Heat, q , is energy transferred between a system and its surroundings. For a pro

Question

Heat, q, is energy transferred between a system and its surroundings. For a process that involves a temperature change

q=mCsT

where Cs is specific heat and m is mass.

Heat can also be transferred at a constant temperature when there is a change in state. For a process that involves a phase change

q=nH

where, n is the number of moles and H is the enthalpy of fusion, vaporization, or sublimation.

The following table provides the specific heat and enthalpy changes for water and ice.

Part A

Calculate the enthalpy change, H, for the process in which 45.7 g of water is converted from liquid at 10.1 C to vapor at 25.0 C .

For water, Hvap = 44.0 kJ/mol at 25.0 C and Cs = 4.18  J/(gC) for H2O(l).

Part B

How many grams of ice at -12.5 C can be completely converted to liquid at 8.1 C if the available heat for this process is 5.27×103 kJ ?

For ice, use a specific heat of 2.01 J/(gC) and Hfus=6.01kJ/mol.

Express your answer to three significant figures and include the appropriate units.

Explanation / Answer

Part A :

moles of water = 45.7 / 18 = 2.539

Q = m Cp dT + n Hvap

= 45.7 x 4.18 x (25 - 10.1) + 2.539 x 44 x 10^3

= 114562 J

= 114.6 kJ

Q = n H

114.6 = 2.539 x H

H = 45.1 kJ/mol

part B)

Q = m Cp dT + n Hfus

5.27×10^6 = m x 2.01 x (0 +12.5) + m/18 x 6.01 x 10^3 + m x 4.18 x (8.1 - 0)

m = 13414 g

mass of ice = 1.34 x 10^4 g

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