28.1.14 Answers to bond energy questions

Please let me know after you have checked your answers if there is a problem.

Grade C Question. 

Hydrogen burns in chlorine.

H2(g) + Cl2(g)          2HCl(g)

a. One each of H–H and Cl–Cl bonds breaks.

b.  Two H–Cl bonds form.

c.  Energy to break bonds = 434 kJ + 242 kJ = 676 kJ

  Energy given out on forming bonds
  = 2 × 431 kJ = 862 kJ

  Overall energy change = -186 kJ given out.

d. Exothermic

Grade A Question 3.

a.  Bonds broken = 1933kJ

  Bonds formed = 1959kJ

  Overall energy = -26kJ

b.  Exothermic as the value is negative

Grade B Question 4

a.Negative amount of energy given out. So products have less energy than the reactants.

b.Is the minimum energy needed to start a reaction.

Grade A  to A* Question 5

c.i. 3466kJ/mol

c.ii. -730 kJ/mol

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