# F322: Enthalpy Changes

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## Enthalpy

• "The heat energy stored in a chemical system" (Symbol = H).
• Enthalpy change = △H.
• Can be measured by temperature changes in reactions.
• Can be exothermic or endothermic. [Next cards]
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## Exothermic Reactions

• Reactants lose energy to the surroundings.
• The temperature increases.
• Common examples:
-Combustion of fuels.
-Respiration.
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## Endothermic

• Reactants gain energy from surroundings.
• Temperature decreases.

• Common examples:
-Thermal decomposition (needs heat).
-Photosynthesis.
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## Bonds

• 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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## Standard Enthalpy Changes

• 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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## Calculating Enthalpy Changes

• 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.

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## Hess' Law

• "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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