Understanding Points
Reactivity 1.2.1—Bond-breaking absorbs and bond-forming releases energy.
Reactivity 1.2.2—Hess’s law states that the enthalpy change for a reaction is independent of the
pathway between the initial and final states.
Bond enthalpy
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The energy required to break 1 mole of a covalent bond in gaseous state
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X-Y(g) ⇌ X(g) + Y(g)
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Energy is released during the formation of bonds → exothermic → ∆H negative
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Energy is absorbed during the breaking of bonds → endothermic → ∆H positive
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Mnemonic: “Mexo Bendo” (making-exothermic, breaking-endothermic)
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The enthalpy of a reaction can be calculated using bond enthalpy
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∆H = ∑bonds broken – ∑bonds formed
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∆H = ∑reactants – ∑products
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Limitations
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Bond enthalpies are only an average value so the enthalpy change won’t be exact for specific reactions and molecules
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Bond enthalpies can only be used on their own if all the reactants and products are in the gaseous state
Hess’ Law and energy cycles
ΔH3 = ΔH1 + ΔH2
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‘Enthalpy change is independent of the path taken’
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Regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes
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The enthalpy change going from A to B is the same whether the reaction is proceeding directly from A to B or whether it goes from A to C as a intermediate and then to B
Rules
1.
If you multiply a reaction by a certain number , you also multiply ∆H by the same number
e.g.) C -> 2C, ΔH -> 2 x ΔH
2.
If you reverse the reactants and products, you change the sign of ∆H
3.
Cancel out elements that appear of both sides
e.g.) CaCO3 (s) → CaO(s) + CO2 (g)
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Equation 1: CaCO3 (s) + 2HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l) ∆H1= -13kJmol-1
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Equation 2: CaO(s) + 2HCl (aq) → CaCl2 (aq) + H2O (l) ∆H2= -80kJmol-1
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Step 1) write down the full equation following the reaction
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Equation 1+ reverse of Equation 2
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CaCO3 (s) + 2HCl (aq)+ CaCl2 (aq) + H2O (l) → CaCl2 (aq) + CO2 (g) + H2O (l) + CaO(s) + 2HCl (aq)
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Step 2) cancel out species that are in both the reactant and product side (the resulting reaction should match the overall reaction)
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CaCO3 (s) + 2HCl (aq) + CaCl2 (aq) + H2O (l) → CaCl2 (aq) + CO2 (g) + H2O (l) + CaO(s) + 2HCl (aq)
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Step 3) find the enthalpy change by adding the enthalpy changes for the reactions (as flipped for equation 2)
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13- (-80) = +67kJmol-1