Question

C-1 How much heat is required to raise the temperature of 2000 g of water from 50°C to 80°C? The specific heat of water is 4.184 J/g °C a) 110.46 kJ b) 169.45 kJ C ) 251.04 kJ d) 301.25 kJ C-2 An exothermic reaction causes the surroundings to a) warm up b) become acidic c) condense d ) decrease in temperature C-3 Which of the reactions below have a positive entropy change? a) PC13(g) + Cl2(g) PCIs(8) b) NH3(g) + HCI(g) →NH.CI() c) U(s) + 3F2(g) →UF6(s) d) 2NH3(8) ► N2(g) + 3 H2(8)

Answer 1

C-1. Mass of water = 2000 g

Specific heat of water = 4.184 J/g.ºC

Initial temperature of the water = 50ºC

Final temperature of the water = 80ºC

Change in temperature of the water = (80 – 50)ºC = 30ºC

Heat required = (mass of water)*(specific heat of water)*(change in temperature of the water)

= (2000 g)*(4.184 J/g.ºC)*(30ºC)

= 251040 J

= (251040 J)*(1 kJ)/(1000 J)

= 251.04 kJ

(c) 251.04 is the correct answer.

C-2. An exothermic reaction is defined as a reaction which releases heat. The heat is lost from the reaction system to the surroundings. Hence, the temperature of the surroundings increases and the surroundings warm up or get hotter. Therefore, (a) is the correct answer.

C-3. The change in entropy of a system depends on the number of units (atoms, molecules or ions) present in the system. The higher is the number of units present in the system, the greater is the randomness of the system and the higher is the entropy. Consequently, the reaction which proceeds with an increase in the number of molecules will show a positive entropy change. There are two reactant molecules in (d) and four product molecules. Hence, the reaction (d) proceeds with an increase in the number of molecules and hence, will show a positive entropy change. The correct answer is (d).