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Lecture 1: Energy and Enthalpy

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Need to differentiate between the system and surroundings. ... Chemical process in which system evolves resulting in heat transfer to the surroundings ... – PowerPoint PPT presentation

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Title: Lecture 1: Energy and Enthalpy


1
Lecture 1 Energy and Enthalpy
  • Reading Zumdahl 9.1 and 9.2
  • Outline
  • Energy Kinetic and Potential
  • System vs. Surroundings
  • Heat, Work, and Energy
  • Enthalpy

2
  • Energy is the capacity to do work or to produce
    heat
  • Energy is conserved, it can neither be created
    nor destroyed, different forms of energy
    interconvert
  • However, the capacity to utilize energy to do
    work is limited (entropy)

3
Energy Kinetic vs. Potential
  • Potential Energy (PE)
  • Energy due to position or chemical composition
  • Equals (mgh) in example.
  • Kinetic Energy (KE)
  • Energy due to motion.
  • Equals mv2/2 in example.

4
Mechanical Energy KE PE
  • Energy is the sum of kinetic energy and potential
    energy.
  • Energy is readily interconverted between these
    two forms.
  • If the system of interest is isolated (no
    exchange with surroundings), then total energy is
    constant.

5
Example Mass on a Spring
  • Initial PE 1/2 kx2
  • At x 0
  • PE 0
  • KE 1/2mv21/2kx2
  • Units of Energy
  • Joule kg.m2/s2
  • Example
  • Init. PE 10 J
  • M 10 kg
  • Vmax 2(PE)/M1/2 1.4m/s

0
6
Energy Kinetic vs. Potential
  • Potential Energy (PE)
  • Energy due to position or chemical composition
  • Equals (mgh) in example.
  • Kinetic Energy (KE)
  • Energy due to motion.
  • Equals mv2/2 in example.

7
First Law of Thermodynamics
  • First Law Energy of the Universe is Constant
  • E q w
  • q heat. Transferred between two bodies of
    differing temperature. Note q ? Temp!
  • w work. Force acting over a distance (F x d)

8
Applying the First Law
  • Need to differentiate between the system and
    surroundings.
  • System That part of the universe you are
    interested in (i.e., you define it).
  • Surroundings The rest of the universe.

9
Conservation of Energy
  • Total energy is conserved.
  • Energy gained by the system must be lost by the
    surroundings.
  • Energy exchange can be in the form of q, w, or
    both.

10
Heat Exchange Exothermic
  • Exothermic Reaction. Chemical process in which
    system evolves resulting in heat transfer to the
    surroundings
  • Heat flows out of the system
  • q lt 0 (heat is lost)

11
Another Example of Exothermic
12
Heat Exchange Endothermic
  • Endothermic Reaction Chemical process in which
    system evolves resulting in heat transfer to the
    system
  • Heat flows to the system
  • q gt 0 (heat is gained)

13
Another Example of Endothermic
14
  • In exothermic reactions, the potential energy
    stored in chemical bonds is converted into
    thermal energy (random kinetic energy), i.e. heat
  • Once we have done that, we have lost the ability
    to utilize the same potential energy to do work
    or generate heat again (dissipation)

15
Energy and Sign Convention
  • If system loses energy
  • Efinal lt Einitial
  • Efinal-Einitial DE lt 0.
  • If system gains energy
  • Efinal gt Einitial
  • Efinal-Einitial DE gt 0.

16
Heat and Work Sign Convention
  • If system gives heat
  • q lt 0 (q is negative)
  • If system gets heat
  • q gt 0 (q is positive)
  • If system does work
  • w lt 0 (w is negative)
  • If work done on system
  • w gt 0 (w is positive)

17
Example Piston
  • Figure 9.4, expansion against a constant external
    pressure
  • No heat exchange
  • q 0
  • System does work
  • w lt 0

(adiabatic)
18
Example (cont.)
  • How much work does the system do?
  • Pext force/area
  • w force x distance
  • Pext x A x Dh
  • Pext DV
  • w - Pext DV (note sign)

19
  • When it is compressed, work is done to a gas
  • When it is expanded, work is done by the gas
    (e.g. your cars engine)

20
Example 9.1
  • A balloon is inflated from 4 x 106 l to 4.5 x 106
    l by the addition of 1.3 x 108 J of heat. If the
    balloon expands against an external pressure of 1
    atm, what is DE for this process?
  • Ans First, define the system the balloon.

21
Example 9.1 (cont.)
  • DE q w
  • (1.3 x 108 J) (-PDV)
  • (1.3 x 108 J) (-1 atm (Vfinal -
    Vinit))
  • (1.3 x 108 J) (-0.5 x 106 l.atm)
  • Conversion 101.3 J per l x atm
  • (-0.5 x 106 l.atm) x (101.3 J/l.atm)
    -5.1 x 107 J

22
Example 9.1 (cont.)
  • DE (1.3 x 108 J) (-5.1 x 107 J)
  • 8 x 107 J (Ans.)
  • The system gained more energy through heat than
    it lost doing work. Therefore, the overall
    energy of the system has increased.

23
Definition of Enthalpy
  • Thermodynamic Definition of Enthalpy (H)
  • H E PV
  • E energy of the system
  • P pressure of the system
  • V volume of the system

24
Why we need Enthalpy?
  • Consider a process carried out at constant
    pressure.
  • If work is of the form D(PV), then
  • DE qp w
  • qp - PDV
  • DE PDV qp
  • qp is heat transferred at constant
    pressure.

25
Definition of Enthalpy (cont.)
  • Recall H E PV
  • DH DE D(PV)
  • DE PDV (P is constant)
  • qp
  • Or DH qp
  • The change in enthalpy is equal to the heat
    transferred at constant pressure.

26
Changes in Enthalpy
  • Consider the following expression for a chemical
    process
  • DH Hproducts -
    Hreactants
  • If DH gt0, then qp gt0. The reaction is
    endothermic
  • If DH lt0, then qp lt0. The reaction is
    exothermic

27
Enthalpy Changes Pictorially
  • Similar to previous discussion for Energy.
  • Heat comes out of system, enthalpy decreases (ex.
    Cooling water).
  • Heat goes in, enthalpy increases (ex. Heating
    water)
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