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AP/IB Chemistry
Chapter 6 Notes Part 1 – Energy Definitions and Review

What is Energy?
o Share ideas… What is energy? What does it do? How can it be defined? Etc.

Energy in Action. Notes on videos...
o Energy in reactions helps to explain why physical and chemical changes occur

Energy – “Textbook” Definitions
o Energy = capacity to do work or produce heat
o Two forms of mechanical energy
 Potential energy = energy resulting from position or composition
 Kinetic energy = energy resulting from motion
o When energy is transferred into and out of systems, it can have two main effects
o Energy can be transferred as heat or work.
 Generally when heat is transferred, a change in temperature will take place

Energy – an operational definition (a Model approach)
o Energy is a conserved, substance-like quantity that is always involved when a system
undergoes change.
o Guiding Principles:
1. Energy can be viewed as a substance-like quantity that can be stored in a physical
system.
2. Energy can “flow” or be “transferred” from one system to another and so cause
changes.
3. Energy maintains its identity after being transferred.
Energy Storage
o View energy storage as different “accounts” used by the system to store the energy.
 Thermal energy, Eth – energy of motion. The quantity of thermal energy stored
by an object is related to both its mass and velocity.
 Interaction energy, Ei – energy stored in the system due to the arrangement of
molecules that exert attractions on one another.
 Chemical potential energy, Ech- energy due to attractions of atoms within
molecules.
o Etotal = Eth + Ei + Ech


Energy Transfer
W
o Working (referred to as work by the physicists as if it is something different from
energy) is the way in which energy is transferred between macroscopic (large enough
to be seen) objects that exerts forces on one another.
Q
o Heating (referred to as heat by the chemists) is the way in which energy is transferred
by the collisions of countless microscopic objects.
o Energy is always transferred from the “hotter” object (one in which the molecules
have greater Eth) to a colder one (one in which the molecules have lower Eth).
R
o Radiating is the process in which energy is transferred by the absorption or emission
of photons (particles of light).
o Energy Transfer in chemistry is usually associated with a change in temperature
 Temperature = measurement of average kinetic energy of particles (random
motions)
o Temperature is NOT a measure of total energy.
E = Q + W + R

Representing Energy Storage and Transfer – LOL charts
o LOL charts are a representational tool.
o Use to help build a conceptual understanding of energy storage and transfer.
o Energy transfer and storage concepts will be quantified later.
o To study energy storage and transfer, we must separate the system from the
surroundings
o System = the part of the universe we are concerned with; the reactants, the physical
object, etc.
o Surroundings = everything else in the universe; container or lab area
Using LOL Charts
1. Draw the LOL chart. Decide what the system is and write it in the circle.
2. Represent the initial energy storage of the system. Conventions?
3. Represent the transfer of energy into, out of, or within the system using arrows.
4. Represent the final energy storage of the system. Make sure energy is conserved!
o Examples. Work through the following. I’ll do the first one with you.
1.
A cup of hot coffee cools as it sits on the table.
2.
A can of cold soda warms as it is left on the counter.
3.
A tray of water (20 ˚C) is placed in the freezer and turns into ice cubes (- 8 ˚C)
4.
One of the ice cubes described in #3 is placed in a glass of room temperature (25
˚C) soft drink. Do separate bar charts for the ice cube and the soft drink.
Describe how the arrangement and the motion of the molecules in each system
change from the initial to the final state

More with Energy Conservation:
o What’s not so nice with the following “textbook” definition?
 Energy cannot be created or destroyed, but only converted to other forms.
o What’s a better way to describe energy conservation using our model?
o Mathematical statement of energy conservation:
o ∆E = q + w + r
o What does this mean in words?
o How can ∆E be measured???

Measuring Energy Transfer
o Work = a force acting over a distance
o Chemistry focuses on Pressure-Volume work
 This is the work that is done by or on a sample of gas (like in your car!)
o PV work does not affect temperature (temp must remain constant!)
o PV work is calculated using the equation
w = -P∆V
o
w = work
P = pressure DV = change in volume
o As gas expands, it does work on the surroundings
o When gas contracts, work is being done by the surroundings.
o Can focus on changes without radiation so that ∆E = qp + w
o PV work done by expanding gas is not easy to measure, so chemists use the chang in
enthalpy, denoted ∆H, to describe energy transferred at constant pressure:
o ∆H = qp
o Terminology Discussion. Let’s review:
 Q - what you can calculate
 E - what you’d really like to discuss
 ∆H - what you can discuss
o Often used interchangeably in the textbook. Keep in mind benefits of each one.
o
o
o
o
o
During changes, energy can flow one of two ways.
Exothermic process = heat energy flows out of the system; energy is given off
These processes “feel” warm
Endothermic reaction = heat energy flows into the system; energy is absorbed
These processes “feel” cold
AP/IB Chemistry
Chapter 6 Notes Part 2 – Energy associated with Reactions

