Energy Changes &

Phase Changes

Heating & Cooling Curves

Heat Flow: hot to cold

SysteSystem –m – 1 1

phasephase

Environment

What about phase changes?

• Sometimes more than one phase of substance is present–when melting ice: both liquid water and solid

water (ice) are present at same time

• temperature is constant: T=0

All chemical & physical changes are accompanied by energy changes

• Phase changes are physical changes:• sometimes energy is absorbed - endothermic• sometimes energy is released - exothermic

• Energy change for given phase change can be measured/calculated

Remember 6 possible phase changes and their names?

Melting Ice

• Ice water on hot plate: ice is melting • ice is absorbing heat from hot plate and using it to

change phase• temperature of ice-water mix is constant (0˚C)

– heat energy from hot plate is going into phase change (PE is increasing)

• heat energy not going to KE of particles

Identify a phase change as endothermic or exothermic

PE

Solid

Liquid

GasEndothermic

Exothermic

Melting VaporizationSublimation

CondensationFreezingDeposition

Heating & Cooling Curves

• Measure temperature as function of time at constant heating or cooling rate

Time

Tem

pera

tur

eI II III IV V

Solid

Solid & Liquid

Liquid

Liquid & Gas

Gas

K.E. K.E. K.E.

K.E.↔K.E.↔P.E.↔

P.E.↔P.E.↔

P.E. P.E.Melt pt.

Boil pt.

Melting & Boiling Points

• Plateaus = Phase changes = PE changes

• when 2 phases present, temperature is constant

• Melting point, Boiling point

What happens to temperature as heat is added at boiling point?

• Nothing, until only 1 phase is present!

To analyze a heating/cooling curve:

• Does curve go uphill or downhill?• Label phases present in each region• Describe what happens to KE in each region• Describe what happens to PE in each region• Locate melting point and boiling point