- "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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- Reactants lose energy to the surroundings.
- The temperature increases.
- Common examples:
-Combustion of fuels.
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- Reactants gain energy from surroundings.
- Temperature decreases.
- Common examples:
-Thermal decomposition (needs heat).
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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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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.
-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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