Chemistry 30 Thermodynamics: Heat of Formation Worksheet

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.

CO_{2 (g)}, CuCl_{2 (g)}, CuCl_(g), N₂H_{4 (l)}, NH₄Cl_(s).

Answers - either format would be acceptable as an answer

		C _(s) + O_{2 (g)} → CO_{2 (g)} + 393.5 kJ
	or	C _(s) + O_{2 (g)} → CO_{2 (g)}	Δ H = -393.5 kJ

		Cu _(s) + Cl_{2 (g)} → CuCl_{2 (s)} + 220.1 kJ
	or	Cu_(s) + Cl_{2 (g)} → CuCl_{2 (s)}	Δ H = -220.1 kJ

		Cu _(s) + ½Cl_{2 (g)} → CuCl_{2 (s)} + 137.2kJ
	or	Cu _(s) + ½Cl_{2 (g)} → CuCl_{2 (s)}	Δ H = -137.2 kJ

		N_{2 (g)} + 2H_{2 (g)} + 50.6 kJ → N₂H_{4 (l)}
	or	N_{2 (g)} + 2H_{2 (g)} + 50.6 kJ → N₂H_{4 (l)}	Δ H = +50.6 kJ

		½N_{2 (g)} + 2H_{2 (g)} + ½Cl_{2 (g)} → NH₄Cl _(s) + 314.4 kJ
	or	½N_{2 (g)} + 2H_{2 (g)} + ½Cl_{2 (g)} → NH₄Cl _(s)	Δ H = -314.4 kJ

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The standard heat of formation, ΔH^o_f, for sulfur dioxide (SO₂) is -297 kJ/mol. How many kJ of energy are given off when 25.0 g of SO_{2 (g)} is produced from its elements?

Answer:

How you develop your answer will depend on how you set up your calculations in Chemistry 20. If you want to use a different method of solving the question, feel free - there are many acceptable ways to do these calculations, and different people will prefer different methods.

Solution:

Step 1: Calculate moles SO₂

molar mass of SO₂ = 32.1 + 2(16.0) = 64.1 g/mol

moles SO₂ =

25.0 g

64.1 g/mol

0.390 mol

If you have trouble converting mass into moles, better dig out your old Chemistry 20 notes or textbook and review. Chemistry 30 will require you to remember this basic chemistry skill!

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 — the definition of heat of formation. Determine how much energy will be released for 0.390 mol of SO₂:

kJ released = (0.39 mol)(-297 kJ/mol) = - 116 kJ answer

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

C₂H₅OH _(l) + 3 O_{2 (g)} → 2 CO_{2 (g)} + 3 H₂O _(l) + 9.5 × 10² kJ

How much heat is produced when 11.5 g of alcohol is burned?

Answer:

As with Question 2, we first need to 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 =

11.5 g

46.0 g/mol

= 0.250 mol

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:

kJ = (0.250 mol) × (9.50 × 10² kJ/mol) = 2.38 × 10² kJ answer

ΔH for the complete combustion of 1 mol of propane is -2.22 × 10³ kJ:

C₃H_{8 (g)} + 5 O_{2 (g)} → 3 CO_{2 (g)} + 4 H₂O _(l)

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

Answer:

This question is very similar to the previous question. Begin by determining moles of propane actually used. The molar mass of C₃H₈ is :

moles C₃H₈ =

33.0 g

44.0 g/mol

= 0.750 mol

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

kJ = (0.750 mol) × (2.22 × 10³ kJ/mol) = 1.67× 10³ kJ

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Chemistry 30

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