± Enthalpy of a Phase Change Heat, q , is energy transferred between a system an

Question

± Enthalpy of a Phase Change

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 47.0 g of water is converted from liquid at 7.0 Cto 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).

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

Part B

How many grams of ice at -23.9 C can be completely converted to liquid at 11.4 C if the available heat for this process is 4.63×103 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)

step 1 : water at 7 oC to 25 oC

q1 = 47 x 4.18 (25 - 7)/1000 = 3.54 kJ

step 2 : water at 25 oC to vapor at 25 oC

q2 = 47 x 44/18 = 115.00 kJ

Total enthalpy change = (q1+q2) x 18/47 = 45.40 kJ/mol

Part B)

Ice is heated to 0 oC then melted at that temperature. Water is then heated to 11.4 oC

4.63 x 10^6 = g x 2.01 x 23.9 + g x 6.01 x 10^3/18 + g x 4.18 (11.4)

grams of ice to be melted = 10775.160 g

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