Why is water more stable as a solid below 0 o C but as a liquid above it?

Why is water more stable as a solid below 0 oC but as a liquid

above it?

What controls why only some things happen?

Energy is neither created or destroyed during chemical or

physical changes, but it is transformed from one form

to another.

Euniverse = 0

TYPES of ENERGY

Kinetic Potential

Mechanical Gravitational

Thermal Electrostatic

Electrical Chemical

Radiant

Dropping a rock?

1 2 3

0% 0%0%

1. Gravitational Mechanical

2. Gravitational Thermal

3. Gravitational Gravitational

Using a flashlight?

1 2 3 4

0% 0%0%0%

1. Thermal Radiant

2. Chemical Thermal

3. Chemical Radiant

4. Electrostatic Radiant

Driving a Car?

One turn of a giant windmill produces 0.26 kWh of electricity. Howmany kJ is this?

Endo: heat added to system Exo: heat released by system

SYSTEM and SURROUNDINGSSystem: The thing under studySurroundings: Everything else in the universe

Energy transfer between system and surroundings:

Dissolution of KNO3

HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

HEAT: What happens to thermal (heat) energy?

Three possibilities:

• Warms another object

• Causes a change of state

• Is used in an endothermic reaction

Example 1: 5 g wood at 0 oC + 5 g wood at 100 oCExample 2: 10 g wood at 0 oC + 5 g wood at 100 oCExample 3: 5 g copper at 0 oC + 5 g copper at 100 oCExample 4: 5 g wood at 0 oC + 5 g copper at 100 oC

Clicker Choices: 1: 0 oC 2: 33 oC 3: 50 oC 4. 67 oC 5: 100 oC 6: other

Temperature Changes from Heat Exchange

Bill, go to the next slide. You know you want to.

1 2 3 4 5 6

17% 17% 17%17%17%17%

Temperature Changes from Heat Exchange

5 g wood at 0 oC + 5 g wood at 100 oC

1. 0 oC

2. 33 oC

3. 50 oC

4. 67 oC

5. 100 oC

6. other

1 2 3 4 5 6

17% 17% 17%17%17%17%

Temperature Changes from Heat Exchange

10 g wood at 0 oC + 5 g wood at 100 oC

1. 0 oC

2. 33 oC

3. 50 oC

4. 67 oC

5. 100 oC

6. other

1 2 3 4 5 6

17% 17% 17%17%17%17%

Temperature Changes from Heat Exchange

5 g copper at 0 oC + 5 g copper at 100 oC

1. 0 oC

2. 33 oC

3. 50 oC

4. 67 oC

5. 100 oC

6. other

1 2 3 4 5 6

17% 17% 17%17%17%17%

Temperature Changes from Heat Exchange

5 g wood at 0 oC + 5 g copper at 100 oC

1. 0 oC

2. 33 oC

3. 50 oC

4. 67 oC

5. 100 oC

6. other

Conclusion I: What happens to thermal (heat) energy?

When objects of different temperature meet:

• Warmer object cools• Cooler object warms• Thermal energy is transferred

• qwarmer = -qcooler

Conclusion II: What controls the magnitude of an object’s temperature change?

Quantitative: Calculating Heat Exchange: Specific Heat Capacity

Calculate the specific heat capacity of copper:

Calculate the specific heat capacity of wood:

Which has the smallest specific heat capacity?

1 2 3 4

25% 25%25%25%1. Wood

2. Copper

3. Silver

4. Water

Specific Heat Capacity

The energy required to heat one gram of a substance by 1 oC.

Usefulness: #J transferred = S.H. x #g x T

How much energy is used to heat 250 g water from 17 oC to 100 oC?

What happens to thermal (heat) energy?

When objects of different temperature meet:

• Warmer object cools• Cooler object warms• Thermal energy is transferred

• qwarmer = -qcooler

specific heat x mass x T = specific heat x mass x T

warmer object cooler object

Heat transfer between substances:

owood o

Jq = 1.8 5 g (-18 C)

g C

= -160 J

oCu o

Jq = 0.385 5 g (+82 C)

g C

= +160 J

Conceptually Easy Example with Annoying Algebra:

If we mix 250 g H2O at 95 oC with 50 g H2O at 5 oC,

what will the final temperature be?

Thermal Energy and Phase Changes

First: What happens?

Thermal Energy and Phase Changes

First: What happens?

Thermal Energy and Phase Changes

First: What happens?

Warming:

• Molecules move more rapidly

• Kinetic Energy increases

• Temperature increases

Melting/Boiling:

• Molecules do NOT move more rapidly

• Temperature remains constant

• Intermolecular bonds are broken

• Chemical potential energy (enthalpy) increases

But what’s really happening?

Energy and Phase Changes: Quantitative Treatment

Melting:

Heat of Fusion (DHfus) for Water: 333 J/g

Boiling:

Heat of Vaporization (DHvap) for Water: 2256 J/g

Total Quantitative Analysis

Convert 40.0 g of ice at –30 oC to steam at 125 oC

Warm ice: (Specific heat = 2.06 J/g-oC)

Melt ice:

Warm water (s.h. = 4.18 J/g-oC)

Total Quantitative Analysis

Convert 40.0 g of ice at –30 oC to steam at 125 oC

Boil water:

Warm steam (s.h. = 1.92 J/g-oC)

Enthalpy Change and Chemical Reactions

H = energy needed to break bonds – energy released forming bonds

Example: formation of water:

H = ?

Enthalpy Change and Chemical Reactions

H is usually more complicated, due to solvent and solid interactions.

So, we measure H experimentally.

Calorimetry

Run reaction in a way that the heat exchangedcan be measured. Use a “calorimeter.”

Calorimetry Experiment

N2H4 + 3 O2 2 NO2 + 2 H2O

Energy released = E absorbed by water +E absorbed by calorimeter

Ewater =

Ecalorimeter =

Total E =

H = energy/moles =

0.500 g N2H4

600 g water

420 J/oC

Hess’s Law

If reactions can be “added”so can their H values.