F322: Enthalpy Changes

Enthalpy

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In a chemical reaction, bonds are broken in the reactants (endothermic [needs energy]), and then in the products bonds are made (exothermic [releases energy]).
If more energy is needed to break bonds than the energy released = Endothermic overall.
If more energy is released when bonds are made than bonds broken = Exothermic.
Strength of bond is shown by its bond enthalpy.
Average Bond Enthalpy = The enthalpy change when breaking 1 mole of a given type of bond in the molecules of a gaseous species.
E.g. H2 -> 2H △H = +436 kJmol-1
You can use a table of these (e.g. C-H = 413 etc.) to work out whether a reaction is exothermic or endothermic by taking away the products enthalphy from the reactant's; if the number is negative then the reaction is exothermic; if positive, endothermic.

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Symbol = △Hθ
Standard conditions are:
-298K (25 degrees).
-100KPa (1 atm)
-1 mol dm-3 for concentration of solutions.
△Hfθ = The enthalpy change when 1 mole of a substance is made from its elements in their standard states under standard conditions.
△Hcθ = The enthalpy change when 1 mole of a substance is reacted with oxygen under standard conditions.
△ Hrθ = The enthalpy change when chemicals react in the molar quantities as expressed in the reaction equation, under standard conditions.

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Q = -m c △T
Enthalpy change mass of liquid heated 4.18 (specific heat capacity) change in temp.
The use of experimental data to calculate the enthalpy change.
The values calculated experminentally will be lesx than theoretical due to heat loss.
Loses heat:
-Through material of container.
-Through open top.
Fixed by:
-Use insulating material.
-Add a lid.

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"The enthalpy change of a reaction is independant of the route taken, providing the initial and final conditions are the same".
Simple methods:
-If for △Hf = Sum of enthalpies of products - sum of enthalpies of reactants.
-If for △Hc = Sum of enthalpies of reactants - sum of enthalpies of products.

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