Ch6 Enthalpy changes

Which statements about enthalpy changes are correct?

• The enthalpy change of atomisation is always positive

• The enthalpy change when a C–C bond is broken is positive

• The enthalpy change of neutralisation of a weak acid is always negative

At 550 °C nitrogen dioxide reacts with unburnt hydrocarbon fragments such as CH₃ in the catalytic converter of a motor vehicle.

4CH₃ + 7NO₂ → 3.5N₂ + 4CO₂ + 6H₂O

Which row gives the energy change for this reaction and a possible reason for it?

• A

• B

• C

• D

Two reactions and their enthalpy changes are shown.

2C(s) + 2H₂(g) → C₂H₄(g) ∆H = +52.2 kJ mol^–1

C₂H₂(g) + H₂(g) → C₂H₄(g) ∆H = –175.8 kJ mol^–1

These data can be used to calculate the enthalpy change for the reaction shown.

2C(s) + H₂(g) → C₂H₂(g) ∆H = X

• –228.0 kJ mol^–1

• –123.6 kJ mol^–1

• +123.6 kJ mol^–1

• +228.0 kJ mol^–1

Which statements are correct for all exothermic reactions?

• ∆H for the reaction is negative.

• On a reaction pathway diagram the products are shown lower than the reactants.

• The reaction will occur without heating.

Ethanol is increasingly being used as a fuel for cars.

The standard enthalpy change of formation of carbon dioxide is –393 kJmol^–1.

The standard enthalpy change of formation of water is –286kJ mol^–1.

The standard enthalpy change of formation of ethanol is –277 kJ mol^–1.

What is the standard enthalpy change of combustion of ethanol?

• –1921 kJ mol^–1

• –1367 kJ mol^–1

• –956 kJ mol^–1

• – 402kJ mol^–1

A butane burner is used to heat water. The Mr of butane is 58.

● ∆H_c of butane is –2877kJ mol^–1.

● 250 g of water is heated from 12°C to 100°C.

● The burner transfers 47% of the heat released from the burning fuel to the water.

Assume that the butane undergoes complete combustion and none of the water evaporates.

What is the minimum mass of butane that must be burnt?

• 0.068 g

• 1.85 g

• 3.94 g

• 4.48 g

Which statement about enthalpy changes is correct?

• Enthalpy changes of atomisation are always negative.

• Enthalpy changes of combustion are always positive.

• Enthalpy changes of formation are always positive.

• Enthalpy changes of neutralisation are always negative.

For which reactions does the value of ∆H represent both a standard enthalpy change of combustion and a standard enthalpy change of formation?

• C(s) + O₂(g) → CO₂(g)

• 2C(s) + O₂(g) → 2CO(g)

• CO(g) + 0.5(g)O₂ → CO₂(g)

Calcium reacts with water to form calcium hydroxide and hydrogen.

Ca(s) + 2H₂O(l) → Ca(OH)₂(s) + H₂(g)

The standard enthalpy change for this reaction is – 414kJ mol^–1.

What further information is needed in order to calculate the standard enthalpy change of formation of calcium hydroxide, Ca(OH)₂(s)?

• the standard enthalpy change of H₂O(l)

• the standard enthalpy change of H₂(g)

• first and second ionisation energies of Ca

The enthalpy change of reaction 1 is –114 kJ mol^–1.

2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l) reaction 1

By using this information, what is the most likely value for the enthalpy change of reaction 2?

Ba(OH)₂(aq) + 2HCl (aq) → BaCl₂(aq) + 2H₂O(l) reaction 2

• –57 kJ mol^–1

• –76 kJ mol^–1

• –114 kJ mol^–1

• –228 kJ mol^–1