Calculating $\Delta H$ from Data

The two most reliable formulas in Chapter 1. Memorize the arrow directions.

Cycle Logic

Elements are at the BOTTOM.
Arrows point UP to Compounds.

\Delta H = \sum \Delta H_f (Prod) - \sum \Delta H_f (React)

"Products minus Reactants"

Cycle Logic

Combustion Products (

CO_2, H_2O

) are at the BOTTOM.
Arrows point DOWN from Compounds.

\Delta H = \sum \Delta H_c (React) - \sum \Delta H_c (Prod)

"Reactants minus Products"

Deep Think Concept

Data Type	Formula	Why?
$\Delta H_f$	Prod - React	We go against reactant formation arrows and with product formation arrows.
$\Delta H_c$	React - Prod	We go with reactant combustion arrows and against product combustion arrows.
Bond Enthalpy	Broken - Formed	Functionally same as "Reactants - Products" (Since bonds in reactants are broken).

Putting it into Practice

Paper 2 Style

Calculated the Standard Enthalpy of Formation of Propanone ( $C_3H_6O$ ) using the following Combustion Data:

$\Delta H_c (C) = -394 \: kJ/mol$

$\Delta H_c (H_2) = -286 \: kJ/mol$

$\Delta H_c (C_3H_6O) = -1786 \: kJ/mol$

[2 Marks]

Calculate the enthalpy change for the reaction: $2NO_2(g) \rightarrow N_2O_4(g)$

Given: $\Delta H_f (NO_2) = +33.2 \: kJ$ , $\Delta H_f (N_2O_4) = +9.2 \: kJ$