C3.2 - Energetics

Question 1

A substance is added to hydrochloric acid.
The temperature of hydrochloric acid did not change after adding the substance.

Suggest a reason for this. (1)

Answer 1

the substance did not react with hydrochloric acid (1)

Question 2

A substance is added to hydrochloric acid.
The temperature of hydrochloric acid decreased after adding the substance.

Explain what reaction this was (endo or exo). (2)

Answer 2

endothermic (1)

because the temperature (of the surroundings) decreased (1)

Question 3

Hydrogen reacts with chlorine to produce hydrogen chloride. (H-H + Cl-Cl -> 2H-Cl)

Bond Energies (kJ/mol):
H-H: 436
Cl-Cl: 243
H-Cl: 432

a) Calculate the energy change in the reaction. (3)

b) Explain whether this reaction is exothermic or endothermic. (2)

Answer 3

a) -185 kJ/mol
change = reactants - products
change = (436+243) - 2(432) (1)
change = 679 - 864 (1)
change = -185 kJ/mol (1)

b) exothermic (1)
because more energy is transferred to surroundings than from surroundings/ energy change is negative (1)