Hess's Law

Which statements about Hess's law are correct?

I. Itis an expression of the Law of Conservation of Energy

II. It can be used to indirectly determine the enthalpy of formation of a substance.

III. The overall enthalpy change for a reaction is independent of the reaction pathway.

• I and II only

• I, II, and III

• I and III only

• II and III only

Synthesis gas is a mixture of hydrogen and carbon monoxide. It is produced by passing steam over carbon.

H₂O_(g) + C_(s) → H_2(g) + CO_(g)

The standard enthalpies of formation of water and carbon monoxide are - 286 and - 110 kJ mol^-1 respectively.

What additional information is required to calculate the standard enthalpy change for the production of synthesis gas?

• The stoichiometry of the reaction

• The pressure under which the process is carried out.

• The standard enthalpy of formation of hydrogen.

• The enthalpy of vaporization of water.

Tags

NGSS.HS-PS3-1

What is the standard enthalpy change for the reduction of magnesium oxide by hydrogen to produce water and magnesium metal has given that the standard enthalpies of formation of magnesium oxide and water are - 602 and - 286 kJ mol^-1 respectively.

• - 316 kJ mol^-1

• + 316 kJ mol^-1

• - 888 kJ mol^-1

• + 888 kJ mol^-1

Tags

NGSS.HS-PS1-4

The ore magnetite, Fe₃O₄, can be reduced by carbon monoxide

Fe₃O_4(s) + 4CO_(g) → 3Fe_(s) + 4CO_2(g)

The standard enthalpies of formation of magnetite, carbon monoxide, and carbon dioxide are −1118, −110, and −394 kJ mol−1 respectively.

What is the standard enthalpy change for the reduction of magnetite by carbon monoxide?

• + 3134 kJ

• - 18kJ

• +18 kJ

• _3134 kJ

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

Diborane, B₂H₆ has been suggested as possible rocket fuel. The equation for its combustion is

B₂H_6(g) + 3O_2(g) → B₂O_3(s) + 3H₂O_(l) ΔH_c^⦵ = − 2168 kJ mol^-1

The enthalpies of the formation of boron oxide and water are −1274 and −286 kJ mol−1 respectively.

What is the standard enthalpy of the formation of diborane?

• + 36 kJ mol^-1

• -608 kJ mol^-1

• + 608 kJ mol^-1

• - 36 kJ mol^-1

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The copper thermite reaction is used for creating electrical joints:

3Cu₂O_(s) + 2Al_(s) → 6Cu_(s) + Al₂O_3(s)

The enthalpies of the formation of Cu₂O and Al₂O₃ are −170 and −1676 kJ mol-1 respectively.

What is the standard enthalpy change of the copper thermite reaction in kJ?

• - 2186

• - 1506

• -1166

• -1846

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

Ethanol can be oxidized to ethanal,

C₂H₅OH_(l) + ½O_2(g) → CH₃CHO_(l) + H₂O_(l) ΔH^⦵ = a

which can then be further oxidized to ethanoic acid.

CH₃CHO_(l) + ½O_2(g) → CH₃COOH_(l) ΔH^⦵ = b

Ethanol can also be oxidized directly to ethanoic acid

C₂H₅OH_(l) + O_2(g) → CH₃COOH_(l) + H₂O_(l) ΔH^⦵ = c

Which is the correct relationship between a, b and c?

• a = b - c

• c = a - b

• b = a - c

• b = c- a

Tags

NGSS.HS-PS1-4

Which species has a standard enthalpy of formation value equal to zero?

• l(l)

• I_2(g)

• I_2(s)

• I_2(l)

2-methylpropene can be obtained by dehydrating 2-methylpropan-1-ol.

(CH₃)₂CHCH₂OH_(l) → (CH₃)₂C=CH_2(g) + H₂O_(l)

The standard enthalpies of formation of 2-methylpropan-1-ol, 2-methylpropene, and water are −335, −17, and −286 kJ mol−1 respectively.

What is the standard enthalpy change for the dehydration of 2-methylpropene?

• + 638 kJ mol^-1

• -32 kJ mol^-1

• - 638 kJ mol^-1

• + 32 kJ mol^-1

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

The energy cycle shows how Hess's Law can be used to find the standard enthalpy of formation of methane if the enthalpies of combustion of carbon, hydrogen and methane are known.

Which expression will give the correct value for the enthalpy of formation of methane?

• ΔH_c^⦵(methane) + ΔH_c^⦵(carbon) + ΔH_c^⦵(hydrogen)

• ΔH_c^⦵ (carbon) + 2ΔH_c^⦵ (hydrogen) − ΔH_c^⦵(methane)

• ΔH_c^⦵(methane) − ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen)

• ΔH_c^⦵(carbon) + 2ΔH_c^⦵(hydrogen) + ΔH_c^⦵(methane)

Tags

NGSS.HS-PS1-4