Formation Reactions

Question

For which of the following reactions is ΔH°rxn equal to ΔH°f of the product(s)?You do not need to look up any values to answer this question.
Check all that apply.

A) Na(s)+12Cl2(g)→NaCl(s)

B) Na(s)+12Cl2(l)→NaCl(s)

C) CO(g)+12O2(g)→CO2(g) 

D) C(s,graphite)+O2(g)→CO2(g)

E) CaCO3(g)→CaO+CO2(g)

F) 2Na(s)+Cl2(g)→2NaCl(s)

Answer 1

For which of the following reactions is ΔH°_rxn equal to ΔH°_f of the product(s)?You do not need to look up any values to answer this question.

Check all that apply.

Understanding Standard Enthalpies of Formation (ΔH°_f) comes down to a few strict rules:

1. The product must be exactly 1 mole of that compound and is being formed from the elements that make it up. For this reason, some students get stuck on why there would be a fraction as the coefficient. Just keep in mind that the ratios of the moles are still the same, so this is not breaking any chemistry rules.
2. The phases of the elements must be in the standard state. For example, metals are usually solids, while nonmetals can be solids, liquids, or gases.
3. Diatomic molecules must be written as such. There are 7 diatomic molecules from elements on the Periodic Table: N₂, O₂, F₂, Cl₂, I₂, Br₂, H₂. You will not see any of these elements as single, unbonded atoms in nature.

Let's get started!

A) Na(s) + 1/2 Cl₂(g) → NaCl(s)

Rule #1: 1 mole of NaCl is formed

Rule #2: Na is a solid, Cl₂ is a gas, and NaCl is a solid, which are their correct standard states

Rule #3: Na is not diatomic, Cl₂ is diatomic, and NaCl is not diatomic.

This obeys all of the rules, so ΔH°_rxn = ΔH°_f

B) Na(s) + 1/2 Cl₂(l) → NaCl(s)

Rule #1: 1 mole of NaCl is formed

Rule #2: Na is a solid, Cl₂ is a liquid - which is NOT its correct standard state

This does NOT obey all of the rules, so ΔH°_rxn ≠ ΔH°_f

C) CO(g) + 1/2 O₂(g) → CO₂(g) 

Rule #1: 1 mole of CO₂ is formed

Rule #2: CO is a compound and NOT an element. Be careful...this is easy to get tripped up on!

This does NOT obey all of the rules, so ΔH°_rxn ≠ ΔH°_f

D) C(s,graphite) + O₂(g) → CO₂(g)

Rule #1: 1 mole of CO₂ is formed

Rule #2: C is a solid, O₂ is a gas, and CO₂ is a gas, which are their correct standard states

Rule #3: O₂ is diatomic

This obeys all of the rules, so ΔH°_rxn = ΔH°_f

E) CaCO₃(g) → CaO + CO₂(g)

Rule #1: 1 mole of product is not formed. Also CaCO₃ is a solid.

This does NOT obey all of the rules, so ΔH°_rxn ≠ ΔH°_f

F) 2 Na(s) + Cl₂(g) → 2 NaCl(s)

Rule #1: 1 mole of product is not formed

This does NOT obey all of the rules, so ΔH°_rxn ≠ ΔH°_f

A and D are the only cases where ΔH°_rxn = ΔH°_f

I hope this helped clarify Heats of Formations for you!