Energy Flow in Chemical Reactions
Exothermic Reactions
o In exothermic reactions, heat is evolved.
o We consider heat to be one of the products.
o Example – Adding HCl to Mg ribbon generates heat as a product.
Endothermic Reactions
o In endothermic reactions, heat is absorbed by the system from the surroundings.
o We consider heat to be a reactant.
o Examples:
o Decomposition of copper(II) hydroxide
o Solvation of ammonium chloride

Path of Energy Flow
o The amount of energy associated with a chemical reaction does not depend on the
pathway by which the reaction occurs.
o State function = any property that does not depend on pathway.

Using the “Enthalpy” Concept
o Enthalpy (H) = energy per mole of a reaction
o
H = E + PV
o We are concerned with the change in enthalpy (DH) = the change in heat per mole of
a reaction; measured in kJ / mol
o Enthalpy is a state function.
o The change in enthalpy (DH) can be determined by subtracting the enthalpy values of
the reactants from the products.
o The enthalpy change for the reaction is calculated using the equation
o
DH = SHproducts - SHreactants
o Enthalpy values of reactants and products can be looked up in tables. Elements in
natural state have enthalpy values of zero.
o But how can enthalpy values be determined in the first place???
o Enthalpy values are not always known, therefore we have other methods for
calculating DH.

Calorimetry
o Calorimetry is a branch of chemistry devoted to measuring the change in heat
associated with reactions.
o Calorimeter = instrument used to measure the change in temperature that
accompanies a chemical reaction.
o Before a calorimeter can be used, it is important to know how different substances
respond to change in heat.
Heat Capacity
o Specific heat capacity (s) = amount of heat that one gram of a substance must absorb
for the temperature to be raised by 1oC; unit J/goC
o Molar heat capacity = amount of heat that one mole of a substance must absorb for
the temperature to be raised by 1oC; unit J/moloC
o To determine the specific heat of a substance use the equation
s = Q / (m * DT)
s = specific heat
Q = energy (in Joules)
m = mass (grams)
DT = change in temp (oC)
o Try this problem:
It takes 78.2 J to raise the temperature of 125.6 g of Hg from 20.0oC to 53.5oC.
Calculate the specific and molar heat.

Constant Pressure Calorimetry
o To calculate DH for a reaction when pressure is constant we must know
o Specific heat capacity of solution (s)
o Mass of solution (m)
o Change in temperature (DT)
o Knowing these three things, we use the mathematical model:
Q = s * m * DT
Q = energy transferred (J)
m = mass (g)
S = specific heat (J/goC)
DT = change in temp (oC)
o Energy must then be converted to kJ and related to the moles of substance involved in
the reaction.

Constant Volume Calorimetry
o Enthalpy change can also be calculated for reactions when volume is held constant.
o
ΔE = q + w, since V is constant, w = 0 so
o
ΔE = q = qv
o To find energy change, simply multiply the heat capacity of calorimeter by the
change in temperature.
o Not used that often

Units of Enthalpy
o Why is enthalpy measured in kJ / mol?
o Dividing by the number of moles adjusts for the fact that the amount of heat produced
depends on the quantity of reactants used.
o Heat is an extensive property, one that depends on size of sample
o Other properties, like temperature, are intensive and do not depend on size of sample.
AP/IB Chemistry
Chapter 6 Notes Part 3 – Hess’s Law and Enthalpy of Formation
6.3

Hess's law - the change in enthalpy for a change from a particular set of reactants to a particular set of
products is the same regardless of the number of steps involved.
- statement indicates that ΔH is a state function (independent of pathway)
a. Characteristics of Enthalpy Changes
- if a reaction is reversed the sign of the enthalpy change must be reversed
- the magnitude of ΔH is directly proportional to the quantities of reactants and products, i.e.
if a reaction is doubled, ΔH must also be doubled(an extensive property).
6.4

Standard Enthalpies of Formation (ΔHºf) - the change in enthalpy that accompanies the formation of
one mole of a compound from its elements with all substances in their standard states.
a. standard states :
- for compounds
- for gaseous substances, the pressure is exactly 1 atm
- substances must be pure
- solutions must have a concentration of 1 M
- for elements the standard state is that state at which the element normally exists under the
conditions of 1 atm and 25 ºC.
ΔHº reaction   Hº f products -  Hº f reactants
b. Calculation of ΔHºf :
- elements are not included in these calculations because they require no change in form (ΔHºf = 0)