1. A guide for A level students KNOCKHARDY PUBLISHING
ENTHALPY CHANGES
A guide for A level students
KNOCKHARDY PUBLISHING

2. ENTHALPY CHANGES www.argonet.co.uk/users/hoptonj/sci.htm INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
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3. ENTHALPY CHANGES CONTENTS Thermodynamics Enthalpy changes
Standard enthalpy values
Standard enthalpy of formation
Standard enthalpy of combustion
Enthalpy of neutralisation
Bond dissociation enthalpy
Hess’s law
Calculating enthalpy changes using bond enthalpies
Calculating enthalpy changes using enthalpy of formation values
Calculating enthalpy changes using enthalpy of combustion values
Practical measurement
Check list

4. Before you start it would be helpful to…
ENTHALPY CHANGES
Before you start it would be helpful to…
be able to balance equations
be confident with simple arithmetical operations

5. THERMODYNAMICS
First Law Energy can be neither created nor destroyed but
It can be converted from one form to another
Energy
changes all chemical reactions are accompanied by some form of energy change
changes can be very obvious (e.g. coal burning) but can go unnoticed
Exothermic Energy is given out
Endothermic Energy is absorbed
Examples Exothermic combustion of fuels
respiration (oxidation of carbohydrates)
Endothermic photosynthesis
thermal decomposition of calcium carbonate

6. THERMODYNAMICS - ENTHALPY CHANGES
Enthalpy a measure of the heat content of a substance at constant pressure
you cannot measure the actual enthalpy of a substance
you can measure an enthalpy CHANGE
written as the symbol DH , “delta H ”
Enthalpy change (DH) = Enthalpy of products - Enthalpy of reactants

7. THERMODYNAMICS - ENTHALPY CHANGES
Enthalpy a measure of the heat content of a substance at constant pressure
you cannot measure the actual enthalpy of a substance
you can measure an enthalpy CHANGE
written as the symbol DH , “delta H ”
Enthalpy change (DH) = Enthalpy of products - Enthalpy of reactants
Enthalpy of reactants > products
DH = - ive
EXOTHERMIC Heat given out
REACTION CO-ORDINATE
ENTHALPY

8. THERMODYNAMICS - ENTHALPY CHANGES
Enthalpy a measure of the heat content of a substance at constant pressure
you cannot measure the actual enthalpy of a substance
you can measure an enthalpy CHANGE
written as the symbol DH , “delta H ”
Enthalpy change (DH) = Enthalpy of products - Enthalpy of reactants
Enthalpy of reactants > products Enthalpy of reactants < products
DH = - ive DH = + ive
EXOTHERMIC Heat given out ENDOTHERMIC Heat absorbed
REACTION CO-ORDINATE
ENTHALPY
ENTHALPY
REACTION CO-ORDINATE

9. STANDARD ENTHALPY CHANGES
Why a
standard? enthalpy values vary according to the conditions
a substance under these conditions is said to be in its standard state ...
Pressure: kPa (1 atmosphere)
A stated temperature usually 298K (25°C)
• as a guide, just think of how a substance would be under normal lab conditions
• assign the correct subscript [(g), (l) or (s) ] to indicate which state it is in
• any solutions are of concentration 1 mol dm-3
• to tell if standard conditions are used we modify the symbol for DH .
Enthalpy Change Standard Enthalpy Change
(at 298K)

10. STANDARD ENTHALPY OF FORMATION
Definition The enthalpy change when ONE MOLE of a compound is formed in its
standard state from its elements in their standard states.
Symbol DH°f
Values Usually, but not exclusively, exothermic
Example(s) C(graphite) + O2(g) ———> CO2(g)
H2(g) ½O2(g) ———> H2O(l)
2C(graphite) + ½O2(g) H2(g) ———> C2H5OH(l)
Notes Only ONE MOLE of product on the RHS of the equation
Elements In their standard states have zero enthalpy of formation.
Carbon is usually taken as the graphite allotrope.

11. STANDARD ENTHALPY OF COMBUSTION
Definition The enthalpy change when ONE MOLE of a substance undergoes complete
combustion under standard conditions. All reactants and products are in
their standard states.
Symbol DH°c
Values Always exothermic
Example(s) C(graphite) + O2(g) ———> CO2(g)
H2(g) ½O2(g) ———> H2O(l)
C2H5OH(l) O2(g) ———> 2CO2(g) + 3H2O(l)
Notes Always only ONE MOLE of what you are burning on the LHS of the equation
To aid balancing the equation, remember...
you get one carbon dioxide molecule for every carbon atom in the original and one water molecule for every two hydrogen atoms
When you have done this, go back and balance the oxygen.

