Thermochemistry - Dr. VanderVeen
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Transcript Thermochemistry - Dr. VanderVeen
Thermochemistry
Unit theme: Energy
Thermochemistry
Heat capacity
Specific heat
Units of heat
Potential energy diagrams
Endothermic and exothermic reactions
enthalpy
Heats of changes of state
calorimetry
Thermochemical equations
Hess’ Law
calorie
joule
Gibb’s Free energy
entropy
What evidence can you see
that energy is involved?
Energy
Defined as the capacity to do work or
supply heat
Chemical potential energy:
aka chemical energy
Energy stored in chemicals because of their
compositions
Different substances store different amount of
energies
Heat
A form of energy that flows from a warmer
object to a cooler object
Q
Can’t be measured directly
Can only measure its effect on temperature
When heat is added to a system, its
temperature rises
Temperature is not heat!
Temperature is a measure of average kinetic energy
Thermochemistry
The study of heat changes that occur
during chemical reactions and physical
changes of state
Units of Heat
calorie
The quantity of heat needed to raise the
temperature of 1 g of pure water by 1 degree
Celsius
1 kcal = 1000 calories = 1 Calorie (nutrition)
joule
SI unit, named after British physicist
newtonmeter
1 cal = 4.186 joules
1000 J = 1 kJ
Commonly used because joules are so small
Heat Capacity
The amount of heat it takes to change an
object’s temperature by 1 Celsius degree
Depends partly on mass
More mass greater heat capacity
Problem
How many calories are required to heat
32.0 g of water from 25.0oC to 80.0oC?
How many joules is this?
Answer:
1760 calories
7360 joules = 7.36 kJ
Specific Heat
Not all substances
respond the same
way to the input of
heat
Some get hot much
more quickly than
others!
Specific Heat
The amount of heat required to raise the
temperature of 1 g of a substance by 1oC
A measure of how well a substance
stores heat energy
Substances with low specific heat (ex.
metals) heat quickly, cool quickly
Substances with high specific heat (ex.
water) take a long time to heat and cool
Using Specific Heat
C or Cp
Units: J/goC or cal/goC
Q = m C T
where
Q = total heat change
m = mass
T = temperature change
Phase Changes
Melting, freezing
Boiling, condensing
Sublimation, deposition
Phase changes occur without
temperature changes
Calculating Energy Changes
in Phase Changes
Can’t use specific heat, because no
temperature change is involved
Instead, use this formula
Q = m Heat of fusion (or heat of
vaporization)
use heat of fusion for freezing, melting
use heat of vaporization for boiling,
condensation
Heating/Cooling Curves
Regions A, C, E
have temperature
change
Q = mCT
Choose specific heat
to match state of
matter
Regions B, D are
phase changes
B: Q = mHfus
D: Q = mHvap
Enthalpy of the reaction
The total energy change associated with
a chemical or physical change
Given the symbol Hrxn
Hrxn = (energy of products) – (energy of reactants)
Classifying Heat Changes
Thermite reaction
Produces molten iron
Formerly used in
welding railroad
tracks, shipbuilding
Gives off light and
heat!
Highly exothermic
Exothermic reactions
Energy is released to
the surroundings
The temperature of
the surroundings
increases
The products have
less energy than the
reactants
Endothermic Reactions
Energy is absorbed
from the
surroundings
The temperature of
the surroundings
decreases
The products have
MORE energy than
the reactants
Thermochemical
Equations
Exothermic reactions
Energy released by system
Can treat energy as a product
H is negative
2 equivalent ways to write equation:
CaO + H2O Ca(OH)2 + 65.2 kJ
CaO + H2O Ca(OH)2
H = - 65.2 kJ
Thermochemical
Equations
Endothermic reactions
Energy absorbed by system
Can treat energy as a reactant
H is positive
2 equivalent ways to write equation:
2 NaHCO3 + 129 kJ Na2CO3 + H2O + CO2
2 NaHCO3 Na2CO3 + H2O + CO2
H = + 129 kJ
2 ways to manipulate
thermochemical equations
1) Write equation backwards
Sign of H must change
A+BC
H = + 123 kJ
CA+B
H = - 123 kJ
2) Multiply everything by a coefficient
Must multiply H by coefficient, too!
3 A + 3B 3C H = 3 (+123 kJ) = + 369 kJ
Problems with
thermochemical equations
Consider the equation:
2 NaHCO3 Na2CO3 + H2O + CO2
H = + 129 kJ
How many kJ would be released if 4.5
moles NaHCO3 reacted?
Hess’ Law
If you add two or more thermochemical
equations to give a final equation, then
you can also add the heat changes to
give the final enthalpy of reaction.
Example
What is the enthalpy change, Hrxn, for
the decomposition of hydrogen peroxide?
Target: 2 H2O2(l) 2 H2O(l) + O2(g)
Given:
H2(g) + O2(g) H2O2(l)
H2(g) + ½ O2(g) H2O(l)
H = -187.9 kJ
H = -285.8 kJ
Standard Heats of
Formation, Hof
The standard heat of formation of a
compound is the change in enthalpy that
accompanies the formation of one mole
of a substance from its elements in their
standard states.
The heat of formation of elements in their
standard states is arbitrarily set to zero.
Using heats of formation
Thermodynamic stability: a measure of the
energy required to decompose the compound
Compounds with large, negative enthalpies of
formation are thermodynamically stable
Many heats of formation have been measured.
(see Appendix A-6 in textbook)
Another way to do Hess’ Law!
Hrxn = SnHf(products) – SmHf(reactants)
where
n represents the coefficients for the products
m represents the coefficients for the reactants
Example Problems
Compute Hrxn for the following reaction.
(Refer to Appendix A-6)
2NO(g) + O2(g) 2 NO2(g)
Compute Hrxn for the following reaction.
(Refer to Appendix A-6)
4 FeO(cr) + O2(g) 2 Fe2O3(cr)
Ans: -144.14 kJ, -560.0 kJ
Entropy
Symbol: S
A quantitative measure of the degree of
disorder in a system
The greater the disorder, the larger the value
of S
Solids have a high degree of order (low entropy)
Gases have a low degree of order (high entropy)
More particles (moles) results in higher entropy
Entropy, cont.
Systems tend to proceed to higher
disorder
Examples
Stirring sugar into your coffee
The neatness of your locker
The order of cards in a pack of playing cards
after shuffling
J. Willard Gibbs
1839-1903
First American to
earn a Ph.D. in
science from a US
university
Yale, 1863
One of nation’s best
scientists
Will a reaction occur
spontaneously?
The answer depends on the balance
between enthalpy (heat changes) and
entropy
Gibb’s Free Energy
The energy available from the system to do
useful work
Gibb’s Free Energy
G = H – TS
If G is negative, the reaction will occur
spontaneously and can proceed on its own.
If G is positive, the reaction is
nonspontaneous and needs a sustained
energy input to proceed.
If G is zero, the reaction is at equilibrium
(both the forward and reverse reactions take
place!)