Chapter 17 - Thermochemistry
Download
Report
Transcript Chapter 17 - Thermochemistry
Chapter 5
“Thermochemistry”
Mr Daniel : OHS AP Chemistry
Credit to:
Charles Page High School
Stephen L. Cotton
Section 11.1
The Flow of Energy – Heat and Work
OBJECTIVES:
• Explain how energy, heat, and work are
related.
• Classify processes as either exothermic or
endothermic.
• Identify the units used to measure heat
transfer
• Distinguish between heat capacity and
specific heat.
2
Energy Transformations
Thermochemistry – study of heat changes that
occur during chemical and physical changes.
Energy - capacity for doing work or supplying
heat
• weightless, odorless, tasteless
• Energy stored within chemical substances is
called chemical potential energy
Gasoline contains a significant amount of
chemical potential energy
3
Energy Transformations
Heat (“q”) is energy that transfers from one
object to another, because of a temperature
difference between them.
• only changes of heat can be detected!
• flows from warmer cooler object
All chemical reactions and changes in physical
state involve either:
a) release of heat, or
b) absorption of heat
4
Exothermic and Endothermic Processes
When studying heat changes there are two
important categories:
• the system - the part of the universe on
which you focus your attention (a test tube,
a room, a molecule, the Earth etc.)
• the surroundings - includes everything else
in the universe
Together, the system and it’s surroundings
constitute the universe
Exothermic and Endothermic Processes
Thermochemistry
is concerned with the flow
of heat from the system to it’s surroundings,
and vice-versa.
The Law of Conservation of Energy states
that in any chemical or physical process,
energy is neither created nor destroyed.
• All the energy is accounted
for as work, stored energy,
or heat.
Exothermic and Endothermic Processes
Exothermic and Endothermic Processes
Exothermic:
Heat flowing out of a system into
it’s surroundings
• defined as negative change
q has a negative value
• The system loses heat
as the surroundings heat up
Exothermic and Endothermic
Every reaction has an energy change
associated with it. For example, the…
** Gummy Bear Sacrifice!! ** (next week!!!)
Exothermic reactions release energy, usually in
the form of heat.
Endothermic reactions absorb energy
Chemical Energy is stored
in the form of bonds
between atoms.
9
Units for Measuring Heat Flow
1) A calorie is defined as the quantity of heat
needed to raise the temperature of 1 g of pure
water 1 oC.
a Calorie, (written with a capital C), is 1000
calories and refers to the energy in food
(100 Calorie candy bar is 100,000 calories)
1 Calorie = 1 kilocalorie = 1000 calories
10
Units for Measuring Heat Flow
2) The Joule is the SI unit of heat and energy
and is related to the calorie:
4.184 J = 1 cal
Named after James Prescott Joule
11
Heat Capacity and Specific Heat
Heat Capacity (Cp) - the amount of heat
needed to increase the temperature of an
entire object exactly 1 oC
Cop = Joules/oC
Heat capacity is
dependent on both
the object’s mass and
its chemical
composition
Heat Capacity and Specific Heat
Specific
Heat Capacity (c) - the amount of heat
it takes to raise the temperature of 1 gram of
the substance by 1 oC
Often called simply “Specific Heat”
c = Joules/ gram oC
13
Table of Specific Heats
Heat Capacity and Specific Heat
Water has a HUGE specific heat value, when is
compared to other chemicals:
C (H2O) = 4.18 J/(g oC), or
C (H O) = 1.00 cal/(g oC)
2
• It heats slowly and
• It cools slowly
Consequently, water maintains a relatively
constant temperature!
15
Heat Capacity and Specific Heat
Heat calculations for various materials are done
with the formula:
q = mass (g) x c x T
• q = change in heat
• c = specific heat
• T = change in temperature
Units are either J/(g oC) or cal/(g oC)
16
- Page 299
Section 11.2
Measuring and Expressing
Enthalpy Changes
OBJECTIVES:
• Describe how calorimeters are used to
measure heat flow.
• Construct thermochemical equations.
• Solve for enthalpy changes in chemical
reactions by using heats of reaction.
18
Calorimetry
Calorimetry - the precise measurement of the
heat into or out of a system for chemical and
physical processes.
The device used to measure the absorption
or release of heat in chemical or physical
processes is called a Calorimeter
Calorimetry
Foam cups are excellent heat insulators, and
are commonly used as simple calorimeters
For systems at constant pressure, the heat
content is the same as a property called
Enthalpy (H) of the system
A foam cup
calorimeter –
here, two
cups are
nestled
together for
better
insulation
Calorimetry
Changes in heat = Changes in enthalpy = H
q = H These terms are often used
interchangeably
Thus, q = H = m x c x T
H is negative for an exothermic process
H is positive for an endothermic process
Calorimetry
Calorimetry experiments can be performed at a
constant volume using a device called a “bomb
calorimeter” - a closed system
• Used by nutritionists to measure energy
content of food
A bomb calorimeter
A Bomb Calorimeter
Energy
C + O2 → CO2 + 395 kJ
C + O2
395kJ
CO2
Reactants
Products
∆ H= -395 kJ
25
Exothermic
In
an exothermic rxn the products are
lower in energy than the reactants and
energy is released.
