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Investigating Exothermic Reactions at the Molecular Level – Ammonia – 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:
N2(g) + 3H2(g)  2NH3(g)
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 92.4 kJ of energy
into the surroundings.
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)
Question 6:
molecular
nitrogen
+
molecular
hydrogen

individual atoms of
hydrogen and nitrogen

ammonia
N2(g) + 3H2(g)  2N(g) + 6H(g)  2NH3(g)
Question 7:
N2(g) + 3H2(g)  2NH3(g)
Energy required to break 1  NN bond = 1  944 = +944 kJ
Energy required to break 3  H–H bond = 3  436 = +1308 kJ
Total energy required to break bonds = (+944) + (+1308) = +2252 kJ
Energy released when 6  N–H bonds are formed = 6  –388 = –2328 kJ
 H = (+2252) + (–2328) = –76.0 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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