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Energy
Energy is the capacity to do work
 Work = F d
o F – force
o d – distance
 Remember from physics, work is only done when there is motion
Forms of Energy
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KE – Kinetic Energy, energy of motion, KE = ½ mv2
o Mechanical energy - the KE of an object is calculated from mass and
velocity
o Thermal energy – temperature it a measure of the average KE of particles
o Electrical energy – motion of electrons
PE – potential energy
o Gravitational energy - as a result of height, PE = mgh
o Electrostatic energy - potential energy of particles depends on the location
and charges of particles
Chemical Energy – energy stored in atoms due to their arrangement in molecules
or ions
First Law of Thermodynamics – The combined amount of matter and energy in the
universe is constant
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Conservation of Matter
o When Balancing chemical equations - the total number of atoms is equal
on each side
Law of Conservation of Energy – Energy is neither created nor destroyed in
chemical reactions and physical changes
o The difference in internal energy is lost (exothermic) to the surroundings
or gained (endothermic) from the surroundings, but the total amount of
energy is constant
System
 Identify the system vs. the surroundings
 Open system – energy and matter can be transferred
 Closed system – energy, but not matter can be transferred
 Isolated system – neither energy or matter can be transferred
State Functions
State Function – property having a unique value when the state of the system is defined
 Temperature
 Pressure
 Volume
 Composition
 Physical state (solid, liquid, gas)
 Internal Energy
 Enthalpy – heat content
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Extensive: depends on the amount of substance
o Mass
o Volume
o Enthalpy
Intensive Property: does not depend on the amount of substance
o Density
o Melting Point
o Boiling Point
o Specific heat
Change in State Function
 Change is always (final value – initial value)
 The intermediate values are not described
 Ex: for temperature
∆ T = Tfinal – T initial
Units of Energy
Joule = Newton • meter
calorie = heat required to increase the temp of 1 gram of water by 1 degree C
1 calorie = 4.184 Joules
1 Calorie = 1 kcal = 1000 calories
Enthalpy (H)
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Think of enthalpy as chemical potential energy
The quantity of heat (Q) transferred into or out of a system is stored as enthalpy
o In a closed system, Q + Δ Hrxn = 0
 When Q>0, things have gotten hot (Q = mcp∆T)
 If Q>0, then that heat energy came from the chemical reaction
 Therefore, some of the chemical potential energy was converted to
heat
 So, the enthalpy (stored chemical energy) of the system decreased
 In other words, Δ H <0
Heat is NOT a state functions
Heat is NOT stored, the energy transferred is stored
Δ H = H final – H initial
For a chemical reaction: Δ H = H products – H reactants
Enthalpy Changes in Chemical Reactions
o
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o
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For a chemical reaction, the products and the reactants are “the system”
Δ H rxn is called the Heat of Reaction
For a chemical reaction: Δ H rxn = H products – H reactants
The heat of reaction is the heat absorbed or given off during a chemical reaction
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ΔH>0
o Endothermic
o H products > H reactants, more stored chemical energy in the products
o Since Q > 0, heat must enter the system from the surroundings,
making the surroundings colder
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ΔH<0
o Exothermic
o H products < H reactants, more stored chemical energy in the reactants
o Since Q < 0, heat will leave the system, making the surroundings
hotter
Heat Calculations w/ Chemical Reactions
1. SiH4 (g) + 2 O2 (g) → SiO2 (s) + 2 H2O (l) + 1516 kJ
1516 kJ of heat is released when 1 mole of SiH4 reacts with 2 moles of O2 to produce
1 mole of SiO2 and 2 moles of H2O. How much heat is released if 15.0 g of H2O is
formed?
Solution:
15.0 g H2O
1 mole H2O
18.01 g H2O
1516 kJ
2 mol H2O
=
631 kJ
2. When 10.0 g of methane burns in an excess of oxygen gas, 503 kJ of heat is released.
Write the balanced chemical equation including the energy.
Solution:
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O(g) + 807 kJ
503 kJ
10.0 g CH4
16.04 g CH4
1 mole CH4
=
807 kJ
1 mole CH4
Calorimetry
Using the temperature change of a known amount of substance (usually water) to determine
the energy changes in a chemical or physical reaction.
Lab experiments you have done with a calorimeter:
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Calculation of heat of fusion of water
Calculation of heat of solution
Insulated – no heat transfer
Calculate ΔHrxn (this is a direct method)
o Q rxn + Q water + Q calorimeter = 0
o ΔH rxn + m cp ΔT + C calorimeter ΔT = 0
o Choose a rxn w/ no gaseous product which can escape, usually an
aqueous rxn
o Now use to calculate ΔHrxn
o The energy change during the chemical reaction will cause the temp of
the water to change