ΔHrxn = -395 kJ
• The negative sign indicates that
energy is released.
26
CaCO
→ +CaO
CaCO
CaO
CO2+ CO2
3 + 176
3 → kJ
Energy
CaO + CO2
176 kJ
CaCO3
Reactants
Products
∆H = + 176 kJ
27
Endothermic
The products are higher in energy than the
reactants and energy is absorbed.
ΔHrxn = +176 kJ
• The positive sign indicates that energy is
absorbed
28
Heat of Reaction
A
heat of reaction is the heat change for the
equation.
• The physical state of reactants and
products must also be given.
• Standard conditions for the reaction is
101.3 kPa (1 atm.) and 25 oC (different
from STP)
29
Thermochemical Equations
An equation that includes energy is called a
thermochemical equation
CH4 + 2O2 CO2 + 2H2O + 802.2 kJ
• 1 mole of CH4 releases 802.2 kJ of energy.
• When you make 802.2 kJ you also make
2 moles of water
30
CH4 + 2 O2 CO2 + 2 H2O + 802.2 kJ
If
10. 3 grams of CH4 are burned
completely, how much heat will be
produced?
10. 3 g CH4
1 mol CH4
16.05 g CH4
802.2 kJ
1 mol CH4
= 514 kJ
Thus, ΔH = - 514 kJ for this reaction
31
Summary, so far...
Enthalpy
Enthalpy: The heat content a substance has at
a given temperature and pressure
Can’t be measured directly because there is no
set starting point
The reactants start with a heat content
The products end up with a heat content
So we can measure how much enthalpy
changes
H (delta H) is the measure of change in heat
33
Enthalpy
Enthalpy: Amount of heat in a system (H)
.
If heat is released, H is negative (exothermic)
If heat is absorbed, H is positive (endothermic)
34
Energy
Change is down
ΔH is <0
Exothermic
Reactants
Products
35
Energy
Change is up
ΔH is > 0
Endothermic
Reactants
Products
36
Heat of Reaction
Heat of Reaction: The heat that is released or
absorbed in a chemical reaction
Equivalent to H
C + O2(g) CO2(g) + 393.5 kJ
C + O2(g) CO2(g)
H = -393.5 kJ
In a thermochemical equation, it is important to
indicate the physical state
H2(g) + 1/2O2 (g) H2O(g) H = -241.8 kJ
H2(g) + 1/2O2 (g) H2O(l) H = -285.8 kJ
37
Heat of Combustion
The heat from the reaction that completely burns
1 mole of a substance:
C + O2(g) CO2(g) + 393.5 kJ
C + O2(g) CO2(g)
H = -393.5 kJ
38
Section 17.3
Heat in Changes of State
OBJECTIVES:
• Classify the enthalpy change
that occurs when a
substance melts, freezes,
boils, condenses, or
dissolves.
39
Section 17.3
Heat in Changes of State
OBJECTIVES:
• Solve for the enthalpy change that
occurs when a substance melts,
freezes, boils condenses or dissolves.
40
Heat in Changes of State
Heat
of Fusion (Hfus) : the heat absorbed
by a substance in melting from a solid to
a liquid
For H2O: Hfus = 334 J/ g
Molar Hfus = 6.01 kJ/ mol
q = mass x Hfus (there is no temperature change)
41
Molar
Heat of Solidification (Hsolid) =
heat lost when one mole of liquid
solidifies (or freezes) to a solid
For H2O: Hfus = -334 J/ g
Molar Hfus = -6.01 kJ/ mol
q = mass x Hsolid (no temperature change)
Heat in Changes of State
Heat
absorbed by a melting
solid is equal to heat lost
when a liquid solidifies
• Thus, Hfus = -Hsolid
Note Table 17.3, page 522
43
- Page 521
Heats of Vaporization and Condensation
When
liquids absorb heat at their boiling
points, they become vapors.
Molar Heat of Vaporization (Hvap) = the
amount of heat necessary to vaporize
one mole of a given liquid.
q = mass x Hvap (no temperature change)
For H2O: Hfus = 2260 J/ g
Molar Hfus = 40.7 kJ/ mol
45
Heats of Vaporization and
Condensation
Condensation
is the opposite of
vaporization.
Molar Heat of Condensation (Hcond)
= amount of heat released when
one mole of vapor condenses to a
liquid
Hvap = - Hcond
46
Heats of Vaporization and Condensation
Note Figure 17.10, page 523
The large values for water Hvap and
Hcond are the reason hot vapors such
as steam is very dangerous
• You can receive a scalding burn from
steam when the heat of condensation
is released!
H20(g) H20(l)
Hcond = - 40.7kJ/mol
47
-Page 524
Heat of Solution (Ch 11)
Heat
changes can also occur when
a solute dissolves in a solvent.
Molar Heat of Solution (Hsoln) =
heat change caused by dissolution
of one mole of substance
Sodium hydroxide provides a good
example of an exothermic molar
heat of solution:
49
Heat of Solution
NaOH(s)
H2O(l)
Na1+(aq) + OH1-(aq)
Hsoln = - 445.1 kJ/mol
The heat is released as the ions
separate (by dissolving) and
interact with water, releasing 445.1
kJ of heat as Hsoln -thus becoming
so hot it steams!
50
- Page 313
52