Definitions + reaction
pathway diagrams
1. Enthalpy
Definitions
Heat content stored in a chemical system
2. Exothermic
Type of reaction that releases heat energy
3. Endothermic
Type of reaction that absorbs heat energy
4. Activation energy, Ea
Minimum energy colliding particles need to break bonds and start a reaction.
5. Bond energy
ΔH when 1 mole of type of covalent bond in a gaseous molecule is broken
6. Enthalpy change, ΔH
Heat transferred during a chemical reaction
7. Standard conditions, θ
101 kPa and 298 K
8. Combustion
Burning
9. Formation
When a compound is made from its elements
10. Neutralisation
Acid + alkali → salt + water
11. Hess’s Law
ΔH for a reaction is independent of the route
Energy profile diagram
Exothermic
Energy profile diagram
Endothermic
ΔH = positive
Bond energy calculations
for ΔHr
Calculating ΔHr
Enthalpy change
of a reaction
GCSE recap
Bond energy: enthalpy change
when 1 mole of a type of covalent bond
in a gaseous molecule is broken.
Also known as:
✓Bond dissociation energy
✓Bond enthalpy
✓Exact bond energy
System and surroundings
The system refers to the actual chemicals, whose potential
energy content depends on their bonds.
Exothermic reaction
surroundings
chemical system
loses energy
system
H = − ve
Surroundings get warmer
Endothermic reaction
surroundings
chemical system
gains energy
system
H = + ve
Surroundings get cooler
Average bond energies
➢Actual bond enthalpy:
specific to a particular bond in a particular compound.
➢Average bond enthalpy:
Usually averaged over a range of environments and over a range of compounds.
Average bond enthalpy of C−H bond = 413 kJ mol−1
2HBr(g) → H2(g) + Br2(g)
H-Br
→ H-H + Br-Br
H-Br
2x H-Br
2x 366
1x H-H
1x 436
Bond
H-Br
H-H
Br-Br
Bond energy
(kJ/mol)
366
436
193
1x Br-Br
1x 193
Total energy in for breaking bonds =
Total energy out for making bonds =
732 kJ/mol
629 kJ/mol
ΔHr = 732 - 629
ΔH = +103 kJ/mol (endothermic reaction)
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
O=O
O=C=O +
+
→
O=O
4x C-H
4x 435
2x O=O
2x 497
2x C=O
2x 803
4x H-O
4x 464
Total energy in for breaking bonds =
Total energy out for making bonds =
2734 kJ/mol
3462 kJ/mol
ΔHr = 2734 - 3462
ΔH = -728 kJ/mol (exothermic reaction)
Bond
C-H
O-H
O=O
C=O
Bond energy
(kJ/mol)
435
464
497
803
C-O
C-C
360
346
Standard enthalpy changes
θ
θ
θ
θ
H r
H c
H neut H f
Q: Give the formula and state symbol to represent each of the following
substances in their standard states.
Carbon
C(s)
Oxygen
O2(g)
Hydrogen
H2(g)
Water
H2O(l)
Magnesium chloride
MgCl2(s)
Standard state:
Normal physical state
at 298K and 101kPa.
Standard conditions θ
θ
Standard enthalpy change of reaction, H r
Enthalpy change when the reactants in the stoichiometric
equation react to give the products under standard
conditions.
Exothermic or endothermic
Example:
N2(g) + 3H2(g) → 2NH3(g) Hrθ = −92 kJ mol-1
i.e. : 92 kJ energy given out when 1 mol N2 reacts with 3 mol H2 to form 2 mol NH3
Standard enthalpy change of combustion, H c
θ
Enthalpy change when one mole of a substance is burnt in
excess oxygen under standard conditions.
Exothermic
Example:
C2H6(g) + 3½ O2(g) → 2CO2(g) + 3H2O(l)
Hcθ = −1560 kJ mol-1
i.e. : 1560 kJ energy given out when 1 mol C2H6 is burnt in excess O2
Standard enthalpy change of neutralisation,
H neut
θ
Enthalpy change when one mole of water is formed by
reacting an acid and alkali under standard conditions.
Exothermic
Example:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
neutHθ = −57.3 kJ mol−1
i.e. : 57.3 kJ energy given out when 1 mol H2O is formed by reacting HCl + NaOH
θ
Standard enthalpy change of formation, H f
The enthalpy change when one mole of a compound is formed
from its elements under standard conditions.
Exothermic or endothermic
Example:
2 C(s) + 3 H2(g) + ½ O2(g) → C2H5OH(l)
Hfθ = −277 kJ mol−1
i.e.: 277 kJ energy given out when 1 mol C2H5OH is formed from its elements
Practice ANSWERS
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