Question

# Chemical reactions are invariably associated with the transfer of energy either in the form of heat or light. In the laboratory, heat changes in physical and chemical processes are measured with an instrument called Calorimeter. Heat change in the process is calculated as$$q = ms\Delta T$$ ........ $$s= specific\quad heat$$$$= c\Delta T$$ ....... $$c= heat\quad capacity$$Heat of reaction at constant volume is measured using bomb Calorimeter.$$q_V = \Delta U = internal\quad energy\quad change$$Heat of reaction at constant pressure is measured using bomb Calorimeter.$$q_P = \Delta H$$$$q_P = q_V + P\Delta V$$$$\Delta H = \Delta U + \Delta nRT$$The enthalpy of fusion of ice is $$6.02 kJ mol ^{ -1 }$$. The heat capacity of water is $$4.18 J g^{ -1 }C^{ -1 }$$. What is the smallest number of ice cubes at $$0^{ \circ }C$$, each containing one mole of water, that is needed to cool $$500 g$$ of liquid water from $$20^{ \circ }C$$ to $$0^{ \circ }C$$.

A
8
B
7
C
140
D
120

Solution

## The correct option is B 7heat released to cool $$500 g$$ water from $$20^{ \circ }C$$ to $$0^{ \circ }C$$.$$q = ms\Delta T$$$$= 500\times 4.18\times 20 = 41800 J$$$$= 41.8 kJ$$Number of moles of water(ice) that will melt to absorb $$41.8 kJ= \dfrac{q}{Enthalpy\ of\ fusion} =\displaystyle \frac { 41.8 }{ 6.02 }$$$$=6.94 \approx 7$$.Hence, the correct option is $$B$$Chemistry

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