Calculating heat of phase changes

Calculating Heat of Phase Changes

(with a focus on water)

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images on this page come from your NYS Chemistry Reference Tables.

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Table B is on p1. Others are from p12​

​During a phase change, heat is either absorbed (endothermic) or released (exothermic)

​The heat associated with a phase change is directly related to...

• The mass , m, of the sample (units of grams)

• A constant based on the phases the change involves

  • if transition between solid and liquid phases... (a.k.a. freezing or melting)

    • Use H_f , heat of fusion

  • if transition between liquid and gas phases... (a.k.a. vaporizing or condensing)

    • Use H_v, heat of vaporization

  ​ex​ample calculations next... (with specific values for H_f and H_v included)

​Example One:

Calculating the heat absorbed when 100 g of water melts using formula q = mH_f ​

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• m = 100 g

• heat of fusion, H_f = 334 J/g

• q = 100 g x 334 J/g = 33400 J

​Example Two:

Calculating the heat released when 100 g of water freezes using formula q = mH_f ​

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• m = 100 g

• heat of fusion, H_f = 334 J/g

• q = 100 g x 334 J/g = 33400 J

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Yeah... the same amount of heat absorbed when 100 g of water melts.

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Energy is conserved... so opposite processes have equal but opposite energy change.​

​Example Three:

Calculating the heat absorbed when 100 g of water vaporizes (boils) using formula q = mH_v ​

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• m = 100 g

• heat of vaporization, H_v = 2260 J/g

• q = 100 g x 2260 J/g = 226000 J

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It is a very similar equation. Only the value of the constant is different. Notice that it takes more energy per gram of water to boil it than to melt it.​

​Next up... 2 problems as an exit ticket to see how you are doing with these equations...

Multiple Choice

50 grams of water is melted.

( H₂O (s) --> H₂O (l) )

How much energy, q, did the water absorb? (units of Joules, J)

Recall that q = mH_f and H_f = 334 J/g

• 334 J

• 33400 J

• 16700 J

• 113000 J

Multiple Choice

75 grams of water is boiled.

( H₂O (l) --> H₂O (g) )

How much energy, q, did the water absorb? (units of Joules, J)

Recall that q = mH_v

• 2260 J

• 25050 J

• 169500 J

• 226000 J

Energy values can get quite large... so kilojoules (kJ) or even megajoules (MJ) are often used instead of the base unit Joule, J.

For example... 4500 J could be reported as 4.500 kJ