Enthalpy and Thermochemical Equations

When q Equals ΔH

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volume_mute

Under what conditions are the heat (q) and the enthalpy change (ΔH) of a process equal?

At constant volume.

At constant pressure.

Only for endothermic reactions.

At absolute zero.

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volume_mute

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Explanation

Write the Thermochemical Equation

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volume_mute

Hydrogen and chlorine react to form hydrogen chloride gas with an enthalpy change of −184 kJ. Which is the correct thermochemical equation?

H₂(g) + Cl₂(g) → 2HCl(g) ΔH = −184 kJ

2HCl(g) → H₂(g) + Cl₂(g) ΔH = −184 kJ

H₂(g) + Cl₂(g) → HCl(g) ΔH = −184 kJ

H₂(g) + Cl₂(g) → 2HCl(g) ΔH = +184 kJ

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volume_mute

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Explanation

Exothermic or Endothermic?

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volume_mute

Process

×Forming HCl releases 184 kJ

×Forming NaN₃ has ΔH = +21.72 kJ

×Forming NaCl gives off 772 kJ

×Forming NO absorbs 180 kJ

Exothermic

Endothermic

Classify each process as exothermic or endothermic.

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Explanation

Reading the Sign of ΔH

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volume_mute

True or False: A negative value of ΔH indicates an exothermic reaction.

False

True

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volume_mute

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Molar Enthalpy of Neutralization

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volume_mute

In a 250 mL solution, 0.25 mol of KOH and 0.25 mol of HNO₃ react and the temperature rises from 22.5°C to 35.9°C (heat of reaction = −14.0 kJ). What is ΔH on a molar basis?

−56.0 kJ/mol

−224 kJ/mol

−3.5 kJ/mol

−14.0 kJ/mol

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volume_mute

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Explanation

Endothermic Formation of NO

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volume_mute

Nitrogen and oxygen react to make NO(g) while absorbing 180 kJ. Which is the correct thermochemical equation?

N₂(g) + O₂(g) → 2NO(g) ΔH = −180 kJ

N₂(g) + O₂(g) → NO(g) ΔH = +180 kJ

2NO(g) → N₂(g) + O₂(g) ΔH = +180 kJ

N₂(g) + O₂(g) → 2NO(g) ΔH = +180 kJ

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volume_mute

When q Equals ΔH

Correct Answer

Explanation

Write the Thermochemical Equation

Correct Answer

Explanation

Exothermic or Endothermic?

Correct Answer

Explanation

Reading the Sign of ΔH

Correct Answer

Explanation

Molar Enthalpy of Neutralization

Correct Answer

Explanation

Endothermic Formation of NO

Correct Answer

Explanation

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