Thermodynamics

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

Energy Conversion Examples:1. dropping a rock2. using a flashlight3. driving a car

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

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

Energy transfer between system and surroundings:

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

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

Temperature Changes from Heat Exchange

What happens to thermal (heat) energy?

When objects of different temperature meet:

• Warmer object cools

• Cooler object warms

• Thermal energy is transferred

• qwarmer = -qcooler

Quantitative: Calculating Heat Exchange: Specific Heat Capacity

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 TreatmentMelting:

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

Boiling:

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

Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC

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

oo

J2.06 40.0 g (30.0 C) = 2470 J

g C

oo

J4.18 40.0 g (100.0 C) = 16720 J

g C

333 J40.0 g = 13320 J

g

Melt ice:

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

Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC

Boil water:

oo

J1.92 40.0 g (25.0 C) = 1920 J

g C

2256 J40.0 g = 90240 J

g

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

Lots of different types of energy.

We use Enthalpy:

Heat exchanged under constant pressure.

Energy and Chemical Reactions

Energy/Enthalpy Diagrams

Some Examples of Enthalpy Change

2 C(s) + 2 H2(g) C2H4(g) H = +52 kJ

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.”

Bomb 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

Enthalpy Change and Bond Energies

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

Example: formation of water:

H = [498 + (2 x 436)] – [4 x 436] kJ = -482 kJ

General Rule:

SO2 + ½ O2 SO3 dH = -98.9 kJ

2 SO3 2 SO2 + O2 dH = ?

Hess’s Law

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

Thermochemistry Lab Calculations

Goal: What is H for the formation of MgO from Mg(s) and O2(g)?

Mg(s) + ½ O2(g) MgO(s) H = ? kJ/molData:From lab measurements:

Mg(s) + 2 H+(aq) Mg2+(aq) + H2(g) H1 = ___________ kJ/mol

MgO(s) + 2 H+(aq) Mg2+(aq) + H2O(l) H2 = ___________ kJ/mol

From a table: H2(g) + ½ O2(g) H2O(l) H3 = -285.8 kJ/mol

Task: Find a way to add these three reactions to get the desired reaction. Manipulate the H values as needed, and add them.

Calculating Heat Production

Heat of Formation

Heat of Formation: The general idea

Find the enthalpy change for burning ethyl alcohol