Heating/Cooling Curves

Heating/Cooling Curves

Introduction*

In thermochemistry, the relation between the amount of heat absorbed or released by a substance, q, and its accompanying temperature change, ΔT, was introduced:

q=mcΔT

where m is the mass of the substance and c is its specific heat. This relation, however, applies to matter being heated or cooled, but not undergoing a change in state.

When a substance being heated or cooled reaches a temperature corresponding to one of its phase transitions, further gain or loss of heat is a result of diminishing or enhancing intermolecular attractions, instead of increasing or decreasing molecular kinetic energies. While a substance is undergoing a change in state, its temperature remains constant.

The amount of heat required to change one mole of a substance from the liquid state to the gas state is the enthalpy of vaporization, ΔH_vap. For example, the vaporization of water at standard temperature is represented by:

H₂O(l)⟶H₂O(g) ΔH_vap=44.01 kJ/mol

The amount of heat required to change one mole of a substance from the solid state to the liquid state is the enthalpy of fusion, ΔH_fus of the substance. The enthalpy of fusion of ice is 6.0 kJ/mol at 0 °C. Fusion (melting) is an endothermic process:

H₂O(s)⟶H₂O(l) ΔH_fus=6.01 kJ/mol

A typical heating curve for a substance depicts changes in temperature that result as the substance absorbs increasing amounts of heat. Plateaus in the curve (regions of constant temperature) are exhibited when the substance undergoes phase transitions. A cooling curve is the reverse of the heating curve. A cooling curve depictschanges in temperature that result as the substance releases increasing amounts of heat.

In summary, the heat needed to change the temperature of a given substance (the slanted region of a heating/cooling curve) is:

q = m  ×  c  ×  ΔT  (no phase transitions)

The heat needed to induce a given change in phase (the plateaus region of the heating/cooling curve) is given by

q = n  ×  ΔH  (no temperature changed)

In this lab, you will observe the phase changes of water and draw heating and cooling curves of water with a computer simulation.

Procedure

Copy this to your web browser.  You will need to sign in with free account.  You can also sign in with Google, Facebook, Twitter, or Microsoft.

https://interactives.ck12.org/simulations/chemistry/phases-of-matter/app/index.html?screen=sandbox

You will see this on your screen.

Part I Heating Curve

Select the starting state to be Solid and the final state to be Gas, start the heating by clicking  the arrow on the lower left. Observe the molecular change happens within the cup and pay attention to the heating curve on the upper left and the temperature shown on the upper right.  Answer questions asked on the Data Sheet.

Part II Cooling Curve

Select the starting state to be Gas and the final state to be Solid, start the cooling by clicking  the arrow on the lower left. Observe the molecular change happens within the cup and pay attention to the cooling curve on the upper left and the temperature shown on the upper right.   Answer questions asked on the Data Sheet.

Data Sheet

Part I Heating Curve

1. Does the temperature vary when the ice melts? What’s the temperature when the ice melts?

2. Does the temperature vary when the water boils? What’s the temperature when the water boils?

3. What’ the y- axis and x-axis of the heating curve?

4. Sketch a heating curve of water from solid ice to steam. Please don’t use the screen shot. You can use a piece of paper and pencil for sketching. Label both axes, start with a random negative temperature in ^OC and end with a random temperature above 100 ^OC. Label the physical state of each of the five sections of the heating curve. Label the melting point and boiling point of water. Once you are done sketching, take a picture of it, and submit it with your lab report.

Part II Cooling Curve

1. Does the temperature vary when the steam condenses? What’s the temperature when the steam condenses?

2. Does the temperature vary when the water freezes? What’s the temperature when the water freezes?

3. What’ the y- axis and x-axis of the cooling curve?

4. Sketch a cooling curve of water from steam to solid ice. Please don’t use the screen shot. You can use a piece of paper and pencil for sketching. Label both axes, start with a random temperature above 100 ^OC and end witha random negative temperature in ^OC. Label the physical state of each of the five sections of the heating curve. Once you are done sketching, take a picture of it, and submit it with your lab report.

Post Lab Assignment

1. How much heat is required to convert 135 g of ice at –15 °C into water vapor at 120 °C? Show all your work.

c_{liquid water} = 4.184 J/g °C

c_steam= 1.84 J/g °C

c_ice= 2.09 J/g °C

ΔH_vap = 40.65 kJ/mol

ΔH_fus = 6.01 kJ/mo

2. Evaporation of sweat requires energy and thus take excess heat away from the body. Some of the water that you drink may eventually be converted into sweat and evaporate. If you drink a 20-ounce bottle of water (590g) that had been in the refrigerator at 3.8 °C, how much heat is needed to convert all of that water into sweat and then to vapor? (Note: Your body temperature is 36.6 °C. For the purpose of solving this problem, assume that the thermal properties of sweat are the same as for water.

c_{liquid water} = 4.184 J/g °C

c_steam= 1.84 J/g °C

c_ice= 2.09 J/g °C

ΔH_vap = 40.67 kJ/mol at 36.6 °C.

ΔH_fus = 6.01 kJ/mol

3. Upload your sketches of the heating curve and cooling curve of water, completed Data Sheet, and answers to post lab questions to Canvas.