Chemistry

Name: ___________________________

Interpreting Heating and Cooling Curves at the Molecular (or Atomic) Level For both Heating and Cooling Curves 

Pressure is assumed to be constant at all times.



Total Energy = Potential Energy + Kinetic Energy

A.

FOR HEATING CURVES



The continuous input of energy from the surroundings means that the ________________ energy of the system is continuously increasing. Either kinetic or potential energy increases, but not at the same time.



Melting (fusion) and boiling (vaporization) are ____________________ phase changes ie. Energy is needed to overcome attractions between particles.





Intermolecular attractions are the forces that are broken if a molecular substance is being melted of boiled; however, metallic bonds between metal atoms and ionic bonds between ions are broken when metals and ionic substances melt or boil, respectively.



Bond breaking is an endothermic process.

Some substances pass from the solid phase to the gas phase ___________________ passing through the liquid phase.  There is only one phase change in the heating curve for these systems. o  eg. a curve for iodine sublimating at 70 C under normal pressure conditions would show only one “flat” region.

140

l↔g

g

120 100 T o ( C)

80

l

60 40 20

s↔l s

0 -20 Time

1

Chemistry

Name: ___________________________

GENERAL STEPS: 1.

Heating of Solid:

 T =  EK

no change in EP

Increased vibration of particles about their fixed positions in the solid. 2.

Fusion (Melting) of Solid:

no change in T = no change in EK

 EP

Heat absorbed increases the potential energy of the particles in the solid, weakening the attractions between the particles. Particles move more freely as they become free from their fixed positions and the substance changes into the liquid state. 3.

Heating of Liquid:

 T =  EK

no change in EP

Particles move more freely, but the attractions are still strong enough to hold particles in the liquid phase. 4.

Vaporization (Boiling) of Liquid: no change in T = no change in EK

 EP

Heat added is used as potential energy to overcome the attractions between particles as the particles break free and the substance changes into the gas phase. 5.

Heating of Gas:

 T =  EK

no change in EP

Adding heat speeds up the gas particles.

B.

No further changes occur.

FOR COOLING CURVES

140

g 120

g↔l

100 T o ( C)

80

l

60 40

l↔s

20

s

0 -20 Time

2

Chemistry 

Condensation and solidification are exothermic phase changes.  Potential energy is released in the form of heat as intermolecular attractions form between closely approaching molecules. When a liquid metal solidifies, the energy is released as metallic bonds from between atoms. 



Name: ___________________________

Bond formation is an _____________________ process. Energy is released from the system to the surroundings.

When the temperature of a system ___________________, kinetic energy is transferred to the surroundings in the form of heat. This too is an exothermic process.

GENERAL STEPS: 1.

 T =  EK

Cooling of Gas;

no change in EP

Particles are slowing down and coming closer together. 2.

Condensation of Gas:

no change in T = no change in EK

 EP

The decreasing potential energy is released as heat, allowing new attractions between the particles to form as the substance changes to the liquid state. 3.

Cooling of Liquid:

 T =  EK

no change in EP

Particles slow down even more and move closer together. 4.

Solidification of Liquid: no change in T = no change in EK

 EP

The decreasing potential energy is released as heat, allowing stronger attractions to form between the particles as the substance changes to the solid state. 5.

Cooling of Solid:

 T =  EK

no change in EP

As more heat is released, particles vibrate less in the solid. No further changes occur.

NOTE:

Particles

Attractions

The type of particle and attractions depend on the type of substance:

Molecular Substance

Metallic Substance

Ionic Substance

molecules

metal atoms

ions

intermolecular forces

metallic bonds

ionic bonds

Replace the particle and attractions in the description above with the appropriate particle and attraction from the table. Replace the substance with the name of the substance undergoing the state change.

3