Transcript PowerPoint 5.1 - The First Law and Enthalpy

Catalyst
 Complete the exam reflection sheet using the data from the exam.
Remainder of the Year
Monday
Tuesday
Wednesday
Thursday
Friday
15th
16th
Unit 5
Unit 5
NO SCHOOL!
NO SCHOOL!
19th
20th
21st
Unit 5
Unit 5
Unit 5
26th
27th
28th
29th
30th
Unit 5
Unit 5
Unit 5
Unit 5
Unit 5
3rd
Review and
Quest
4th
Review and
Quest
5th
6th
7th
Review
Review
Review
10th
MOCK AP
EXAM
11th
MOCK AP
EXAM
12th
Review Exam
13th
14th
Last day of classes
Whoosh Bottle
Justify – TPS
 Why is heat generated during this reaction:
CH3CH2OH + 3O2  3H2O + 2CO2
Lecture 5.1 – 1st Law of
Thermodynamics and Enthalpy
Today’s Learning Targets
 LT 5.1 – I can discuss the energy associated with a system using the ideas of
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work and heat.
LT 5.2 – I can compare and contrast the idea of system and surrounding
when examining a given substance
LT 5.3 – I can discuss the First Law of Thermodynamics, how it relates to the
quantitative description of the energy of a system, and how it describes
endothermic/exothermic reactions.
LT 5.4 – I can define what a state function is and apply this definition to
thermodynamic problems
LT 5.5 – I can relate the 1st law of thermodynamics to the concept of
enthalpy and I can apply this concept to chemical reaction and
endothermic/exothermic.
What is Energy?
 Energy is the capacity of a system
to do work and/or transfer heat
 All processes require some form of
energy
 Energy is classified as being either:
 Kinetic Energy – The energy
of motion
 Potential Energy – The energy
associated based on an objects
position.
Two Energy Forms: Work and Heat
 Work is the energy used to cause
an object to move against a force
WorkForceDistance
 Heat is the energy used to cause
the temperature of an object to
increase
 ALWAYS transfers from one
object to another object

Two Types of Energy: Kinetic and Potential
 There are both energy of motion and energy of location
 Kinetic Energy is energy of motion and is calculated by:
1
2
KE 
mv
2
 Potential Energy is energy based on location/position of an
object. Bonds have energy based on composition.
 All energy is measured in Joules and can be converted to
calories:
1 calorie = 4.184 J
1 Cal = 1 kcal = 1000 calories
System and Surrounding
 When studying the movement of energy we need to define
system and surroundings
 System – The portion that we will be studying
 Surrounding – Everything else that is not be studied
Types of Systems
 Open System – Matter and Heat can be exchanged
 Closed System – Heat can be exchanged, but matter cannot
 Isolated System – No exchange of heat and matter
1st Law of Thermodynamics
 1st Law of Thermodynamics states that energy cannot be
created or destroyed, therefore it must be conserved.
 Used to analyze energy changes in chemical systems
 An increase in energy of system leads to an equal decrease of
surroundings, and vice versa
Internal Energy (E)
 Internal Energy is the sum of all kinetic and potential energies
of the components of a system.
 We are concerned with the change in internal energy (ΔE)
E  E final  Einitial
Esurroundings Esystem Euniverse 0
 Efinal > Einitial indicates system gained from surroundings and ΔE is
positive
 Efinal < Einitial indicates system lose to the surroundings and ΔE is
negative

Heat, Work, and the 1st Law
 ΔE is exchanged in the form of either heat or work
 If you remove heat (q < 0) and/or work is done by the system (w
< 0) , then ΔE is negative
 If you add heat (q > 0) and/or work is done by the system (w >
0), then ΔE is positive
Sign Conventions
 When the system increases, then values are positive
 When the system loses, the values are negative
Heat, Work, and the 1st Law
 We, therefore, can describe the internal energy by:
ΔE = q + w
You must decide on signs for q and w!
Class Example
 Gases A (g) and B (g) are confined in a cylinder with a moveable
piston. These gases react by the following reaction:
A (g) + B (g)  C (s)
As the reaction occurs, the system loses 1150 J of heat to the
surroundings. The piston moves downward as the gases react to
form a solid. As the volume of the gas decreases under the
constant pressure of the atmosphere, the surroundings do 480 J of
work on the system. What is the change in internal energy?
Table Talk
 Calculate the change in the internal energy for a process in which
a system absorbs 140 J of heat from the surroundings and does 85
J of work on the surroundings.
Relay Races
 Two points back on your exam to the winning team!
Relay Race Questions
 What does the 1st Law of Thermodynamics State
 You add heat to a glass of water. Define system and surrounding in
this scenario.
 Work is done by the system. Should w be positive or negative?
 Calculate ΔE when q = 0.763 kJ and w = -840 J. Is it
endothermic or exothermic?
 Calculate ΔE when a system releases 66.1 kJ of heat to its
surroundings while the surroundings do 44.0 kJ of work on the
system
Justify – TPS
 Using the idea of heat, system and surrounding, why was it
beneficial for the three men to cuddle?
Endothermic/Exothermic
 When heat is transferred to a system from the surroundings, it is
endothermic
Flow of Heat
 When heat is transferred from a system to the surroundings, it is
exothermic
Flow of Heat
Road Trip!
5280 ft
524 ft
For each route,
what is the
change in
altitude?
State Functions
 State functions are values that depend only on the present state of
the system, not on the path the system took to reach that state
 ΔE is a state function, but q and w are not
 This means that if q is increased, then w decreases by the same
amount
 Think of q and w being the different paths taken
 Other state functions are T, P, and V
Enthalpy
 We combine three state functions (P, V, and E) to create the state
function of enthalpy that is used to describe the flow of energy
into/out of a system
H = E + PV
 The change in enthalpy equals the heat gained or lost at constant
pressure
 See derivation on the board
ΔH = qP
Meanings of Enthalpy Values
 +ΔH = system gained energy; endothermic process
 -ΔH = system lost energy; exothermic process
Class Example
 Indicate the sign of ΔH in these processes carried out under
atmospheric pressure and indicate whether it is exothermic or
endothermic:
 An ice cube melts
Table Talk
 Indicate the sign of ΔH in these processes carried out under
atmospheric pressure and indicate whether it is exothermic or
endothermic:
 1 g of butane is combusted in sufficient oxygen to give complete
combustion to CO2, H2O, and a release of energy
1st Law Advertisement
 You are working for an advertisement company and some
scientists have asked you to create an advertisement for the first
law
 Your clients have required that you include the following
terms/ideas:
 Energy
 Work
 Heat
 System/Surrounding
 Internal Energy
 Enthalpy
 State Functions
Exit Ticket
Closing Time
 Read 5.1 – 5.3
 Do book problems: 5.3, 5.4, 5.6, 5.9, 5.15, 5.25, 5.26, 5.27,
and 5.28