Quiz of the Week

Quiz of the Week - Chemistry 221 Hess's Law

Master the concepts from this week's material by trying these quiz questions which will NOT affect your grade in any way. All scoring is anonymous and known only to you, the user! If you have any questions, please contact me. Good luck!

A Quick Review of Hess's Law

Hess's Law:

If a reaction is the sum of two or more other reactions, then ΔH for the overall process must be the sum of the ΔH values of the constituent reactions.

Some practical rules to follow:

When a reaction is reversed, the magnitude of ΔH remains the same, but its sign changes.
When the balanced equation for a reaction is multiplied by an integer, the value of ΔH for that reaction must be multiplied by the same integer.
The change in enthalpy for a given reaction can be calculated from the enthalpies of formation of the reactants and products, namely ΔH^°_reaction = ΣΔH^°_f(products) - ΣΔH^°_f(reactants)
Elements in their standard states are not included in the ΔH_reaction calculations; that is, ΔH^°_f for an element in its standard state is zero.

Chemistry 221 - Hess's Law

For the following problems, you will need these standard enthalpy of formation (ΔH^°_f) values:

CO_2(g) = -393.15 kJ	H₂O_(g) = -241.8 kJ	H₂O_(l) = -285.8 kJ	CaCO_3(s) = -1206.9 kJ	CaO_(s) = -635.1 kJ
NH_3(g) = -46.1 kJ	CH_4(g) = -74.8 kJ	PCl_3(l) = -319.7 kJ	C₆H_6(l) = +49.0 kJ	NO_2(g) = +34.0 kJ
CH₃OH_(l) = -239.0 kJ	C₈H_18(l) = -269.0 kJ	Al₂O_3(s) = -1675.7 kJ	SiO_2(s) = -910.9 kJ	CuSO_4(s) = -771.4 kJ

1. What is ΔH^°_f for the reaction C_(s) + 2 H_2(g) --> CH_4(g)? 2. Is this equation correct? P_4(s) + 6 Cl_2(g) --> 4 PCl_3(l) ΔH^°_f = -319.7 kJ 3. What is ΔH^° for this reaction: C₆H_6(l) + ¹⁵/₂ O_2(g) --> 6 CO_2(g) + 3 H₂O_(l) 4. Find ΔH^°_f for Ca(OH)_2(s) if ΔH^°_rxn = -65.2 kJ for the reaction CaO_(s) + H₂O_(l) --> Ca(OH)_2(s). 5. Calculate ΔH^°_rxn for the reaction 4 NH_3(g) + 7 O_2(g) --> 4 NO_2(g) + 6 H₂O_(l). 6. Calculate ΔH_rxn for the combustion of methanol, CH₃OH_(l). Assume all products are gaseous and assume only one mole of methanol is used. 7. Calculate ΔH_rxn for the combustion of octane, C₈H_18(l). Assume all products are gaseous and assume only one mole of octane is used. 8. Using your answer in question #7, how many kJ of energy will be released per gram of octane burned? 9. Given the following data:

	2 O_3(g) --> 3 O_2(g)	ΔH^° = -427 kJ
	O_2(g) --> 2 O_(g)	ΔH^° = +495 kJ
	NO_(g) + O_3(g) --> NO_2(g) + O_2(g)	ΔH^° = -199 kJ

Calculate ΔH^° for the reaction NO_(g) + O_(g) --> NO_2(g). 10. Given the following data:

S_(s) + ³/₂O_2(g) --> SO_3(g) ΔH^° = -395.2 kJ

2 SO_2(g) + O_2(g) --> 2 SO_3(g) ΔH^° = -198.2 kJ

Calculate ΔH^° for the reaction S_(s) + O_2(g) --> SO_2(g).

	S_(s) + ³/₂O_2(g) --> SO_3(g)	ΔH^° = -395.2 kJ
	2 SO_2(g) + O_2(g) --> 2 SO_3(g)	ΔH^° = -198.2 kJ