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Investigating Exothermic Reactions at the Molecular Level – Hydrogen Peroxide – Answers
Question 1:
a)
An exothermic chemical reaction releases energy into the surroundings, usually in the form of
heat and light.
b) For an exothermic chemical reaction, the energy change is negative, −H.
Question 2:
a)
An endothermic chemical reaction absorbs energy from the surroundings, usually in the form of
heat and light.
b) For an endothermic chemical reaction, the energy change is positive, +H.
Question 3:
a)
Bond breaking absorbs energy. It absorbs energy from the surroundings and is therefore
endothermic.
b) Bond formation releases energy. It releases energy into the surroundings and is therefore
exothermic.
Question 4:
2H2O2(l)  O2(g) + 2H2O(l)
Question 5:
a)
H means change in energy (or change in enthalpy).
b) A negative value for H signifies that the reaction is exothermic and releases 196.4 kJ of energy
into the surroundings.
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)
Question 6:
hydrogen
peroxide

individual atoms of
hydrogen and oxygen

molecular
oxygen
+
water
2H2O2(l)  4H(g) + 4O(g)  O2(g) + 2H2O(l)
Question 7:
2H2O2(l)  2H2O(l) + O2(g)
Energy required to break 2  O–O bond = 2  +146 = +292 kJ
Energy required to break 4  O–H bonds = 4  +463 kJ = +1852 kJ
Total energy required to break bonds = (+292) + (+1852) = +2144 kJ
Energy released when 1  O=O bonds form = 1  –496 = –496 kJ
Energy released when 4  O–H bonds form = 4  –463 = –1852 kJ
Total energy released during bond formation = (–496) + (–1852) = –2348 kJ
 H = (+2144) + (–2348) = –204 kJ
 The reaction is exothermic because the energy that is released during bond formation is greater
than the energy that is absorbed during bond breaking.
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