Enthalpy Changes

Which of the following is an endothermic process?

• Condensation of water vapor

• Freezing of water

• Melting of ice

• Combustion of methane

Which statement is true for an exothermic reaction?

• The system absorbs heat from the surroundings

• The products have higher enthalpy than the reactants

• The enthalpy change (ΔH) is positive

• Heat is released to the surroundings

In an enthalpy profile diagram for an exothermic reaction, the curve shows:

• Products at a higher energy level than reactants

• Reactants at a higher energy level than products

• Products and reactants at the same energy level

• Products absorbing more energy than reactants

The activation energy (Ea) on an enthalpy profile diagram is represented as:

• The difference between the energy of products and reactants

• The difference between the peak of the curve and the energy of the reactants

• The overall enthalpy change (ΔH)

• The energy level of products

The standard enthalpy change of formation is defined as the enthalpy change when:

• 1 mole of compound is formed from its elements in their standard states

• 1 mole of reactant is completely burned in oxygen

• 1 mole of acid reacts with 1 mole of base

• 1 mole of product is formed at 298 K and 100 kPa

Which reaction represents the standard enthalpy change of neutralization?

• HCl(g) → H⁺(aq) + Cl⁻(aq)

• HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

• CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

• 2H₂(g) + O₂(g) → 2H₂O(l)

50 cm³ of 1.0 mol/dm³ HCl is mixed with 50 cm³ of 1.0 mol/dm³ NaOH. The temperature rises by 6.0 °C. Assume the solution has a mass of 100 g and specific heat capacity 4.18 J g⁻¹ K⁻¹. What is the heat released?

• 251 J

• 1,880 J

• 2,510 J

• 4,180 J

50 cm³ of 1.0 mol/dm³ HCl is mixed with 50 cm³ of 1.0 mol/dm³ NaOH. The temperature rises by 6.0 °C. Assume the solution has a mass of 100 g and specific heat capacity 4.18 J g⁻¹ K⁻¹. What is the enthalpy change of neutralization per mole of water formed?

• -25.1 kJ mol⁻¹

• -50.2 kJ mol⁻¹

• -100.4 kJ mol⁻¹

• -12.6 kJ mol⁻¹

A spirit burner loses 0.45 g of methanol (CH₃OH) when heating 100 g of water by 10 °C. (c = 4.18 J g⁻¹ K⁻¹). What is the energy absorbed by the water? Ar of C:12; H:1; O:16

• 418 J

• 4,180 J

• 8,360 J

• 41,800 J

A spirit burner loses 0.45 g of methanol (CH₃OH) when heating 100 g of water by 10 °C. (c = 4.18 J g⁻¹ K⁻¹). What is the enthalpy change of combustion of methanol (Mr = 32 g/mol)?

• -297 kJ mol⁻¹

• -464 kJ mol⁻¹

• -555 kJ mol⁻¹

• -832 kJ mol⁻¹