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Questions and Answers
What type of reaction does the enthalpy of formation typically represent?
The enthalpy change during combustion is endothermic.
False
What is the enthalpy change when one mole of water is formed in a reaction between an acid and an alkali?
Enthalpy of neutralisation
Bond dissociation enthalpy is ________ when one mole of covalent bonds is broken.
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Which of the following represents an endothermic process?
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Match the enthalpy change with its corresponding description:
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What is the enthalpy change when one mole of an ionic solid dissolves in water?
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The first electron affinity is always exothermic.
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Study Notes
Enthalpy Change Definitions
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Enthalpy of Formation (ΔH): Energy change when one mole of a substance is formed from its elements in standard states; typically exothermic with a negative value. Example: 2 Na(s) + ½ O2(g) → Na2O(s).
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Enthalpy of Combustion (ΔH): Energy change during the complete combustion of one mole of a substance in oxygen; always exothermic. Example: H2(g) + ½ O2(g) → H2O(l).
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Enthalpy of Neutralisation (ΔH): Energy change when one mole of water is produced from the reaction between an acid and an alkali; exothermic. Example: H2SO4(aq) + NaOH(aq) → Na2SO4(aq) + H2O(l).
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Ionisation Enthalpy (Δ):
- First ionisation: Energy change when one mole of gaseous atoms loses one electron to form 1+ ions; endothermic.
- Second ionisation: Energy change when one mole of 1+ ions loses one electron to form 2+ ions; also endothermic. Example: Mg(g) → Mg+(g) + e⁻, Mg+(g) → Mg²+(g) + e⁻.
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Electron Affinity (ΔH):
- First electron affinity: Energy change when gaseous atoms gain an electron to form 1- ions; often exothermic.
- Second electron affinity: Energy change when 1- ions gain an electron to form 2- ions; typically endothermic. Example: O(g) + e⁻ → O⁻(g), O⁻(g) + e⁻ → O²⁻(g).
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Enthalpy of Atomisation (ΔH): Energy change when one mole of gaseous atoms is produced from an element in its standard state; endothermic. Example: ½ I2(s) → I(g).
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Hydration Enthalpy (ΔH): Energy change when one mole of gaseous ions is dissolved in water; exothermic. Example: Mg²+(g) + aq → Mg²+(aq).
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Enthalpy of Solution (ΔH): Energy change when one mole of an ionic solid dissolves in water; values can vary. Example: MgCl2(s) + aq → Mg²+(aq) + 2 Cl⁻(aq).
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Bond Dissociation Enthalpy (ΔH): Energy change when one mole of covalent bonds is broken in the gaseous state; endothermic. Example: I2(g) → 2 I(g).
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Lattice Enthalpy of Formation (ΔLEFH): Energy change when one mole of solid ionic compound forms from its gaseous ions; exothermic. Example: Mg²+(g) + 2 Cl⁻(g) → MgCl2(s).
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Description
This quiz focuses on key definitions related to enthalpy changes in chemistry, specifically the enthalpy of formation and combustion. You'll explore the differences between exothermic and endothermic reactions with relevant examples. Test your knowledge on these fundamental concepts in thermodynamics!
