Standard Enthalpy of Formation, ΔH°f (Text Section 5.5, Table C6 in Appendix C)
The standard enthalpy of formation is defined as ______
The standard enthalpy of formation depends on:
–The phase of the substance
–Temperature and pressure
–The number of mols that are reacting
Example 1: the standard enthalpy of formation of CO2 is -393.5 kJ/mole. The reaction would be as follows:
C (s) + O2 (g) CO2 (g) + 393.5 kJ
Example 2: the standard enthalpy of formation of NO is 90.37 kJ/mole. The reaction would be as follows:
½ N2 (g) + ½ O2 (g) + 90.37 kJ NO (g)
But if two mols of NO are formed the enthalpy change is +180.8 kJ, since the equation is:
N2 (g) + O2 (g) + 180.8 kJ 2NO (g)
How is this calculated?
We use given values on a table, to calculate the amount of heat or the molar enthalpies, using calorimetry
–Table – Handouts and Appendix – Table 6
Reminders
–Most diatomic elements have ΔH°f =0 because they are already in their standard states
–- ΔH°f means that the reaction is exothermic
–+ ΔH°f means that the reaction is endothermic
Molar Enthalpies
–Ex. The standard enthalpy of formation for sulfur dioxide is – 297 kJ / mol. How many kilojoules of energy are given off when 25.0 g of sulfur dixoide is produced from its elements?
–Think Stoich!!!
Ex. 2 – page 317
We need to take into account the number of moles of a substance and the ΔH°f is per mol.
Hess’s Law of Heat Summation (Text Section 5.4)
–When determining the enthalpy change of a reaction, it is often not necessary to evaluate every reaction experimentally.
–Since enthalpy is a state property, the change only depends on a comparison of the initial and final conditions.
–This allows us to set up a hypothetical pathway for the reaction.
–As long as the initial and final states are the same, the enthalpy change will also be the same.
Hess’s Law –
–If a reaction can be expressed as the algebraic sum of two or more smaller reactions, then the enthalpy change of the large reaction will be the algebraic sum of the enthalpy changes of the smaller reactions.
Mathematically
ΔH overall reaction ( net ) = ΔH step 1 + ΔH step 2 + ΔH step 3 etc.
Many chemical reactions occur in a series of steps rather than a single step.
For example, the following reaction describes the burning (combustion) of carbon:
C (s) + O2(g) → CO2(g) ΔH = -393.5 kJ
If not enough oxygen is present, CO rather than CO2 is produced:
C (s) + ½ O2(g) → CO (g) ΔH = -110.5 kJ
If more oxygen is now added, CO will undergo further combustion with oxygen:
CO (g) + ½ O2(g) → CO2 (g) ΔH = -283.0 kJ
Watch what happens if we add together the second and third reaction:
(2) C (s) + ½ O2(g) → CO (g)
(3) CO (g) + ½ O2(g) → CO2 (g)
(1) C (s) + O2(g) → CO2(g)
Now compare the total energy released in the second and third reactions with the amount of energy released in the original reaction:
ΔHReaction 2 / + / ΔHReaction 3 / = / ΔHReaction 1-110.5 kJ / + / (-283.0 kJ) / = / -393.5 kJ
-393.5 kJ / = / -393.5 kJ
How did we get these answers?
Using the standard enthalpies of formation and the chemical equation
One of the underlying principles for Hess’s Law is the First Law of Thermodynamics, often called the Law of Conservation of Energy. (already in notes!!!)
If energy could appear or disappear, there would be no reason to expect that the enthalpy change for an overall reaction could be obtained simply by adding the enthalpy changes of a series of thermochemical equations that combine to give the overall equation.
When applying Hess’s Law we must take into account the number of moles involved in the reaction and if we reverse the reaction, the sign of the enthalpy change will be changed as well.
A shorthand algorithm for applying Hess’s Law is as follows
ΔHreaction = Σ ΔH°f products - Σ ΔH°f reactants
–Σ means “the sum of ”
Ex.
What is the enthalpy of formation for the production of ammonia? How could we check this?
(Step 1) N2 (g) + 2 H2 (g) N2H4 (g) ΔH = 95.4 kJ
(Step 2) N2H4 (g) + H2 (g) 2 NH3 (g) ΔH = -187.6 kJ
Ex. 2
The equation for the decomposition of mercury (II) oxide is as follows:
2HgO(s) + 182 kJ 2Hg(l) + O2(g)
What is the ΔH°f HgO(s)?
Ex. 3
Calculate the heat of reaction for:
CH4(g) + 3Cl2(g) CHCl3(l) + 3HCl (g)
Questions
- When iron rusts in air, the following reaction occurs:
4Fe(s) + 3O2(g)2Fe2O3(s)ΔH = -1643 kJ
What is the heat of formation of Fe2O3(s)?
- The standard heats of formation of HCl(g) and HBr(g) are -92 kJ/mol and -36.4 kJ/mol. Using this information, calculate the ΔH for the following reaction:
Cl2(g) + 2HBr(g) 2HCl(g) + Br2(g)
- Do questions 5 (p. 329), 7 (p. 338)