Hess’ Law and Enthalpy of Formation

State Function revisited….

Dependent ONLY on a system’s state at a given moment in time. Only initial and final states Not based on the path to get to a given

condition

Ex. Energy, Enthalpy

Standard Conditions

Represented by a “ ° “ beside ΔH or ΔE, etc.

25°C

1 atm

Methods for determining ΔH

1) Calorimetry

2) Application of Hess’ Law

Hess’ Law

Enthalpy change for a chemical reaction is the same whether it occurs in multiple steps or one step ΔHrxn = ΣΔHA+B+C (sum of ΔH for each step)

Allows us to break a chemical reaction down into multiple steps to calculate ΔH Add the enthalpies of the steps for the enthalpy for

the overall chemical reaction

Guidelines for using Hess’ Law

Must use data and combine each step in a way that gives the chemical reaction with the unknown ΔH

Set up steps so chemical compounds not in the final reaction are cancelled

Reverse a reaction if necessary and change the sign on ΔH

Check for correct mole ratios

Example 1:

H2O(l) H2O (g) ΔH° = ?

Based on the following: H2 + ½ O2 H2O(l) ΔH° = -285.83 kJ/mol

H2 + ½ O2 H2O(g) ΔH° = -241.82 kJ/mol

Example 2:

C(s) + 4H2 C3H8 (g) ΔH° = ?

Based on the following: 2H2 + O2 2H2O ΔH° = -571.7 kJ/mol

C3H8 + 5 O2 3CO2 + 4H2O(g) ΔH°= -2220.1kJ/molC(s) + O2 CO2 (g) ΔH° = -393.5 kJ/mol

Methods for determining ΔH

1) Calorimetry

2) Application of Hess’ Law

3) Enthalpies of Formation

Enthalpy of Formation (ΔHf°)

Enthalpy for the reaction forming 1 mole of a chemical compound from its elements in a thermodynamically stable state.

Elements present in “most thermodynamically stable state” 25°C°, 1atm

Example 3

MgO (s) + CO2 (g) MgCO3 (s)

MgO + CO2 Mg (s) + C (s) + 3/2 O2

Mg (s) + C (s) + 3/2 O2 MgCO3

Apply Hess’ Law----

Really ΔHf (products) - ΔHf (reactants)

Calculate ΔHrxn based on enthalpy of formation (ΔHf)

aA + bB cC + dDΔH° =[c (ΔHf°)C + d(ΔHf°)D] - [a (ΔHf°)A + b (ΔHf°)B ]

Example 4

Calculate the ΔH° for the combustion of 1 mole of pentaborane (B5H9) reacting with excess oxygen

2B5H9 + 12O2 5B2O3 + 9H2O ΔH° = ?

Example 5

Isopropyl alcohol (rubbing alcohol) undergoes a combustion reaction

2(CH3)2CHOH + 9O2 6CO2 + 8H2O

ΔH° = -4011 kJ/mol

Calculate the standard enthalpy of formation for isopropyl alcohol.

Homework

pp. 253 #65-67, 72