EXAM 1

1
EXAM 1

• Class average 84
• Exams will be handed back at the end of
  classgrade is on page 2.
• Answer key is on class website.
• If you have questions, e-mail me.
• If you need to see me, come during office hours
  on Thursday or e-mail me to set-up an appointment.

2
Chapter 4 Energy
Chem 100 Lecture 7 February
15, 2005
3
What are the issues?

• Many of our problems result from our increased
  use of fossil fuels
• Air pollution
• Global warming
• Limited supply of fossil fuels
• Shortages have a significant impact on our lives
• Ex In 1970s, during the Arab oil embargo,
  rationing occurred
• Could only buy gas on even or odd days
• Lots of stations closed on weekends
• Locks on gas tanks

4
How much energy we get from a chemical reaction?

• What is Energy?
• Weve already talked about energy
• E hv light energy can be used to break bonds
• More generally, energy exists as work or heat
• Work movement against an opposing force
• Work (force) x (distance moved against the
  force)
• Examples of forces?

5
More definitions

• Heat Molecular motion.
• Hotter objects have greater molecular motion
• Heat is that which flows from a hotter to a
  colder object
• Temperature Molecular speedometer
• The higher the temperature, the faster the
  molecules are moving
• Determines the direction of heat flow (heat
  always flows from the object at higher
  temperature to the one at lower temperature

6
Units

• Metric system unit of energy The joule (J)
• Typical unit of heat calorie
• 1 calorie is the amount of heat necessary to
  raise the temperature of 1 g of water, 1oC
• 1 calorie 4.184 J

7

• Heat and work are different forms of energy, but
  we can easily convert back and forth.
• Example How much work can you accomplish after
  eating a donut (and releasing the heat stored in
  the donut as calories)?
• To solve this problem, we will set
• Work Heat

8
How much work can you do after eating a donut?

• How high a mountain can you climb?
• You will need to move your body against the force
  of gravity
• W F x d
  
• Force (F) mass of body x pull of gravity F mg
• W m x g x d

9
Given information

• 1 donut 425 kcal
• 1 kcal 1 C 1 food calorie 1000 calories
• The person weighs 70 kg (150 lbs)
• Assumption
• Digestion does not require any calories
• Goal
• How high a mountain can we climb? (our answer
  should be a distance)

10
Lets do it!

• First set Heat Work
• (Remember that W m x g x d)
• Heat m x g x d

11

• First law of thermodynamics
• Energy is neither created nor destroyed
• However, energy SOURCES are consumed

12
Sources of energy

• Wood
• Coal
• Natural gas
• Oil
• Nuclear (discussed later)
• Alternative energies
• Hydropower
• Solar
• Wind

13
How do we get energy from fuels?

• Combustion
• Ex CH4 2O2 ? CO2 2H2O energy

This reaction is said to be exothermic (any
chemical or physical change that is accompanied
by the release of heat).
14
Heat of combustion

• Heat of combustion heat evolved when a
  specified amount of a substance burns in O2.
• Typically expressed as kJ/mole or kJ/g
• It has been shown experimentally that the energy
  given during the combustion of 1 mole of methane
  is 802.3 kJ.
• What is this in kJ/g?

15
Exothermic Reactions
The fact that heat is evolved implies Potential
energy of reactants gt products (conservation of
energy)

• Change in energy is calculated
• Energy final energy initial energy
• DEnergy (energy of products) (energy of
  reactants)

The change in energy of ALL exothermic reactions
is negative
16
Endothermic Reactions

• Endothermic reaction (any chemical or physical
  change that absorbs energy)
• In endothermic reactions,
• Potential energy of reactants lt products
• D Energy (energy of products) (energy of
  reactants)
• D Energy is POSITIVE for endothermic reactions

17
Where does the energy come from?

• A reaction involves breaking making bonds
• CH4 2O2 ? CO2 2H2O
• Had to break 4 C-H bonds and 2 OO bonds to form
  2 CO bonds and 4 H-O bonds.

18
Energy in bond breaking/formation

• It takes energy to break bonds.
• Similarly, it takes energy to break wood, tear
  paper, etc.
• These are Endothermic reactions!
• BUT
• Energy is released when bonds are formed
• An Exothermic reaction
• Knowing bond energies (the amount of energy
  required to break a chemical bond) allows us to
  calculate how much energy will be released during
  a reaction.

19
Table of bond energies
20
Question

• It takes 346 kJ to break 1 mole of C-C bonds.
• How much energy is released when 1 mole of C-C
  bonds form?

21
Okay, how to calculate total energy of reaction?

• So it takes energy to break bonds and energy is
  released when bonds form
• In order to calculate the overall energy of the
  reaction, well assume that all the reactant
  bonds are broken first, then all the product
  bonds are formed.
• In reality, bonds are broken and formed
  simultaneously, but the energy depends solely on
  the initial and final statesnot the pathway.

22
Calculation for combustion of CH4

• CH4 2O2 ? CO2 2H2O

Energy absorbed (kJ/mole) 4 x(416) 1664 2
x(498) 996 Energy released - 2 x(803)
-1606 - 4 x(467) -1868
Bonds broken 4 C-H 2 OO Bonds formed 2 CO 4
H-O
Net Energy change (1664 996 1606 1868)
-814 kJ/mol
Exothermic!
23

• Heat of combustion of methane is
• 814 kJ/mole
• Convention is that you dont give sign for
  combustion because combustion reactions are
  ALWAYS exothermic
• Why is combustion exothermic?
• Because the OC bond in CO2 is so strong

24
Energy diagrams
Breaking 2 moles of OO bonds 996 kJ
Forming 2 moles of CO bonds -1606 kJ
Breaking 4 moles of C-H bonds 1664 kJ
Forming 4 moles of H-O bonds -1868 kJ
CH4 O2
Net energy change - 814 kJ
CO2 H2O
25
Compare energy released per gram of fuel octane
vs. ethanol
(gasohol)

• Combustion of octane
• C8H18 O2 ? 8CO2 9H2O

25
2
Bonds broken
Energy change
26
For ethanol

• C2H5OH 3O2 ? 2CO2 3 H2O

27

• Calculations show that you get much more energy
  during combustion of
• 1 g of octane vs. 1 g of ethanol
• Note calculations arent exact
• Bond energies vary slightly depending on the
  chemical environment
• Assumes all reactants and products are gases (not
  true for octane)

28
Activation Energy

• If combustion is an exothermic process, why does
  it require a match to start a fire?
• Activation energy The energy required to start
  the reaction (to start breaking the reactant
  bonds)

The match provides the energy that allows the
initial reactants to get over the barrier
29
Reaction time

• How fast a reaction goes depends on the
  activation energy
• The larger the activation energy, the slower the
  reaction
• How to speed up a reaction
• 1) Increase temperature
• At higher temps, reactants move faster, so when
  they collide, the collision energy is enough to
  overcome the activation energy
• 2) Add a catalyst
• The catalyst provides another reaction
  pathwayanother way to break and make bonds
  

30
A Catalyst lowers the activation energy
A catalyst increases reaction rates by providing
an alternate reaction pathway with lower
activation energy
31
Important distinction

• The total change in energy does NOT depend on the
  pathway
• But, the activation energy does depend on the
  pathway.
• An analogy The change in altitude one
  experiences when traveling from Chicago to Denver
  will always be the same no matter how you get
  there. But if you travel by plane you will get
  there faster than if you travel by car.