12. ENTHALPY OF NEUTRALISATION
Definition The enthalpy change when ONE MOLE of water is formed from
its ions in dilute solution.
Values Exothermic
Equation H+(aq) OH¯(aq) ———> H2O(l)
Notes A value of -57kJ mol-1 is obtained when
strong acids react with strong alkalis.
See later slides for practical details of measurement

13. BOND DISSOCIATION ENTHALPY
Definition Energy required to break ONE MOLE of gaseous bonds to form gaseous atoms.
Values Endothermic - Energy must be put in to break any chemical bond
Examples Cl2(g) ———> 2Cl(g) O-H(g) ———> O(g) + H(g)
Notes • strength of bonds also depends on environment; MEAN values quoted
• making bonds is exothermic as it is the opposite of breaking a bond
• for diatomic gases, bond enthalpy = 2 x enthalpy of atomisation
• smaller bond enthalpy = weaker bond = easier to break

14. BOND DISSOCIATION ENTHALPY
Definition Energy required to break ONE MOLE of gaseous bonds to form gaseous atoms.
Values Endothermic - Energy must be put in to break any chemical bond
Examples Cl2(g) ———> 2Cl(g) O-H(g) ———> O(g) + H(g)
Notes • strength of bonds also depends on environment; MEAN values quoted
• making bonds is exothermic as it is the opposite of breaking a bond
• for diatomic gases, bond enthalpy = 2 x enthalpy of atomisation
• smaller bond enthalpy = weaker bond = easier to break
Mean Values H-H H-F N-N 163
C-C H-Cl N=N 409
C=C H-Br NN 944
CC H-I P-P 172
C-O H-N F-F 158
C=O H-O Cl-Cl 242
C-H H-S Br-Br 193
C-N H-Si I-I 151
C-F P-H S-S 264
C-Cl O-O Si-Si 176
Average (mean) values are quoted because the actual value depends on the environment of the bond i.e. where it is in the molecule
UNITS = kJ mol-1

15. “The enthalpy change is independent of the path taken”
HESS’S LAW
“The enthalpy change is independent of the path taken”
How The enthalpy change going from
A to B can be found by adding the
values of the enthalpy changes for
the reactions A to X, X to Y and Y to B.
DHr = DH DH DH3

16. “The enthalpy change is independent of the path taken”
HESS’S LAW
“The enthalpy change is independent of the path taken”
How The enthalpy change going from
A to B can be found by adding the
values of the enthalpy changes for
the reactions A to X, X to Y and Y to B.
DHr = DH DH DH3
If you go in the opposite direction of an arrow, you subtract the value of
the enthalpy change.
e.g DH2 = - DH DHr DH3
The values of DH1 and DH3 have been subtracted because the route
involves going in the opposite direction to their definition.

17. “The enthalpy change is independent of the path taken”
HESS’S LAW
“The enthalpy change is independent of the path taken”
Use applying Hess’s Law enables one to calculate enthalpy changes from
other data, such as...
changes which cannot be measured directly e.g. Lattice Enthalpy
enthalpy change of reaction from bond enthalpy
enthalpy change of reaction from DH°c
enthalpy change of formation from DH°f

18. Enthalpy of reaction from bond enthalpies
Theory Imagine that, during a reaction, all the bonds of reacting species are broken
and the individual atoms join up again but in the form of products. The
overall energy change will depend on the difference between the energy
required to break the bonds and that released as bonds are made.
energy released making bonds > energy used to break bonds EXOTHERMIC
energy used to break bonds > energy released making bonds ENDOTHERMIC
Step 1 Energy is put in to break bonds to form separate, gaseous atoms
Step 2 The gaseous atoms then combine to form bonds and energy is released
its value will be equal and opposite to that of breaking the bonds
Applying Hess’s Law DHr = Step Step 2

19. Enthalpy of reaction from bond enthalpies
Alternative view
Step 1 Energy is put in to break bonds
to form separate, gaseous atoms.
Step 2 Gaseous atoms then combine
to form bonds and energy is released; its value will be equal
and opposite to that of breaking the bonds
DHr = Step Step 2
Because, in Step 2 the route involves
going in the OPPOSITE DIRECTION
to the defined change of bond enthalpy,
it’s value is subtracted.
SUM OFTHE BOND ENTHALPIES OF THE REACTANTS
REACTANTS
PRODUCTS
ATOMS DH
SUM OFTHE BOND ENTHALPIES OF THE PRODUCTS
DH =  bond enthalpies –  bond enthalpies
of reactants of products

