1. 5.1 Enthalpy of Formation
IB Chemistry

2. Lesson Objectives Define standard enthalpy change of formation
Write formation equations.
Calculate standard enthalpy change of formation

3. Definitions Standard Enthalpy change of formation:
The amount of energy change to form 1 mole of a substance from elements in their standard states.
Formation equations must represent the formation of 1 mole of product and therefore fractions may be required to write the formation equations

4. Writing Formation Equations
Must result in 1 mole of product being formed
All reactants must be in standard state 25°C/298K and 100kPa
Ex: Write the equation for the formation of carbon monoxide
Write reactants in standard states and product in standard state
C(s) O2(g)  CO(g)
Balance equation ensuring 1 mole of product
𝟏 𝟐

5. Writing Formation Equations
Write the formation equations for:
Silver (I) bromide
Ammonia (nitrogen trihydride)
CH3OH
Propane (C3H8)
Glucose

6. Writing Formation Equations
Silver (I) bromide
Ag(s) Br2(g)  AgBr(s)
Ammonia (nitrogen trihydride)
N2(g) H2(g)  NH3(g)
CH3OH
C(s) H2(g) O2(g)  CH3OH(l)
𝟏 𝟐
𝟏 𝟐
𝟑 𝟐 2
𝟏 𝟐

7. Writing Formation Equations
Propane (C3H8)
C(s) H2(g)  C3H8
Glucose
C(s) + H2(g) O2(g)  C6H12O6 (g) 3 4 6 6 3

8. Calculating Enthalpy Change of Reactions:
∆Hө reaction = ∑∆ Hfө products - ∑∆ Hfө reactants
The standard enthalpy of formation of all elements in their standard states is zero!
When a reaction is reversed the magnitude of ∆H stays the same but the sign changes

9. Calculating Enthalpy Change of Reaction:
Calculate ∆H° for the reaction;
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Compound
∆HfӨ (kJ/mol)
H2O(l)
-286
NaOH(aq)
-470
∆Hө reaction = ∑∆ Hfө products - ∑∆ Hfө reactants
= [2(-470)+0] – [0+2(-286)]
∆H° = –368 kJ

10. Calculating Enthalpy Change of Reaction:
Calculate ∆H° for the following reactions;
Photosynthesis
Cellular respiration
The complete combustion of propane
CaCO3(s)  CaO(s) + CO2(g)