Energetics Definitions and Equations Practice

The standard enthalpy change of formation of a compound is the enthalpy change which occurs when one mole of the compound is formed from its elements under standard conditions, and with everything in its standard state.

Does the following equation represent an enthalpy of formation reaction?

Ca_(s) + F_2(g)    →    CaF_2(s)

• Yes

• No

The standard enthalpy change of formation of a compound is the enthalpy change which occurs when one mole of the compound is formed from its elements under standard conditions, and with everything in its standard state.

Does the following equation represent an enthalpy of formation reaction?

H₂ (g) + ½ O₂ (g) → H₂O (g)

• Yes

• No

Answer explanation

Water is not in its standard state. At 25 degrees Celcius and 1 atmospheric pressure (standard conditions) water is a liquid.

The standard enthalpy change of formation of a compound is the enthalpy change which occurs when one mole of the compound is formed from its elements under standard conditions, and with everything in its standard state.

Does the following equation represent an enthalpy of formation reaction?

C _{(s, graphite) +} ½ O₂ + 2H_2(g) →  CH₃OH_(l)

 

• Yes

• No

Answer explanation

Carbon's standard state is solid and the more stable allotrope is graphite.

The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one mole of the compound is burned completely in oxygen (in excess O₂) under standard conditions.

Does the following equation represent an enthalpy of combustion reaction?

2Mg (s)+O₂(g)⟶ 2MgO(s)

• Yes

• No

Answer explanation

This shows combustion of 2 moles of Magnesium, not 1 mole.

The standard enthalpy change of combustion of a compound is the enthalpy change which occurs when one mole of the compound is burned completely in oxygen (in excess O₂) under standard conditions.

Does the following equation represent an enthalpy of combustion reaction?

CH₃OH_(l)+ 3⁄2O_2(g)⟶CO_2(g)+2H₂O_(l)

• Yes

• No

Standard Enthalpy of Solution, ∆H^θ_solution is the energy change when 1 mole of a solute dissolves in solvent to form an infinitely dilute solution under standard conditions. Which equation represents enthalpy of solution of Aluminium sulphate?

• Al₂(SO₄ )_3(s) + aq → 2Al_(aq)³⁺  +  3SO_4(aq)^2-

• Al₂(SO₄ )_3(s) + aq → Al_(aq)³⁺  +  SO_4(aq)^2-

• Al₂(SO₄ )_3(g) + aq → 2Al_(aq)³⁺  +  3SO_4(aq)^2-

• Al₂(SO₄ )_3(s) + aq → 2Al_(aq)²⁺  +  3SO_4(aq)^3-

Answer explanation

The solute must be solid. Ions need to be balanced and the charges must be correct.

Al³⁺ and SO₄^2-

Standard Enthalpy change of Lattice formation (Lattice Energy) ∆H_L^θ    

is the energy released when 1 mole of an ionic solid is formed from its gaseous ions at standard conditions (298K and 1 atmospheric pressure).

            Identify the equation for lattice of formation for Magnesium chloride.

• Mg_(g)²⁺ + Cl_(g)^- → MgCl_2(s)

• Mg_(g)²⁺ + 2Cl_(g)^- → MgCl_2(s)

• Mg_(g)⁺ + Cl_(g)^- → MgCl_(s)

• Mg_(g)²⁺ + Cl_(g)^- → MgCl_2(g)

Answer explanation

Ions must be gaseous with correct charges and product must be solid. The equation must be balanced. The formula of substance must be correct, MgCl_2.

Standard Enthalpy change of Lattice formation (Lattice Energy) ∆H_L^θ    

is the energy released when 1 mole of an ionic solid is formed from its gaseous ions at standard conditions (298K and 1 atmospheric pressure).

            Identify the equation for lattice of formation for sodium oxide.

• 2Na_(g)⁺ + O_(g)^2- → Na₂O_(s)

• Na_(g)⁺ + O_(g)^2- → NaO_(s)

• 2Na_(g)⁺ + O_(g)^2- → Na₂O_(g)

• 2Na_(s)⁺ + O_(g)^2- → Na₂O_(s)

Answer explanation

Ions must be gaseous with correct charges and product must be solid. The equation must be balanced. The formula of substance must be correct, Na₂O_.

The standard atomisation enthalpy is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state at standard conditions.

Does the following equation represent an enthalpy of atomisation reaction?

Ca(s) → Ca(g)

• Yes

• No

The standard atomisation enthalpy is the enthalpy change when 1 mole of gaseous atoms is formed from the element in its standard state at standard conditions.

Does the following equation represent an enthalpy of atomisation reaction?

O₂ (g) → 2O (g)

• Yes

• No

Answer explanation

1 mol of atoms must be formed.

1/2O₂ (g) → O (g)