Chemistry 30 Thermodynamics

What are STP and SATP? Give definitions and appropriate values.

Identify each of the following reactions as endothermic or exothermic.
Also, write each one in an equivalent way.

For example: S_(g) + 2 Cl_{2 (g)} → SCl_{2 (g)} ΔH ° = -510 kJ

Answer: Exothermic (ΔH° is negative). The reaction may also be written as:

S_(g) + 2 Cl_{2 (g)} → SCl_{2 (g)} + 510 kJ

Notice the sign in front of 510 kJ. When the energy term is included in the equation, it always has a + sign in front.

2 Hg_(l) + I_{2 (s)} → Hg₂I_{2 (s)} ΔH ° = -28.9 kcal

N_{2 (g)} + 3 F_{2 (g)} → 2 NF_{3 (g)} + 27.2 kcal

NH₄NO_{3 (s)} + 6.1 kcal → NH₄⁺_(aq) + NO₃^-_(aq)

Na_(s) → Na_(g) ΔH ° = 25.98 kcal

Consider the following two potential energy graphs:

Identify the following graphs as representing endothermic or exothermic reactions:

potential energy curve - endothermic or exothermic?

Which graph above will have a value for ΔH that is negative ?

Using a table of thermochemical data, write heats of formation reactions for the following compounds. Include the energy term as part of the equation.

Example: Write the heat of formation reaction for KOH, including the energy term as part of the equation.

Answer: K + ½ O₂ + ½ H₂→ KOH + 428.8 kJ

IMPORTANT: Remember the seven diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

SO_{2 (g)}

C₃H_{8 (g)}

N₂O_(g)

Na₂CO_{3 (s)}

On the basis on energy changes, select the three reactions from the list below that are most likely to occur spontaneously:

H_{2 (g)} + O_{2 (g)} → H₂O_(g) + 220.1 kJ

3 CO_{2 (g)} + 4 H₂O_(l) + 2220 kJ → C₃H_{8 (g)} + 5 O_{2 (g)}

2 HCl_(g) → H_{2 (g)} + Cl_{2 (g)} ΔH ° = +185 kJ

CH_{4 (s)} + 2 O_{2 (g)} → CO_{2 (g)} + 2 H₂O_(l) ΔH ° = -890 kJ

Chemistry 30

Thermodynamics

Teacher Resources

Other Links