What happens to thermal (heat) energy?

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```					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
Energy Conversion Examples:
1. dropping a rock
2. using a flashlight
3. driving a car
SYSTEMS and SURROUNDINGS
System:        The thing under study
Surroundings:  Everything else in the universe

Energy transfer between system and surroundings:

Endo: heat added to system                     Exo: heat released by system
HEAT: What happens to
thermal (heat) energy?
Three possibilities:
• Warms another object
• Causes a change of state
• Is used in an endothermic reaction
Temperature Changes from Heat Exchange

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

q wood =

q Cu =

Specific heats: Cu = 0.385 J/goC Wood = 1.8 J/goC
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?
But what’s really happening?
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
Energy and Phase Changes:
Quantitative Treatment
Melting:

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

Boiling:

Heat of Vaporization (Hvap) 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)
Energy and Chemical Reactions

Lots of different types of energy.

We use Enthalpy:

Heat exchanged under constant
pressure.
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 exchanged
can 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 =
0.500 g N2H4
Total E =
600 g water
H = energy/moles =                               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

Enthalpy is a
State Function.
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/mol
Data:
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

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