Phase Change Thermochemistry

​Heating Curve

• A heating curve is a graphic diagram in which the energy of the system is shown as a substance progresses from one state of matter to another.

• This is a case in which the system can be directly measured to determine temperature change.

• An example of a heating curve is provided next. Each section has its own equation for calculating the energy required for the substance to progress through the phase change.

Multiple Choice

Given the following problem: How much heat energy is needed for 47.0 g of ice to melt at STP?

Which equation will you use to calculate the energy?

• $$q\ =\ m\Delta H_{fus}$$

• $$q\ =\ m\Delta H_{vap}$$

• $$q\ =\ mc_s\Delta T$$

• $$q\ =\ mc_l\Delta T$$

• $$q\ =\ mc_g\Delta T$$

Match

Match the equation for energy with its phase change.

• melting

• condensation

• freezing

• vaporization

• $$q=m\Delta H_{fus}$$

• $$q=-m\Delta H_{vap}$$

• $$q=-m\Delta H_{fus}$$

• $$q=m\Delta H_{vap}$$

Fill in the Blanks

How much heat energy is needed for 47.0 g of ice to melt at STP?

Fill in the Blanks

What quantity of heat energy is released when 0.506 mol of liquid water freezes?

Fill in the Blanks

What mass of steam is required to release 4.97 x 10⁵ J of heat energy on condensation?

Fill in the Blanks

How much heat (in joules) is needed to heat a 50.0 g sample of aluminum (c = 0.982 J/g∙°C) from 30.0 °C to 45.0 °C?

Fill in the Blanks

How much heat is given off when a 100.0 g sample of water (c = 4.184 J/g⋅°C) cools from 100.0 °C to 25.0°C?

Reorder

Reorder the following energy equations for a substance that is changing from its gas phase to its solid phase.

• $$q=mc_g\Delta T$$

• $$q=m\Delta H_{vap}$$

• $$q=mc_l\Delta T$$

• $$q=m\Delta H_{fus}$$

• $$q=mc_s\Delta T$$

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