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Enthalpy Changes
H = The heat energy transferred
in a reaction at a constant
pressure, measured in Kj.mol-1…read more

Slide 2

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· This means the change in energy was
transferred under standard conditions
(100KPa - 298K)…read more

Slide 3

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Negative H
· An exothermic reaction means energy is
given out to the surroundings so the
overall enthalpy change is negative.
· Examples of exothermic reactions:
· Combustion:
· CH4(g) + 2O2(g) CO2(g) + 2H2O(l)
· Oxidation of carbohydrates like glucose:
· C6H12O6…read more

Slide 4

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Positive H
· Endothermic reactions absorb energy so
the overall enthalpy change is positive.
· Examples include:
· Thermal decomposition:
· CaCO3(s) CaO(s) + CO2(g)
· Photosynthesis…read more

Slide 5

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Breaking bonds
· Energy needs to be put into bonds to
break them so H is positive and the
reaction is endothermic.
· Stronger bonds need more energy to
break so have higher bond enthalpies…read more

Slide 6

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Making bonds
· Energy is released when bonds are
formed so the H is negative and the
reaction is exothermic.
· Stronger bonds release more energy when
they form so their bond enthalpy is higher.…read more

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