IB CHEMISTRY: Topics 5/15 – ENERGETICS
*Opening Reflection Questions:
1. Consider the following changes: ice melting, water evaporating, water vapour condensing,
photosynthesis, respiration, and combustion of gasoline. Classify these changes as absorbing or
releasing thermal energy.
2. Based on your current understanding of energy, how is electrical power produced in Ontario?
What are the sources of energy that produce this power?
3. How is nuclear power different from hydroelectric power? How is it similar?
5.1 EXOTHERMIC AND ENDOTHERMIC REACTIONS
IMPORTANT TERMS:
 Thermodynamics - the study of energy and energy transfer
 THERMOCHEMISTRY - the study of energy changes accompanying matter and its transformations
 LAW OF CONSERVATION OF ENERGY(or mass) - total energy of the universe is ___________; energy can
neither be _____________ nor ______________ [E=mc2…mass is just “frozen” energy]
 Chemical SYSTEM - the _____________ and ____________ of a chemical reaction; represented by a chemical
equation
 SURROUNDINGS - the system’s _________________
 Universe = System + Surroundings
 KINETIC ENERGY(Ek)
the energy associated with motion [Ek = ½ mv2]….i.e.. a massive object travelling at a high speed has lots of
kinetic energy
THERMAL ENERGY
- a type of kinetic energy
- an object possesses thermal energy
 HEAT (q)
refers to the amount of ___________ energy ______________ between substances; heat is transferred, not
possessed;
- heating an object = transferring thermal energy to a substance and thereby increasing the kinetic energy of
the molecules
 TEMPERATURE
- a measure of the ___________________ kinetic energy of the particles that make up a substance or system
[measured in °C or Kelvin (K)]
- Water molecules, in a sample of water for example, possess a wide range of kinetic energies…because they
are all moving at different speeds. Temperature = the average of all of these kinetic energies.
*Consider the following: A glass of water at 80°C has a higher temperature than a swimming pool full of water at 20°C,
but the pool has more thermal energy than the glass of water. Explain.
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HEAT TRANSFER AND ENTHALPY CHANGE (add POE)
*The HEAT TRANSFER FORMULA:
q=
i.e., the quantity of heat transferred depends on THREE things:
1. the _______________ of substance present
2. the ___________ of substance involved [i.e. the SPECIFIC HEAT CAPACITY (c) of the substance]
*c = the quantity of heat required to raise the temperature of a unit mass (1 gram or 1 kg) of a substance
_____°C (_____ Kelvin) (e.g. The specific heat capacity, c, of water is 4.18 J g-1 K-1 OR 4.18 kJ kg-1 K-1.)
Explain what this means in one sentence:
3. the _________________change
Q. When 600 g of water in an electric kettle is heated from 20°C to 95°C to make a cup of tea, how much
heat is transferred into the water?
MORE TERMS:
 ENTHALPY(H)
– the total internal energy of a substance at constant pressure (the sum of many different forms of energy, both kinetic
and potential, present in a chemical system)
i)
ii)
iii)
iv)
v)
These include the energies of:
moving _______________ within atoms
the vibration of _______________ connected by chemical bonds; and
the rotation and translation of molecules that are made up of these atoms.
the _________________ potential energy of protons and neutrons in atomic nuclei; and
the electronic potential energy of atoms connected by chemical bonds.
*It is IMPOSSIBLE to measure enthalpy of a chemical system!!
*However, it IS POSSIBLE, to determine enthalpy ___________________!...and this is largely what this unit is
about…the different methods that can be used to determine enthalpy changes…i.e.determining the net amount of energy
either released or absorbed when a physical, chemical or nuclear change occurs.
 ENTHALPY CHANGE (∆H)
- the energy _____________ or ___________ to the surroundings when a system changes from ___________
to ____________
*Enthalpy changes result from chemical bonds and/or intermolecular forces(sources of stored chemical potential energy)
being __________and __________. The formation of a bond ________________ energy; breaking a bond
_________________energy.
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 ENDOTHERMIC CHANGE
net absorption of energy by the chemical system (i.e. the energy absorbed in the breaking of bonds is
____________ than the energy released in the formation of product bonds). For an endothermic change the
temperature of surroundings ______________ and the sign of ∆H is ____________________.
 EXOTHERMIC CHANGE
net release of energy by the chemical system (i.e. the energy absorbed in the breaking of bonds is
__________ than the energy released in the formation of product bonds). For an exothermic change the
temperature of surroundings _____________ and the sign of ∆H is ____________________.
MORE ON SIGN CONVENTIONS:
 Enthalpy changes for exothermic reactions are given a ____________ sign. The negative sign refers to the fact that
energy has _____________ the system; the energy of the system is decreasing.
 Enthalpy changes for endothermic reactions are given a ____________ sign. (i.e. Energy has been ______________
by the system; the energy of the system has increased.)
