Heat and Phase Change

What affects heat transfer?

•The material: conductor or insulator?•Area. Ex: a bigger window vs. a smaller one.•Thickness. Ex: heat escapes a thinner styrofoam cup

than a thicker one.

Heat and State of Matter

•Does the absorption or release of heat always cause a temp change? NO

•Ex: para-dichlorobenzene (chemical in mothballs) has a melting point of 54 oC.

• If you were to heat this to 80 oC, then insert it into room temperature water, you’d see the temp steadily decrease until you hit 54 oC.

•At this point it is becoming a solid.

• It will stay at 54 oC until the last of the liquid becomes a solid. Then it will decrease again.

•Was heat being transferred during that time? YES

•Phase transitions occur at a constant temperature.•Heat transfers with no change in temp are referred to

as latent heat.

•For melting and freezing:

Q= mΔHFusion

•For vaporization and condensation:

Q= mΔHVaporization

•ΔHFusion = specific heat of fusion per gram

•ΔHVaporization= specific heat of vaporization per gram

•Solid Water: Cp=2.00 J/g°C

•Liquid Water: Cp = 4.18 J/g°C

•Gaseous Water: Cp = 2.01 J/g°C

Examples

•Elise places 48.2 grams of ice in her beverage. What quantity of energy would be absorbed by the ice (and released by the beverage) during the melting process? The heat of fusion of water is 333 J/g.

•What is the minimum amount of liquid water at 26.5 degrees that would be required to completely melt 50.0 grams of ice? The specific heat capacity of liquid water is 4.18 J/g°C and the specific heat of fusion of ice is 333 J/g.

Given info about ice:m = 50.0 gΔHfusion = 333 J/g

Given info about water:C = 4.18 J/g°C Tinitial = 26.5°C Tfinal = 0.0°CΔT = -26.5°C (Tfinal - Tinitial )

The energy gained by the ice is equal to the energy lost from the water.

Qice = -Qliquid waterQice = mΔHfusion = (50.0 g)(333 J/g)

Qice = 16650 J

16650 J = -Qliquid water

16650 J = -mliquid waterCliquid waterΔTliquid water

16650 J = -mliquid water (4.18 J/g°C)(-26.5°C)

16650 J = -mliquid water(-110.77 J/g)

mliquid water = 150.311 g

A 100 g sample of water’s temperature is raised from - 20oC to 120oC. What is the total heat added to the sample over this period?