Download Honors Chemistry Chapter 17 Student Notes

Honors Chemistry
Chapter 17 Notes - Thermochemistry
(Student’s edition)
Chapter 17 problem set: 43,47, 62, 63, 68, 73, 75, 84-86
Chapter 17 figures:
17.1
2, 8-11
The Flow of Energy – Heat and Work
Energy: the ability to do
Formula:
W=
Forms of Energy:
macroscopic particles - relatively large particles that don’t need a microscope to
energy.
be seen - they possess
potential energy -
- available to work at a later time
kinetic energy - energy of
nonmechanical energy - includes chemical, electrical,
electromagnetic (radiant), sound, magnetic, heat.
some is kinetic, some potential (chemical, magnetic)
SI unit for energy - joule (J) (capital - named for James P. Joule - British)
The Law of Conservation of 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
Law of Conservation of Energy - energy
ordinary chemical reaction.
be created or destroyed in an
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:
H2(g)
+
O2 (g)
reactants

products

sometimes reactants have more energy, sometimes products
H2 (g)
+
O2 (g)

heat is given off (
) and the products have less energy than the reactants
Exothermic Graph:
25
ΔH = Products – Reactants
heat 15
Ea = Activation Energy
5
time
2 H2O (l)
+
572 kJ
Endothermic Graph:
2 H2 (g)
+
O2 (g)
25
ΔH = Products – Reactants
heat 15
Ea = Activation Energy
5
time
simpler examples:
H2O (l)

H2O (g)
ΔH is
The reaction is
heat
time
C8H18(l)
+
12 ½ O2(g)

8 CO2(g)
+
9 H2O(g)
+
ΔH is
The reaction is
heat
time
activation energy - minimum energy to get a chemical reaction
ex. gas exploding
Heat and Temperature:
Heat is a form of energy.
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
Answer:
Question 2: Which of the above has more heat?
a. They are equal
b. The gallon of boiling water
c. The drop of boiling water
Answer:
Heat depends on
. It is an
property.
Temperature is measured in degrees. Itt 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
Temperature scales:
°F
of the total energy.
. It usually contains Hg or alcohol.
°C
K
Water Boils
212
100
373
Body Temp
98.6
37
310
Water Freezes
32
37
273
Absolute Zero
-460
-273
0
Celsius - devised by a Swedish astronomer - Anders Celsius – 1742
Kelvin - named for Lord Kelvin - English physicist
Absolute Zero: temperature at which all molecular motion
reached, but we have gotten within 1/1000 K.
temperature = 0 K or -273.15 Co
particles are in constant motion - faster particles have
at absolute zero - there is no
, therefore no energy
higher temperatures =
- never been
energy
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 =
o
F =
o
C =
Examples:
Ex1 Convert 100.0 oC to K
Ex2 Convert 293.0 K to oC
Ex3 Convert 90.0 oF to oC
Ex4 Convert 20.0 oC to oF
Heat and its measurement:
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
Amount of heat in a reaction depends on:
a)
b)
c)
in a chemical reaction
Specific Heat ( Cp ): the amount of energy required to raise the temperature of
q = (m) (Cp ) (t )
where t = tf - ti
or
Cp = q/ (m)(t)
q =
Cp = specific heat
m =
t = change in
For Water:
Cp = 1.000 cal/goC
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?
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?
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:
First find the heat gained (endothermic) by the water:
Now, find the specific heat of the metal sample (heat loss is exothermic):
method 2:
The Conservation of Energy Method (heat gained must equal heat lost):
+[(m)(Cp )(t)] = -[(m')(Cp')(t')]
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.
+[(m)(Cp )(t)] = -[(m')(Cp')(t')]
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
calories/g
% water, sugar less than
%
carbohydrates, proteins provide
calories/g
NIB -
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
electrical 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
electric current -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
and any other form of
energy.
, stored in
Substances also have
of the molecules).
The
energy (
of the energy is the heat content known as
change): H, heat
Heat of reaction (
Reactants
in a chemical reaction.
Products
Products
Reactants  Products +
The reaction is
.
,
Reactants
.
Reactants +
 Products
The reaction is
Heat of Formation (Hf):
Heat
when
mole of compound is
from its
.
Standard Heat of Formation (Hfo) – same as heat of formation, only at 1 atm and 25 oC
Ex.
2 H2
+
O2

2 H2O + 572 kJ
( not Hfo )
H2
+
½ O2 
H2O + 286 kJ
( yes! Hfo) Hfo =

½ O2
- 286 kJ
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?
Na
+
½ Cl2 
NaCl
Convert 11.50 g Na to moles Na to moles NaCl
then multiply by 411.0 kJ
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 changing phase.
the heating curve for water:
100 oC
Temp
0 oC
Energy
flat sections at boiling/melting:
why?
the heating curve for water:
100 oC
Temp
0 oC
Energy
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 ,
moles)
Heat of vap.
why?


needed to vaporize a unit of substance (mass or
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 = 2.2 x 103 J/g - higher - vaporizes
easily - lower
easily -
heat of condensation - same thing - opposite direction - heat is released
formula
q = (unit) (ΔHvap )
unit = gram or mole

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 released when changing a unit of substance from liquid to solid.
same formula: q = (unit)(Δ Hfus)
unit = g or mole
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?
17.4 Calculating Heats of Reaction
Hess’ Law
The overall enthalpy change in a reaction
enthalpy change is equal to the
individual steps in the process.
depend on the number of
. The
of the enthalpy changes for the
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) 
2CO2(g)
Hf
= ? kJ
Related Hess’ Law problem:
Hf reaction = Hf products - Hf reactants
Hf = ___ for an element.
Why?
Ex: CuO(s)
+
H2(g)

Cu(s)
+
H2O(g)
NIB Bond Energy Problems
Ex:
2 H2(g)
+
O2(g) 
2 H2O(g)