Transcript Chemistry and Energy

How are they related?
CHEMISTRY AND ENERGY
Energy Encountered Daily
What is Energy?
 Defined as the ability to do work or create
heat.
 Many types of energy
 Thermal
 Light
 Gravitational
 Kinetic
 Potential
Light Energy Review
 How is light energy produced?
 Electrons release light energy when they fall from
a high energy level to a lower energy.
 We’re now going to talk about energy
released or used in a chemical reaction. Heat
energy
Thermochemistry
 The study of heat used or released in a
chemical reaction.
 Let’s investigate heat as it compares to
temperature using the Heat vs. Temperature
Handout
Specific Heat Calculations
 q = mCΔT
q = heat (J or cal or Cal)
4.184 cal = 1 Joule
1000 cal = 1 Cal (dietary calorie)
m = mass (g)
C = specific heat (J/g oC or cal/g oC)
ΔT = change in temperature (o C or K)
= T f - Ti
Specific Heat
 Specific heat of water = 1 cal /g o C or
= 4.184 J / goC
 Specific heat of most metals = < 1 J / goC
 Do metals heat slowly or quickly compared to
water?
 Do metals stay warm longer or shorter than
water?
Practice Problem
 How much energy is required to heat 120.0 g
of water from 2.0 oC to 24.0oC?
q = mCΔT
m= 120.0 g
C = 4.184 J/goC
ΔT= (24.0 – 2.0)oC = 22.0oC
q = (120.0g)(4.184 J/goC)(22.0oC) =
Practice Problem
 How much heat (in kJ) is given off when 85.0
g of lead cools from 200.0oC to 10.0 oC?
(Specific heat of lead = 0.129 J/g oC)
q = mCΔT
m = 85.0 g
C = 0.129 J/g oC
ΔT = (10.0 – 200.0)oC = - 190.0oC
q = (85.0 g)(0.129 J/g oC)(- 190.0oC) = -
How Do Chemical Reactions
Create Heat energy?
 Consider the combustion of gasoline (octane)
2 C8H18 +25 O2  16 CO2 +18 H2O
 Potential Energy: Stored energy
 Potential energy is stored in the bonds of the
reactant s and the products
 When bonds are broken, the energy is available
 When produce bonds form, some energy is used in
these bonds
 The excess energy is released as heat
Kinetic Energy
 Directly related to temperature
Is Heat Used or Released?
 Endothermic reactions used heat from the
surroundings
 Sweating
 Refrigeration
 Exothermic heat releases heat to the
surroundings
 Hot hands
 Combustion
 Exercise
Endothermic Reactions
 Decrease in kinetic energy  decrease in
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temperature  heat will transfer from the
environment to the system resulting in a
cooler environment
Absorbs heat from its surrounding.
The system gains heat
Positive value for q
H = q = 0
Hproducts  Hreactants
Exothermic Reactions
 Increase in kinetic energy  increase in
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temperature of system heat released to
the environment resulting in a hotter
environment
Releases heat to its surroundings
The system loses heat
Negative value for q
H = q = 0
Hproducts  Hreactants
Enthalpy
 Heat content for systems at constant
pressure
 Symbol is H
 Terms heat and enthalpy are used
interchangeably for this course
 H = q = m C T
 Heat moves from ________ to ___________.
Law of Conservation of
Energy
 Energy is not lost or gained in a chemical
reaction
 In a chemical reaction potential energy is
transferred to kinetic energy
Thermochemical Equations
 An equation that includes the heat change
 Example: write the thermochemical equation
for this reaction
 CaO(s) + H2O(l) Ca(OH)2(s)
H = -65.2 kJ
CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ
Stoichiometry and Thermochemistry
Tin metal can be extracted from its oxide
according to the following reaction:
SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ 
Sn(s) + 4HNO3(aq)
How much energy will be required to
extract 59.5 grams of tin?
How to solve
1. Use your stoichiometry
2. Treat heat as a reactant or product
SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ 
Sn(s) + 4HNO3(aq)
59.5 g Sn 1 mol Sn 192 kJ
1
g Sn 1 mol Sn
If an Object feels hot, it means it is
undergoing a change with a H that
is:
a. positive
b. negative
c. whether the object feels hot or not is
unrelated to its H
d. I don’t know
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If the object feels hot, it means it is
undergoing:
a. an exothermic reaction
b. an endothermic reaction
c. whether it feels hot or not is unrelated
to whether it is undergoing an
exothermic or an endothermic change
How does ice melt?
HEAT DURING A CHANGE OF
STATE
Molar Heat of Fusion
 Heat absorbed by one mole of a substance
during melting
 Constant temperature
 Hfus
 H2O(s)  H2O(l)
H = 6.01 kJ/mol
Molar Heat of Solidification
 Heat lost when 1 mole of a liquid solidifies
 Temperature is constant
 Hsolid
 Hfus = -Hsolid
 H2O(l)  H2O(s) H = -6.01 kJ/mol
Molar Heat of Vaporization
 Heat needed to vaporize 1 mole of a liquid
 Hvap
 H2O(l)  H2O(g)
Hvap = 40.7 kJ/mol
Molar Heat of Condensation
 Heat released when 1 mole of vapor
condenses
 Hcond
 H2O(g)  H2O(s)
 Hvap = -Hcond
Hcond = -40.7 kJ/mol
Phase Change Diagram for Water
Phase Change Diagram
The House that Heats Itself
 http://www.sciencefriday.com/videos/watch/
10007
CALORIMETRY
Calorimetry
 Method used to determine the heat involved
in a physical or chemical change.
 Relies on the law of conservation of energy
Calorimeter
Simple Calorimeter
Calorimetry Math
 Heat gained by the water = q
 Heat lost by the system = -q
mCT = q
T = Tf –Ti , m = mass, C = specific heat
q gained by water = q lost by system
 q water = - q system
 mCT = -mCT
(mass H2O)(spec. heat H2O)(T H2O) = - (mass
sys)(spec. heat sys)(T sys)
Standard Heat of Reaction
 Heat change for the equation as it is written
H = Hf(products) - Hf(reactants)
Standard Heats of Formation (Hf)
 Change in enthalpy when 1 mole of the
compound is formed from its elements in
their standard states at 25oC and 101.3 kPa
Hess’s Law
 A way to calculate the heat of a reaction
that may be too slow or too fast to collect
data from.
 Add together several reactions that will
result in the desired reaction. Add the ΔH
for these reactions in the same way.
 Htotal = Hproducts - Hreactants