20. Enthalpy of reaction from bond enthalpies
Calculate the enthalpy change for the hydrogenation of ethene

21. Enthalpy of reaction from bond enthalpies
Calculate the enthalpy change for the hydrogenation of ethene
DH2 1 x C=C bond @ = kJ
4 x C-H bonds @ = kJ
1 x H-H bond @ = kJ
Total energy to break bonds of reactants = kJ
DH3 1 x C-C bond @ = kJ
6 x C-H bonds @ = kJ
Total energy to break bonds of products = kJ
Applying Hess’s Law DH1 = DH2 – DH3 = (2699 – 2824) = – 125 kJ

22. Enthalpy of reaction from enthalpies of formation
If you formed the products from their elements you should need the same amounts of every substance as if you formed the reactants from their elements.
Enthalpy of formation tends to be an exothermic process

23. Enthalpy of reaction from enthalpies of formation
Step 1 Energy is released as reactants
are formed from their elements.
Step 2 Energy is released as products
DHr = - Step Step 2
or Step Step 1
In Step 1 the route involves going in the OPPOSITE DIRECTION to the defined enthalpy change, it’s value is subtracted.
SUM OFTHE ENTHALPIES OF FORMATION OF THE REACTANTS
REACTANTS
PRODUCTS
ELEMENTS DH
SUM OFTHE ENTHALPIES OF FORMATION OF THE PRODUCTS
DH =  DHf of products –  DHf of reactants

24. Enthalpy of reaction from enthalpies of formation
Sample calculation
Calculate the standard enthalpy change for the following reaction, given that the standard enthalpies of formation of water, nitrogen dioxide and nitric acid are -286, and -173 kJ mol-1 respectively; the value for oxygen is ZERO as it is an element
2H2O(l) NO2(g) O2(g) ———> 4HNO3(l)
DH =  DHf of products –  DHf of reactants
By applying Hess’s Law ... The Standard Enthalpy of Reaction DH°r will be...
PRODUCTS REACTANTS
[ 4 x DHf of HNO3 ] minus [ (2 x DHf of H2O) + (4 x DHf of NO2) + (1 x DHf of O2) ]
DH°r = x (-173) x (-286) x (+33)
ANSWER = kJ

25. Enthalpy of reaction from enthalpies of combustion
If you burned all the products you should get the same amounts of oxidation products such a CO2 and H2O as if you burned the reactants.
Enthalpy of combustion is an exothermic process

26. Enthalpy of reaction from enthalpies of combustion
Step 1 Energy is released as reactants
undergo combustion.
Step 2 Energy is released as products
DHr = Step Step 2
Because, in Step 2 the route involves going in the OPPOSITE DIRECTION to the defined change of Enthalpy of Combustion, it’s value is subtracted.
SUM OFTHE ENTHALPIES OF COMBUSTION OF THE REACTANTS
REACTANTS
PRODUCTS
OXIDATION DH
SUM OFTHE ENTHALPIES OF COMBUSTION OF THE PRODUCTS
DH =  DHc of reactants –  DHc of products

27. Enthalpy of reaction from enthalpies of combustion
Sample calculation
Calculate the standard enthalpy of formation of methane; the standard enthalpies of combustion of carbon, hydrogen and methane are -394, -286 and -890 kJ mol-1 .
C(graphite) H2(g) ———> CH4(g)
DH =  DHc of reactants –  DHc of products
By applying Hess’s Law ... The Standard Enthalpy of Reaction DH°r will be...
REACTANTS PRODUCTS
[ (1 x DHc of C) (2 x DHc of H2) ] minus [ 1 x DHc of CH4O ]
DH°r = x (-394) x (-286) x (-890)
ANSWER = kJ mol-1

28. MEASURING ENTHALPY CHANGES
Calorimetry involves the practical determination of enthalpy changes
usually involves heating (or cooling) known amounts of water
water is heated up reaction is EXOTHERMIC
water cools down reaction is ENDOTHERMIC
Calculation The energy required to change the temperature
of a substance can be calculated using... q = m x c x DT
where q = heat energy kJ
m = mass kg
c = Specific Heat Capacity kJ K -1 kg -1 [ water is 4.18 ]
DT = change in temperature K