Exothermic:
Endothermic:
Blue line (Ek) = change to surroundings
Red line (∆H) = change to system
*When a change occurs in a system, the chemical potential energy
change, ∆H, is numerically __________ to the heat (q) transferred to
the _________________. (e.g. If 50 kJ of energy is ______ by the
system, 50 kJ of energy is _________ by the surroundings and vice
versa.) This is represented mathematically by the following formula:
∆Hsystem =
 q surroundings
SUMMARY:
Sign of
∆Hsystem
Potential
energy of
System
Stability of
System
Kinetic Energy
of
Surroundings
Temperature of
Surroundings
Bonding
EXOTHERMIC
ENDOTHERMIC
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REPRESENTING ENTHALPY CHANGES(SUMMARY)
The FOUR METHODS:
1. Thermochemical Equations with Energy Terms
2. Thermochemical Equations with ∆H values
3. Molar Enthalpies of Reaction
4. Potential Energy Diagrams:
Ex. 1  An EXOTHERMIC PROCESS (Respiration i.e. combustion of glucose)
*Method 1:
*Method 2:
*Method 3:
*Method 4:
Ex. 2  An ENDOTHERMIC PROCESS (Photosynthesis i.e. reverse of respiration reaction)
*Method 1:
*Method 2:
*Method 3:
*Method 4:
*More Practice(Nelson Text ) pp. 319-320 #1-5; p. 320 (bottom) #1-3 (4 – challenge)
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TYPES OF ENTHALPY CHANGES THAT MATTER CAN UNDERGO:
1. __________________ changes
 usually no chemical bonds are broken (no new substance produced)
Examples:
i) change of state:
ii) dissolving:
iii) dissolving:
H2O(l) + energy  H2O(g)
2
CaCl2(s) 

 CaCl2(aq) + energy
OR
CaCl2(aq) 
H O
NH4Cl(aq) + energy 
2. _________________ changes
 chemical bonds between atoms are rearranged resulting in new substances
Examples:
i) *combustion of propane:
ii) acid- base *neutralization:
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) + energy
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l) + energy
*combustion & neutralization = always____________________
3. ________________ changes
 a change in the protons or neutrons in an atom, resulting in the formation of new atoms
Examples:
i) nuclear fission(atom “splitting”):
238
92
U + n0  24 He +
ii) nuclear fusion(atoms fusing into larger atoms): 12 H +
2
1
234
90
Th + energy
H  24 He + energy
*Enthalpy changes associated with each of these changes:
Change
Enthalpy Change Range (kJ/mol)
Physical
100 to
Chemical
102 to
Nuclear
*1010 to
*E = mc2 ; where m = mass & c = 3 x 108 (i.e. the speed of light)
 Therefore, a small amount of _______ contains a massive amount of __________. Mass is really just “frozen” energy.
This equation describes the amounts of energy evolved via nuclear _____________ reactions in the ________.
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MOLAR ENTHALPIES
*MOLAR ENTHALPY, (∆Hx)  the enthalpy change associated with a physical, chemical, or nuclear change involving
_______ mole of a substance (The variable,x , indicates the type of change occurring .)
EXAMPLES:
REACTION/CHANGE
TYPE OF RXN/CHANGE
EXO OR
ENDO?
∆Hx NOTATION
Br2(l)  Br2(g)
H2O(s)  H2O(l)
H2O(l)  H2O(s)
CO2(g)  CO2(s)
KCl(aq)  K+(aq) + Cl-(aq)
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l)
NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l)
Al2O3(s)  2Al(s) + 3 O2(g)
2
Mg(s) + O2(g) + H2(g)  Mg(OH)2(s)
PRACTICE:
Ex. 1: Combustion of hydrogen
H2(g) + ½O2(g)  H2O(g) + 285.8 kJ
**∆Hcombustion = -285.8 kJ/mol
i) How much energy would be released to the surroundings if:
- 2 moles of hydrogen were combusted? ________________
- 6.2 moles? ___________________
*Equation used:
(*For this example, the subscript x = __________________)
Ex. 2: Vapourization of water:
H2O(l) + 40.8 kJ  H2O(g)
**∆Hvapourization = +40.8 kJ/mol
i) How much energy does the water absorb from the surroundings if:
- 3 moles of water are vapourized? ____________
*Equation used:
*Try the following:
1. When 4.0 g of methane burns in oxygen, 200 kJ of heat is evolved. Calculate the amount of heat (in kJ) evolved when
1 mol of methane burns.
2. Hydrazine, N2H4(l), is used in rocket fuel. The thermochemical equation for the combustion of hydrazine is:
N2H4(l) + O2(g)  N2(g) + 2H2O(l) + 622.4 kJ
What quantity of heat is liberated by the combustion of 1.00 g of N 2H4(l)?
3. Glucose, C6H12O6(s), is converted into ethanol, C2H5OH(l), in the fermentation of fruit juice to produce wine:
C6H12O6(s)  2C2H5OH(l) + 2CO2(g) + 67.0 kJ
What quantity of heat is liberated when a 0.500 L of wine containing 47.5 g of C 2H5OH(l) is produced?
4. Determine the quantity of heat required to completely vaporize 5.4 g of water.
*More Practice(Nelson Text)  p. 308 #1-3
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