5.1 Enthalpy of Formation IB Chemistry

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

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

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

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

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

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

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

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

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)