Thermodynamics

1.2.2 Heat of Formation

Standard Heat of Formation ΔHof

the amount of energygained or lost when

1 mole of the substance isformed from its elementsunder standard conditions(25°C, 1 atm = 101.3 kPa)

Ex. the formation reaction for liquid water is described by the following equation:

H2 (g) + ½O2 (g) → H2O(l) + 285.8 kJ The standard heat of formation

is: 285.8 kJ. Since the reaction is exothermic: ΔHo

f -285.8 kJ.

A heat of formation is a type of reaction where one mole of the compound forms from its elements

The heat of formation for pure elements, such as H2(g), O2(g), Al(s), etc. is 0 kJ/mole. You'll find it useful to remember this.

Look over the Heat of formation table

See values in outline

Keep the following points in mind: 1. Balance the equation so that one mole of

the compound is produced.2. Remember the diatomic (7) molecules and

write them correctly (H2, N2, O2, F2, Cl2, Br2, I2).

3. The reactants must be elements, not polyatomic ions.

Examples of polyatomic ions are hydroxide, OH-, carbonate, CO3

2-, and ammonium, NH4+.

H2(g) + ½O2(g) → H2O(l) + 285 kJ. If 285.8 kJ of energy are released during

the formation of one mole of H2O(l), how much energy do you imagine would be released if two moles of water were produced?

If you predicted 571.6 kJ of energy you're right! Our formation reaction tells us that 285.8 kJ of energy are released for every one mole of H2O. This can be written mathematically as:

285.8 kJ/mol H2O×2 mol H2O =571.6 kJ

Our new equation looks like this: 2 H2(g) + O2(g) → 2 H2O(l) + 571.6 kJ

1. Write heats of formation reactions for each of the following compounds. Be sure to include the energy term with the equation, either as part of the equation or separately as Δ H. You will need to refer to a Table of Thermochemical Data.

CO2 (g),

CuCl2 (g), CuCl (g), N2H4 (l),

NH4Cl (s).

C (s) + O2 (g) → CO2 (g) + 393.5 kJ orC (s) + O2 (g) → CO2 (g) Δ H = -393.5 kJ

Cu (s) + Cl2 (g) → CuCl2 (s) + 220.1 kJ or Cu (s) + Cl2 (g) → CuCl2 (s) Δ H = -220.1 kJ

Cu (s) + ½ Cl2 (g) → CuCl2 (s) + 137.2kJ or Cu (s) + ½ Cl2 (g) → CuCl2 (s) Δ H = -137.2 kJ

N2 (g) + 2H2 (g) + 50.6 kJ → N2H4 (l) orN2 (g) + 2H2 (g) → N2H4 (l) Δ H = +50.6

kJ ½N2 (g) + 2H2 (g) + ½Cl2 (g) → NH4Cl (s)+

314.4 kJ or ½N2 (g) + 2H2 (g) + ½Cl2 (g) → NH4Cl (s) Δ H = -314.4 kJ

The standard heat of formation, ΔHof,

for sulfur dioxide (SO2) is -297 kJ/mol. How many kJ of energy are given off when 25.0 g of SO2 (g) is produced from its elements?

Step 1: Calculate moles SO2a. Find the molar mass of SO2 = 32.1 +

2(16.0) = 64.1 g/molb. moles SO2 =

Step 2: Determine kJ for 0.390 mol We know from the question that 297 kJ of

energy is released for 1 mole of SO2 Determine how much energy will be released for 0.390 mol of SO2:

mol390.0g1.64

mol1

1

g0.25

kJ116mol

kJ297

1

mol39.0kJ

The heat of reaction for the combustion of 1 mol of ethyl alcohol is -9.50 × 102 kJ:

C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2O (l) + 9.5 × 102 kJHow much heat is produced when 11.5 g of alcohol is burned?

First determine how many moles of ethyl alcohol are being combusted. You need to begin by finding the molar mass of C2H5OH, which is 46.0 g/mol.

moles C2H5OH =

From our balanced equation we see that 9.50 × 102 kJ of energy are released for every 1 mole of C2H5OH . Now we determine how much energy will be released for 0.250 mol:

mol250.0g0.46

mol1

1

g5.11

kJ1038.2mol

kJ1050.9

1

mol250.0kJ 2

2

ΔH for the complete combustion of 1 mol of propane is -2.22 × 103 kJ:C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (l)

Calculate the heat of reaction for the combustion of 33.0 g of propane.

4. determine moles of propane actually used. The molar mass of C3H8 is:

moles C3H8 =

From our balanced equation we see that 2.22 × 103 kJ of energy are released for every 1 mole of C3H8. Now determine how much energy will be released for 0.750 mol:

33.00.750

144.0 /

gmol

g mol

kJ1067.1mol

kJ1022.2

1

mol750.0kJ 3

3

Assignment- practice problems