Practice Problems Answers

(1) Identify the standard state (solid, liquid or gas) for the following elements:

(a) liquid
(b) solid
(c) gas
(d) liquid
(e) solid

(2) White

(3) This question uses the NIST Chemistry WebBook. What is the value for the standard enthalpy of formation, $\text{ΔH}{}_{\mathrm{f}}^{\mathrm{o}}$, for the following substances:

(a) $\text{ΔH}{}_{\mathrm{f}}^{\mathrm{o}}$ = −277.0 kJ/mol
(b) $\text{ΔH}{}_{\mathrm{f}}^{\mathrm{o}}$ = −483.52 kJ/mol
(c) $\text{ΔH}{}_{\mathrm{f}}^{\mathrm{o}}$ = −411.12 kJ/mol

The trick: in the WebBook, the first value given is for NaCl as a liquid. However, the standard state for NaCl is solid, so the standard enthalpy is actually NOT the first one listed.

(4) Write the full chemical equation of formation for the substances in question 3.

(a) 2C (s, graphite) + 3H₂(g) + ¹⁄₂O₂(g) ---> C₂H₅OH(ℓ)
(b) 2C (s, graphite) + 2H₂(g) + O₂(g) ---> CH₃COOH(ℓ)
(c) Na(s) + ¹⁄₂Cl₂(g) ---> NaCl(s)

Reminder that in the answers for number 4, the product is always written with a coefficient of one.

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