In: Physics
A 31 g ice cube at -15.0oC is placed in 139 g of water at
48.0oC. Find the final temperature of the system when equilibrium
is reached. Ignore the heat capacity of the container and assume
this is in a calorimeter, i.e. the system is thermally insulated
from the surroundings. Give your answer in oC with 3 significant
figures.
Specific heat of ice: 2.090 J/g K
Specific heat of water: 4.186 J/g K
Latent heat of fusion for water: 333 J/g
Heat energy = mass * specific heat * ∆T
AND
Heat energy = mass * heat of fusion
specific heat of ice = 2.09 J/(g * ˚C)
Heat of fusion if ice = 333 J/g
specific heat of water = 4.186 J/(g * ˚C)
Heat energy will flow from the hot water to the cold ice, until the
temperature of the ice = 0. Then heat energy will flow from the
water to the ice until all the ice is melted. Then heat energy will
flow from the hot water to the cold water, until the temperature of
all the water is the same! Since we do not know whether the hot
water can melt all the ice, we better determine the temperature of
the hot water after the ice is melted!
1st the temperature of the ice increases from -15.0 to 0
Heat energy = 31 * 2.09 * 15.0 = 971.85 J
2nd the ice melts
Heat energy = 31 * 333 = 1.0323 J
Total heat energy = 971.85 + 10323 = 11294.85 J
At this point, all the ice has melted. Now you have 31 grams of
water at 0˚. The temperature of the 131 grams of hot water has
decreased as the ice warmed up and melted. Let’s determine the
temperature of the hot water.
11294.85 = 131 * 4.186 * ∆ T
11294.85 = 548.366 * ∆ T
∆ T = 20.60˚C
The temperature of the hot water = 48 – 20.6 = 27.4˚C
This is the temperature of the 139 grams of hot water
3rd the temperature of the water increases
In the 3rd phase, temperature of the cold water increases as the
temperature of hot water decreases, until all the water is at the
same temperature.
Now the temperature of the 31 grams of 0˚ water increases and the
temperature of the 139 grams of 27.4˚C water decreases, until the
temperature of all the water is the same, Tf.
31 * 4.186 * (Tf – 0) = 139 * 4.186 * (27.4 –
Tf)
Divide both sides by 4.186
31 * Tf = 139 * (27.4 – Tf)
31 * Tf = 3808.6 – 139 * Tf
170 * Tf = 3808.6
Tf = 22.4˚C
This is the final temperature of the water!