SS2: Chemistry Theory Questions – Energy and Energy Effect I

Question 1

[a]  Explain the terms exothermic and endothermic reactions. What is the sign of ∆H in each of the reactions?

Answer:

Exothermic reaction is one which releases energy as heat to the environment ∆H= -ve

Endothermic reaction is one which gains energy as heat from the surrounding ∆H= +ve

[b] Draw an energy profile diagram for each of the following reactions.

i. H_2[g] + I_2[s] → 2HI_[g]  ∆H = +53.6KJmol^-1

Answer:

ii. C_[s] + O_2[g] → CO_2[g]   ∆H = -480KJmol^-1

Answer:

Question 2

[a]
(i). Define enthalpy of combustion.

Answer: This is the amount of heat evolved or liberated when 1mole of a substance is burnt completely in excess oxygen under standard conditions

(ii) Why is the heat of combustion always negative

Answer:  Because its an exothermic reaction (Energy is lost)

(iii)Name the instrument used for the determination of heat of combustion in the laboratory.

Answer: Bomb calorimeter

(iv)When 0.06 mole of methane is burnt in air, the heat liberated raises the temperature of 100g of water from 70°C to 85°C.
What is the heat of combustion of methane? [ specific heat capacity = 4.2Jg^-1 °C^-1 ]

Solution:

Q = mC∆T

Q = 100 x 4.2 x (85 – 70)

= 6300Jmol-1

= 6.3KJmol-1