenerated
Does not undergo any permanent
change
Lowers the Ea of a reaction by
providing an alternative route
which has a lower energy

REDUCING ENERGY CONSUMPTION AND HELPING THE ENVIRONMENT
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It is thought that as the enzyme holds the substrate in place, other bonds in the molecule
are weakened and therefore easier to break

BENEFITS OF ENZYMES
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Lower temperatures and pressures can be used
Allows a reaction to take place which forms pure products (no side reactions)
Conventional catalysts are often poisonous (disposal problems)
Enzymes are biodegradable
Saves energy and reduces costs

THE BOLTZMANN DISTRIBUTION
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Collisions are assumed to be elastic (no energy is lost in collisions)
Molecules have kinetic energy
Some molecules move fast and have high energy
Some molecules move slowly and have low energy
The majority of the molecules have an average energy

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EFFECT OF A CATALYST ON REACTION RATE

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Ea is reduced
More molecules will overcome the new lower Ea
More successful collisions
Rate of reaction will increase

DYNAMIC EQUILIBRIUM
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The rate of the forward reaction is equal to the rate of the reverse reaction
System is dynamic (in constant motion) but no observable change
The concentration of the reactants and the products remain the same

LE CHATELIER’S PRINCIPLE



le Chatelier’s principle states that when a system in dynamic equilibrium is subjected to a
chance, the position of equilibrium will shift to minimise the change
Position of equilibrium can be altered by changing:
 Concentrations of reactants or products
 Pressure in reactions involving gases
 Temperature

EFFECT OF CONCENTRATION ON EQUILIBRIUM
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



CH3CH2OH (l) + CH3CH2COOH (l) ⇌ CH3CH2COOCH2CH3 (l) + H2O (l)
Increasing concentration of a reactant causes the position of equilibrium to move in the
direction that decreases this increased reactant concentration
 Position of equilibrium moves to the right-hand side
 Forms more products
Increasing concentration of a product causes the position of equilibrium to move in the
direction that decreases this increased product concentration
 Position of equilibrium moves to the left-hand side
 Forms more reactants

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