Thermochemistry

1
Chapter 6

• Thermochemistry

2
Thermochemistry

• Deals with the energy changes
• Chemical reactions
• Determines whether a reaction is exothermic or
  endothermic
• Physical Changes
• Energy changes

3
Vocabulary

• Energy - The ability to do work (E) in J
• Potential Energy Energy of position (PE)
• Kinetic Energy Energy of motion (KE)
• Temperature Random motion of particles
• Heat Transfer of energy (q) in J
• Work Force x Distance (W) in J
• Enthalpy Energy Change for reactions
• (?H) in J

4
Vocabulary

• Pathway How energy is transferred
• State Function Properties that depend on
  current state. (Independent of pathway)
• Energy is a state function
• Temperature, Volume, Pressure
• Heat and Work are not state functions

5
Example of Energy Change
6
Endothermic Reactions

• Energy is put into a reaction
• Product bonds are weaker than reactant bonds

http//www.bbc.co.uk/scotland/education/bitesize/h
igher/img/chemistry/calculations_1/pe_diags/fig10.
gif
7
Exothermic Reactions

• Energy is released from a reaction
• Product bonds are stronger than reactant bonds

http//www.bbc.co.uk/scotland/education/bitesize/h
igher/img/chemistry/calculations_1/pe_diags/fig03.
gif
8
Signs for Exo vs. Endo.
9
Calorimetry

• Measuring heat associated with a reaction
• Uses a calorimeter
• Coffee Cup Calorimeter
• At constant pressure q -?H
• qms ?T
• q heat, m mass, ?T Change in Temp
• sspecific heat capacity

10
(No Transcript)
11
Specific Heat Capacity

• Energy required to heat one gram of a substance 1
  degree Celcius
• Units J/gºC
• Molar Heat Capacity Just per mole
• Metals have low heat capacities

12

• Example
• How much energy is required to heat
  55.3 grams of Aluminum metal from 22.0ºC to 100.0
  ºC? Specific heat of water is 4.18 J/gºC
  the specific heat of aluminum is 0.89 J/gºC

13

• Example
• 46 p. 282

14
Homework

• P. 281 s 32, 37,40, 43, 48

15
Hesss Law

• In going from a particular set of reactants to a
  particular set of products, the change in
  enthalpy (?H) is the same whether the reaction
  takes place in one step or in a series of steps
• Energy is a state function

16
Comparison

• One step
• N2(g) 2O2(g) ? 2NO2(g) ?H1 68kJ
• Two step
• N2(g) O2(g) ? 2NO(g) ?H2 180kJ
• 2NO(g) O2(g) ? 2NO2(g) ?H3 -112kJ
• N2(g) 2O2(g) ? 2NO2 (g) ?H2 ?H3 68kJ

17
Hesss Law Rules

• Start with the end reaction in mind and work
  backward rearrange your equations
• Work to cancel like terms
• When reversing the equation the sign of ?H must
  be changed
• When multiplying an equation by a coefficient ?H
  must be multiplied by the same coefficient

18

• Example 54 p. 283
• NH3? ½ N2 3/2 H2 ?H 46 kJ
• 2H2 O2 ? 2H2O ?H -486 kJ
• 2N2 6H2O ? 3O2 4NH3 ?H ?
• Based on enthalpy is this a useful synthesis?

19

• Example 52 p. 283
• C4H4 5O2 ? 4CO2 2H2O ?H -2341kJ
• C4H8 6O2 ? 4CO2 4H2O ?H -2755kJ
• H2 ½ O2 ? H2O ?H -286kJ
• C4H4 2H2 ? C4H8 ?H ?

20
Homework

• P. 283 s 51,53,55,57

21

• Calculate how much energy (kJ) is required to
  heat 50.0 L of tap water from 10.0 ºC to 45.0 ºC.
  Assume the density of water is 1.00 g/mL and
  specific heat of water is 4.18 J/g ºC
• 7320 kJ
• A McDonalds hamburger contains 250.
  Calories (thats 2.50x105 calories). How many
  burgers would it take to supply the same amount
  of energy? 1 calorie
  4.18 J
• 7.00 burgers

22
Enthalpy of Formation

• The change in enthalpy that accompanies the
  formation of one mole of a compound from its
  elements with all elements in their standard
  state
• Symbol ?Hfº
• f is formation
• Degree symbol means standard states

23
Standard States

• Standard States
• Temperature is 25 ºC
• Pressure 1.00 atm
• Concentration 1.00 M
• The standard state is how something exists at
  these conditions
• O2(g), Na(s), Br2(l)

24
Reactions

• 4C(s) 2H2(g) ? CH4(g) ?Hfº -75 kJ/mol
• H2(g) ½ O2(g) ? H2O(l) ?Hfº -286 kJ/mol
• Write the enthalpy of formation reaction for
  gaseous carbon dioxide. Dont worry about the
  value
• C(s) O2(g) ? CO2(g)
• Values are in Appendix 4 (A21)
• ?Hfº -393.5 kJ/mol

25
Enthalpy of Formation Values

• Most values are negative
• Compounds are more stable than element
• All elements are zero
• Even diatomic elements!
• States matter
• Liquid water is -286
• Gaseous water is -242
• Use this information to calculate enthalpy of the
  reaction!

26
(No Transcript)
27
Standard Enthalpy of Reaction

• Standard Enthalpy of Reaction is the energy
  change at standard conditions
• Symbol ?Hº
• ?Hº S ?Hfº(Products) - S ?Hfº(Reactants)
• Remember to include coefficients
• Remember elements are zero!

28

• Calculate the standard enthalpy of reaction for
  the following
• Al(s) Fe2O3(s) ? Al2O3(s) Fe(s)
• Al 0, Fe 0, Fe2O3 -826, Al2O3 -1676

29
Homework

• P. 284 63