Download Chapter 17 Notes

Honors Chemistry
Chapter 17 Notes - Thermochemistry
(Student edition)
Chapter 17 problem set: 43, 47, 62, 63, 68, 73, 75, 84-86
17.1
The Flow of Energy – Heat and Work
Energy: the ability to do
Formula:
W=
macroscopic particles - relatively large particles that don’t need a microscope to be seen they possess
energy.
potential energy -
- available to work at a later time
kinetic energy - energy of
nonmechanical energy electromagnetic (radiant), sound, magnetic, heat.
- includes chemical, electrical,
some is kinetic (heat, sound), some potential (chemical, magnetic)
SI unit for energy - joule (J) (capital - named for James P. Joule - British)
The Law of Conservation of Energy:
energy can be converted from one form to another.
example - food is chemical energy - converted to mechanical - limb motion
example - light energy enters our eyes - converted to electrical energy in nerves
- energy cannot be created
or destroyed in an ordinary chemical reaction.
exception - nuclear, But....if mass is considered as a form of energy through the equation
E = mc2, then the law really has no exceptions.
Energy and Chemical Reactions:
Chemical reactions involve:
2 H2(g)
+
O2 (g)

reactants

products
2 H2O (l)
sometimes reactants have more energy, sometimes products…
1
2 H2 (g)
+
O2 (g)

2 H2O (l)
+
heat is given off (
572 kJ
) and the products have less energy than the reactants
Exothermic Graph:
25
ΔH =
heat 15
Ea =
5
time
Endothermic Graph:
2 H2O (l)
→
+ 572 kJ
2 H2 (g)
+ O2 (g)
25
ΔH =
heat 15
Ea =
5
time
simpler examples:
H2O (l)
+
heat

H2O (g)
ΔH is
The reaction is
heat
time
C8H18(l)
+
12 ½ O2(g)

8 CO2(g)
+
9 H2O(g)
+
heat
ΔH is
The reaction is _________________
heat
time
activation energy - minimum energy to get a chemical reaction
. (match - gas explosion)
2
Heat and Temperature:
Heat is a form of
.
1 gallon of boiling water
1 drop of boiling water
Question 1: Which of the above has a higher temperature?
a. They are equal
b. The gallon of boiling water
c. The drop of boiling water
Question 2: Which of the above has more heat?
a. They are equal
b. The gallon of boiling water
Heat depends on
c. The drop of boiling water
. It is an
property.
Temperature is
in degrees. It is a measure of the
of the particles. Temperature does not depend on size (
Heat is
in Joules or calories. It is a
A thermometer works by
property).
of the total energy.
. It usually contains Hg or alcohol.
Temperature scales: know boiling point and freezing point of water, body temperature, etc.
Celsius - devised by a Swedish astronomer - Anders Celsius – 1742
Kelvin - named for Lord Kelvin - English physicist
Absolute Zero: temperature at which all molecular motion
- never been reached,
but we have gotten within 1/1,000,000,000 K. Temperature = 0 K or -273.15 Co
particles are in constant motion - faster particles have
energy
at absolute zero - there is no
, therefore no energy
higher temperatures =
energy
ex - hot glass on fingers - heat from the glass moves into the fingers the energy change is sensed
by the nerves
Conversion formulas:
K = oC + 273
o
F = 9/5(oC)
+ 32
C = 5/9(oF - 32) or….remember the rule of 40!
o
3
Heat and its calculation:
Heat transfers between objects example - ice cube in a thermos of hot water - ice melts, water cools - same amount of heat
SI unit of heat:
(
)
calorie is also used frequently
calorie: the amount of energy required to raise the temperature of
1.000 calorie =
Joules
Calorimetry: measurement of the heat
in a chemical reaction
Amount of heat in a reaction depends on:
a)
b)
c)
Specific Heat ( Cp ): the amount of energy required to raise the temperature of
q = (m) (Cp ) (t )
q = heat
where t = tf - ti
Cp = specific heat
For Water:
m = mass
Cp = 1.000 cal/goC
t = change in temperature
or 4.184 J/goC
sample problems:
example 1 - How many calories does it take to heat 20. g of water from 10.0 to
40.0 oC? also how many J?
4
example 2 - How much heat is required to heat 75 g of Iron (Cp = 0.444 J/g oC)
from 15.5 to 57.0 oC?
example 3 - What is the specific heat of an object if 250 calories will heat 55 g of
it from 25.0 to 100.0 oC?
example 4 - If a 100.0 g sample of silver (Cp = 0.237 J/g oC) at 80.0 oC loses 50.0
calories, what will its final temperature be?
5
example 5 - A 107 g sample of metal at 80.0 oC is placed in 122 g of water at 15.0 oC. The final
temperature of the system is 26.3 oC. Calculate the specific heat of the metal. This problem can be solved
using two methods:
method 1:
method 2:
example 6 - Calculate the final temperature of the system if 25.5 g of a metal (Cp = .975 J/g oC)
at 75.0 oC is placed in 125 g of water at 21.3 oC.
6
NIB - Energy values of different foods:
Why are some foods more fattening than others? Or… (said another way)
Why do some foods provide more energy than others?
Food types
- water, cellulose (fiber), vitamins, minerals - all provide
- carbohydrates, protein, fat - all provide
metabolism
- breaking down of digestible parts of food into
- this process releases
left over energy is stored as _______
some foods are high in water example - lettuce fat provides
NIB -
calories/g
% water, sugar less than
carbohydrates, proteins provide
%
calories/g
Interactions between electrical charges
2 kinds of charge - positive and negative
opposites
, likes
electrostatic forces - forces of attraction or repulsion
these forces change with amount of
charge is always conserved - law of conservation of energy
energy - potential energy created when charged bodies are moved against an
electrostatic force.
needs to be done to move opposite charges apart or like charges together
-flow of electrical charge - happens between 2 oppositely charged objects
metals - conductors
some
conduct
17.2 Measuring and Expressing Enthalpy Changes
Thermodynamics: the study of
.
Substances have energy stored in them…
It is stored in
, stored in
Substances also have
The
, and any other form of
energy (
of the energy is the heat content known as
energy.
of the molecules).
(
).
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change): H, heat
Heat of reaction (
in a chemical reaction.
Reactants
Products
Products
Reactants
Reactants  Products + heat
The reaction is
.
Reactants + heat  Products
The reaction is
.
Heat of Formation (Hf):
Heat
when
mole of
is formed from its
.
) – same as heat of formation, only at 1 atm and 25 oC
Standard Heat of Formation (
Ex.
2 H2
+
O2

2 H2O + 572 kJ
( not
H2
+
½ O2 
H2O + 286 kJ
( yes! Hfo) Hfo =

½ O2
)
reverse reaction:
H2O
+
286 kJ
H2
+
(it takes energy to split water)
Trick question time…
The Hfo for NaCl is – 411.0 kJ/mole. How much energy is absorbed when 11.50 g
of Sodium reacts with an excess of Chlorine?
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NIB Stability of Compounds:
A high negative Hf means that the compound is very
.
Why? The compound releases a lot of energy during its formation so it
takes just as much to break the compound apart.
A high positive Hf means that the compound is very
.
Why? The compound took quite a bit of energy to make the elements join
and will probably require very little activation energy to make the
compound break apart to go to lower energy.
Which is more stable: CS2(g), FeCl3(s), or SO3(g)?
+117 -399
-395
17.3
Heat in Changes of State ….
Temperature and Phase Change: It is usually assumed that more heat means higher
temperature, but not when
.
the heating curve for water:
100 oC
Temp
0 oC
Energy
flat sections at boiling/melting:
why?
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Heat of Vaporization:
when....
a liquid evaporates, it absorbs
energy is used to overcome
the energy doesn’t increase the average energy of the particles
so, the
doesn’t change
when...
a liquid evaporates, it takes energy from its
that’s why alcohol feels cool to the skin
its also why we get cold when getting out of the shower
Heat of vaporization, ΔHvap ,
needed to vaporize a unit of substance (mass or moles)
Heat of vap.
why?


as temperature increases
ΔHvap usually listed for boiling point
ΔH vap for alcohol = 8.6 x 102 J/g - lower - vaporizes
ΔH vap for water = 2.26 x 103 J/g - higher - vaporizes

easily - lower
easily -
heat of condensation - same thing - opposite direction - heat is released
formula
q=
unit =
Example: How much heat does it take to vaporize 50.0 g of water at 100.0 C0?
Example: If 300.0 g of liquid boils for 20.0 min and absorbs 2.26 x 105J, what is
the heat of vaporization if 200.0 grams of liquid remains?
Heat of Fusion
Δ Hfus = heat of fusion
The heat required to change a unit of substance from solid to liquid.
Δ H crystallization
The heat
when changing a unit of substance from liquid to solid.
same formula: q = (unit)(Δ Hfus)
unit = g or mole
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Example: How much ice can be melted by 2.9 x 104 J?
NIB
- Heat Calculations: Example: How much heat is required (in calories) to
transform 50.0 g of ice at - 10.0 °C to steam at 110.0 °C?
100 °C
Temp
0 °C
Energy
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17.4 Calculating Heats of Reaction
Hess’ Law - The overall enthalpy change in a reaction
depend on the
number of
. The enthalpy change is equal to the
of the
enthalpy changes for the individual steps in the process.
The steps (in English):
- Write equations to form compounds
- Multiply the equation by the number of moles needed for the equation
- Change the sign of H if the equation is written in the reverse
- Add up the H values to get the overall value for the equation
- Add up the equations to check your answer
Ex: Calculate the heat change using Hess’ Law:
2 CO(g)
O2(g) 
+
Hf
2CO2(g)
= ? kJ
2 equations must be written in order to include the molecules CO and CO2
C
C
½ O2 
O2

+
+
CO
CO2
we’ll need to switch the sign, reverse the equation, and multiply
we’ll need to multiply
Thus:
Related Hess’ Law problem:
Hf reaction =
Hf =
for an element.
Why? It doesn’t take energy to form an element (it’s been done).
Ex: CuO(s)
+
H2(g)

Cu(s)
+
H2O(g)
Hf Cu = 0 Hf H2 = 0 Hf CuO = - 155 kJ Hf H2O = - 242 kJ or
12
NIB Bond Energy Problems
Ex:
2 H2(g)
+
O2(g) 
2 H2O(g)
must draw correct Lewis pictures:
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