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Question
How much heat energy is released when 5.0 g of water at 20℃ changes into ice at 0℃? Take specific heat capacity of water = 4.2 J g-1 K-1, Specific latent heat of fusion of ice = 336 J g-1.
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Solution
Given,
Mass (m) = 5.0 g
Specific heat capacity of water (c) = 4.2 J g-1 K-1
Specific latent heat of fusion of ice (L) = 336 J g-1
(i) Heat energy released when water lowers it's temperature from 20° C to 0° C
= m × c × change in temperature
Substituting the values in the formula we get,
Q1 = 5 × 4.2 × (20 − 0)
Q1 = 5 × 4.2 × 20
⇒ Q1 = 420 J
(ii) Heat energy released when water at 0° C changes into ice at 0° C = m × L
Substituting the values in the formula we get,
Q2 = 5 × 336 J
⇒ Q2 = 1680 J
From relation,
Q = Q1 + Q2
Q = 420 + 1680
Q = 2100
Total heat released = 2100 J
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