29. MEASURING ENTHALPY CHANGES
Calorimetry involves the practical determination of enthalpy changes
usually involves heating (or cooling) known amounts of water
water is heated up reaction is EXOTHERMIC
water cools down reaction is ENDOTHERMIC
Calculation The energy required to change the temperature
of a substance can be calculated using... q = m x c x DT
where q = heat energy kJ
m = mass kg
c = Specific Heat Capacity kJ K -1 kg -1 [ water is 4.18 ]
DT = change in temperature K
Example On complete combustion, 0.18g of hexane raised the temperature of
100g water from 22°C to 47°C. Calculate its enthalpy of combustion.
Heat absorbed by the water (q) = x x = kJ
Moles of hexane burned = mass / Mr = / =
Enthalpy change = heat energy / moles = / ANS = kJ mol -1

30. MEASURING ENTHALPY CHANGES
Example 1 - graphical
The temperature is taken every half
minute before mixing the reactants.
Reactants are mixed after three minutes.
Further readings are taken every half
minute as the reaction mixture cools.
Extrapolate the lines as shown and
calculate the value of DT.

31. MEASURING ENTHALPY CHANGES
Example 1 - graphical
The temperature is taken every half
minute before mixing the reactants.
Reactants are mixed after three minutes.
Further readings are taken every half
minute as the reaction mixture cools.
Extrapolate the lines as shown and
calculate the value of DT.
Example calculation
When 0.18g of hexane underwent complete combustion, it raised the temperature of
100g (0.1kg) water from 22°C to 47°C. Calculate its enthalpy of combustion.
Heat absorbed by the water (q) = m C DT = 0.1 x x = kJ
Moles of hexane burned = mass / Mr = / 86 =
Enthalpy change = heat energy / moles = / = kJ mol -1

32. MEASURING ENTHALPY CHANGES
Example 2
25cm3 of 2.0M HCl was added to 25cm3 of 2.0M NaOH in an insulated beaker. The initial temperature of both solutions was 20°C. The highest temperature reached by the solution was 33°C. Calculate the Molar Enthalpy of Neutralisation. [The specific heat capacity (c) of water is 4.18 kJ K -1 kg -1]
NaOH HCl ——> NaCl H2O

33. MEASURING ENTHALPY CHANGES
Example 2
25cm3 of 2.0M HCl was added to 25cm3 of 2.0M NaOH in an insulated beaker. The initial temperature of both solutions was 20°C. The highest temperature reached by the solution was 33°C. Calculate the Molar Enthalpy of Neutralisation. [The specific heat capacity (c) of water is 4.18 kJ K -1 kg -1]
NaOH HCl ——> NaCl H2O
Temperature rise (DT) = K – 293K = 13K
Volume of resulting solution = = 50cm3 = dm3
Equivalent mass of water = 50g = kg
(density is 1g per cm3)
Heat absorbed by the water (q) = m x c x DT = x x 13 = kJ

34. MEASURING ENTHALPY CHANGES
Example 2
25cm3 of 2.0M HCl was added to 25cm3 of 2.0M NaOH in an insulated beaker. The initial temperature of both solutions was 20°C. The highest temperature reached by the solution was 33°C. Calculate the Molar Enthalpy of Neutralisation. [The specific heat capacity (c) of water is 4.18 kJ K -1 kg -1]
NaOH HCl ——> NaCl H2O
Temperature rise (DT) = K – 293K = 13K
Volume of resulting solution = = 50cm3 = dm3
Equivalent mass of water = 50g = kg
(density is 1g per cm3)
Heat absorbed by the water (q) = m x c x DT = x x 13 = kJ
Moles of HCl reacting = x 25/1000 = mol
Moles of NaOH reacting = x 25/1000 = mol
Moles of water produced = mol
Enthalpy change per mol (DH) = heat energy / moles of water = / 0.05
= kJ mol -1

35. What should you be able to do?
REVISION CHECK
What should you be able to do?
Explain the difference between an endothermic and exothermic reaction
Understand the reasons for using standard enthalpy changes
Recall the definitions of enthalpy of formation and combustion
Write equations representing enthalpy of combustion and formation
Recall and apply Hess’s law
Recall the definition of bond dissociation enthalpy
Calculate standard enthalpy changes using bond enthalpy values
Calculate standard enthalpy changes using enthalpies of formation and combustion
Know simple calorimetry methods for measuring enthalpy changes
Calculate enthalpy changes from calorimetry measurements
CAN YOU DO ALL OF THESE? YES NO

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38. © 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
ENTHALPY CHANGES
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© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING