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Definitions: recall physics
 Energy

(E):
The ability to do work; measured in Joules (J)
 Work:

Amount of energy applied or transferred over a
distance
 Potential

Energy of object due to its position or composition;
stored energy
 Kinetic

Energy (Ep):
Energy (Ek):
Energy of object due to its motion; movement
energy
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Definitions: recall physics
 Thermal

Total quantity of kinetic & potential energy within a
substance, Ep + Ek = H
 Heat

Energy (H):
(q):
The transfer of thermal energy from a warm body
to cooler body
 Temperature:

A measure of the average kinetic energy of
particles in a substance/object
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Definitions: new ones
 Chemical

System:
The group of reactants & products being studied
 Surroundings:

The environment in which the chemical reaction
takes place; substance that is not part of the
reaction
 Open

System
Matter & Energy able to leave system
 Closed

System
Energy able to leave system
 Isolated

System
Neither matter nor energy able to leave system
System and Surroundings
universe = system + surroundings
• the system is the sample being observed
• the surroundings is everything else
• interactions between a system and its surroundings involve exchange of
energy and matter
Three types of systems based on this type of exchange are:
can exchange
energy and
matter with
surroundings
can exchange energy,
not matter, with
surroundings
cannot exchange
matter or energy with
surroundings
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Calorimetry
Measuring energy changes
in a reaction
 We
measure the ΔT
of the surroundings
and calculate the
thermal energy, q,
lost or gained by
the surroundings
-qsurroundings = +qsystem
q = mcΔT
Calculating the Amount of Heat
Entering and Leaving a System
Measurable properties of a system include:
volume, mass, pressure, temperature, and specific heat capacity.
A calculated property of a system includes heat (Q) that enters or
leaves an object.
ΔT = Tfinal – Tinitial
When heat enters a system, ΔT is positive
and so is Q.
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Specific Heat Capacity,(c);
LEARNING CHECK
A 1.0 g sample of copper is heated from 25.0°C to
31.0°C.
How much heat did the sample absorb?
The First Law of Thermodynamics:
Energy is Conserved
The first law of thermodynamics states that:
Energy can be converted from one form to another but cannot be
created or destroyed.
Since any change in energy of the universe must be zero,
ΔEuniverse = ΔEsystem + ΔEsurroundings = 0
ΔEsystem = –ΔEsurroundings
• if a system gains energy, that energy comes from
the surroundings
• if a system loses energy, that energy enters the
surroundings
USING THE FIRST LAW OF THERMODYNAMICS
A 400 g iron rod is heated in a bunsen flame and then plunged into an
insulated beaker containing 1.00 L of water (1000 g). The original
temperature of the water was 20ºC. After the iron rod and the water
have reached the same temperature, it is 32.8ºC. (specific heat
capacity of iron is 0.45 J/gºC , and water is 4.18J/gºC) What was the
original temperature of the iron rod?
Enthalpy
One way chemists express thermochemical changes is by a variable
called enthalpy, H. The change in enthalpy, ΔH, of a system can be
measured.
It depends only on the initial and final states of the system, and is
represented by
ΔH = ΔE + Δ(PV)
For reactions of solids and liquids in solutions, Δ(PV) = 0
If heat enters a system
• ΔH is positive
• the process is endothermic
If heat leaves a system
• ΔH is negative
• the process is exothermic
The Second Law of Thermodynamics
The second law of thermodynamics states that:
When two objects are in thermal contact, heat is
transferred from the object at a higher temperature to
the object at the lower temperature until both objects
are the same temperature (in thermal equilibrium)
When in thermal contact, energy
from hot particles will transfer to
cold particles until the energy is
equally distributed and thermal
equilibrium is reached.
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Definitions: new ones
 Enthalpy
(H): another term for thermal energy
 Enthalpy Change (ΔH): the change in thermal
energy from reactants to products; this equals q
under constant pressure:
ΔHsystem = qsystem
ΔHsystem = Hproducts – Hreactants
Where qsurroundings is the measurable quantity
 Endothermic:
 intake
of energy, -qsurroundings & +qsystem & + ΔH
 Exothermic:
 release
of energy, +qsurroundings & -qsystem & - ΔH
Comparing Categories of Enthalpy Changes:
Enthalpy of Solution
Three processes occur when a substance dissolves, each with a ΔH
value.
1. bonds between solute molecules or ions break
2. bonds between solvent molecules break
3. bonds between solvent molecules and solute molecules or ions form
Sum of the enthalpy changes: enthalpy of solution, ΔHsolution
The orange arrow
shows the overall ΔH.
Heat must be added to or removed from a substance in order for the
phase of the substance to change.
The ΔH for each phase change has a particular symbol.
For example, ΔHmelt is called the enthalpy of melting.
The ΔH for one
phase change is
the negative of the
ΔH for the opposite
phase change.
Thermochemical Equations
and Calorimetry
Chemical reactions involve
• initial breaking of chemical bonds (endothermic)
• then formation of new bonds (exothermic)
ΔHr is the difference
between the total
energy required to break
bonds and the total
energy released when
bonds form.
Thermochemical Equations
Thermochemical equations include the enthalpy change.
Exothermic reaction:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + 890.8 kJ
Endothermic reaction:
N2(g) + 2O2(g) + 66.4 kJ → 2NO2(g)
The enthalpy term can also be written beside the equation.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
N2(g) + 2O2(g)
→ 2NO2(g)
ΔHr = –890.8 kJ
ΔHr = +66.4 kJ
Enthalpy Diagrams
Enthalpy diagrams clearly show the relative enthalpies of reactants and
products.
• For exothermic reactions, reactants have a larger enthalpy than
products and are drawn above the products.
• For endothermic reactions, the products have a larger enthalpy and
are drawn above the reactants.
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Molar Enthalpy Change ΔHn:
 Molar
Enthalpy Change (ΔHn): enthalpy change
associated with any change (physical, chemical
or nuclear) in one mole of a substance; J/mol or
kJ/mol
ΔHrxn = nΔHn
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Common Molar Enthalpies
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Example:
 Ethanol
is used to disinfect skin before
receiving a flu shot. How does this feel on
your skin? The enthalpy of vaporization
(ΔHvap)of ethanol is 38.6 kJ/mol. What is
the enthalpy change (ΔHrxn) of this
reaction if 1.0g of ethanol is rubbed on
your skin?
LEARNING CHECK
Determine ΔHcomb for 15.0 g of propane.
The molar mass of propane: 44.1 g/mol
Therefore, 15.0 g is 0.340 mol
ΔHcomb = ΔH°comb
ΔHcomb = (0.340 mol) (–2219.2 kJ/mol)
ΔHcomb = 755 kJ/mol
The symbol for a standard enthalpy change is ΔH°, read as "delta H standard" or,
perhaps more commonly, as "delta H nought“. The standard enthalpy change of a
reaction is the enthalpy change which occurs when equation quantities of materials
react under standard conditions, and with everything in its standard state (SATP (25° C
and 100 kPa))
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Expressing Molar Enthalpy
Changes:
1.
As part of chemical equation:
2.
Associated ΔH value:
3.
Potential Energy Diagram:
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3) Potential Energy Diagram: