Energetics and Hess’s Law

The enthalpy change for the reaction

C(s, graphite) + 1⁄2O₂(g) --> CO(g) cannot be measured directly since some carbon dioxide is always formed in the reaction.

It can be calculated using Hess’s Law and the enthalpy changes of combustion of graphite and of carbon monoxide.

C(s, graphite) + O₂(g) --> CO₂ ΔH=-394 kJmol^–1

CO(g) + 1⁄2O₂(g) --> CO₂ ΔH=-283 kJmol^–1

The enthalpy change for the reaction of graphite with oxygen to give carbon monoxide is

• -677 kJmol^–1

• -111 kJmol^–1

• +111 kJmol^–1

• +677 kJmol^–1

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The enthalpy change for the reaction, ∆H_r , is equal to

• ∆H₁ + ∆H₂

• -∆H₁ - ∆H₂

• ∆H₁ - ∆H₂

• -∆H₁ + ∆H₂

Tags

NGSS.HS-PS1-4

The enthalpies of combustion of C(s), H₂(g) and C4H9OH(l) (in kJmol^-1) are as follows

C(s) + O₂(g) -> CO₂(g) ∆H=a

H₂(g) + ½O₂(g) -> H₂O(l) ∆H=b

C₄H₉OH(l) + 6O₂(g) -> 4CO₂(g) + 5H₂O(l) ∆H=c

What is the enthalpy change for the reaction shown below?

4C(g) + 5H₂(l) + ½O₂(g) -> C₄H₉OH(l)

• c – 4a – 5b

• 4a + 5b - c

• 2a + 10b - c

• 2a + 5b + c

The standard enthalpy changes of combustion of carbon, hydrogen and methane are shown in the table.

Which one of the following expressions gives the correct value for the standard enthalpy change of formation of methane in kJ mol^–1?

C(s) + 2H₂(g) → CH₄(g)

• 394 + (2 × 286) – 891

• –394 – (2 × 286) + 891

• 394 + 286 – 891

• –394 – 286 + 891

Tags

NGSS.HS-PS1-4

The table shows the standard enthalpy of formation, Δ_fH^θ, for some of the substances in the reaction

C₂H₆(g) + 6F₂(g) ⟶ C₂F₆(g) + 6HF(g) ΔH^θ = −2898 kJ mol⁻¹

What is the standard enthalpy of formation, in kJ mol⁻¹, for HF(g)?

• −1638

• −273

• +273

• +1638

Tags

NGSS.HS-PS1-4

Using the information given, answer this question.

Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(g)

ΔH^⦵= +96 kJ mol⁻¹, ΔS^⦵ = +138 J K⁻¹ mol⁻¹

The standard enthalpy of formation of steam is

• +286 kJ mol⁻¹

• +242 kJ mol⁻¹

• −242 KJ mol⁻¹

• −286 kJ mol⁻¹

Tags

NGSS.HS-PS1-4

Use the information in the image to answer this question.

The standard enthalpy of combustion of butane, in kJ mol⁻¹, is

• −2880

• −2590

• −806

• −554

Tags

NGSS.HS-PS1-4

Use the information in the image to answer this question.

The value in kJ mol⁻¹ of the enthalpy of thermal dissociation when butane forms propane, hydrogen and carbon is

• −26.3

• −17.5

• +17.5

• +21.2

When ethanamide (CH₃CONH₂) burns in oxygen the carbon is converted into carbon dioxide, the hydrogen is converted into water and the nitrogen forms nitrogen gas.

Using the data above, which one of the following is a correct value for the enthalpy of combustion of ethanamide?

• −1823 kJ mol⁻¹

• −1183 kJ mol⁻¹

• −1000 kJ mol⁻¹

• −360 kJ mo1⁻¹

Tags

NGSS.HS-PS1-4

Given the data in the image

which one of the following is the enthalpy change, in kJ mol⁻¹, of the reaction below?

CH₄(g) → C(s) + 4H(g)

• −947

• +511

• +797

• +947

Tags

NGSS.HS-PS1-4