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Chapter 9 - Thermochemistry
Heat and Chemical Change
PRAIRIE SCHOOL
C HE M I S T RY
SPRING 2015
The Flow of Energy - Heat
 OBJECTIVES:
Explain
the relationship between
energy and heat.
The Flow of Energy - Heat
 OBJECTIVES:
Distinguish
between heat capacity
and specific heat.
Energy and Heat
 Thermochemistry - concerned with
heat changes that occur during
chemical reactions
 Energy - capacity for doing work or
supplying heat
Energy and Heat
5
 Heat - represented by “q”, is energy
that transfers from one object to
another, because of a temperature
difference between them.
 How does heat travel?
Exothermic and Endothermic Processes
 In studying heat changes, think of
defining these two parts:
the system
the surroundings
Exothermic and Endothermic Processes
 The Law of Conservation of Energy
states that in any chemical or
physical process, energy is neither
created nor destroyed.
All the energy is accounted for as
work, stored energy, or heat.
Examples?
Exothermic and Endothermic Processes
 Review: if heat is flowing into a system
from it’s surroundings:
 defined as positive
 q has a positive value (we will discuss
this later)
 called endothermic
system gains heat as the
surroundings cool down
Exothermic and Endothermic Processes
 Review: When heat is flowing out of
a system into it’s surroundings:
defined as negative
q has a negative value
called exothermic
system loses heat as the
surroundings heat up
C + O2  CO2
Energy
10
+ 395 kJ
C + O2
395kJ
C O2
Reactants

Products
Energy
CaCO
 CaO
CaCO
CaO
+ CO+2 CO2
3 + 176
3 kJ
CaO + CO2
176 kJ
CaCO3
Reactants

11
Products
Heat Capacity and Specific Heat
12
 A calorie is defined as the quantity of
heat needed to raise the temperature
of 1 g of pure water 1 oC.
Used except when referring to food
a Calorie, written with a capital C,
always refers to the energy in food
1 Calorie = 1 kilocalorie = 1000 cal.
Heat Capacity and Specific Heat
13
 Joule-- the SI unit of heat and energy
4.184
J = 1 cal
 Specific Heat Capacity - the
amount of heat it takes to raise the
temperature of 1 gram of the
substance by 1 oC (abbreviated “C”)
 This is found in a chart in your books
Heat Capacity and Specific Heat
14
 For water, C = 4.18 J/(g oC), and also
C = 1.00 cal/(g oC)
 Thus, for water:
it takes a long time to heat up, and
it takes a long time to cool off!
Heat Capacity and Specific Heat
15
 To calculate, use the formula:
 q = mass (g) x T x C
 heat abbreviated as “q”
 T = change in temperature
 C = Specific Heat
 Units are either J/(g oC) or cal/(g oC)
Calorimetry
 Calorimetry - the accurate and precise
measurement of heat change for
chemical and physical processes.
Calorimetry
 For systems at constant pressure, the
heat content is the same as a property
called Enthalpy (H) of the system
Calorimetry
 Changes in enthalpy = H
 q = H These terms will be used
interchangeably in this textbook
 Thus, q = H = m x C x T
 H is negative for an exothermic
reaction
 H is positive for an endothermic
reaction
In terms of bonds
19
C
O
O
O
C
O
Breaking this bond will require energy.
O
C
O C O
O
Making these bonds gives you energy.
In this case making the bonds gives you
more energy than breaking them.
Chemistry Happens in
20
MOLES
 An equation that includes energy is called a
thermochemical equation
 CH4 + 2O2  CO2 + 2H2O + 802.2 kJ
 1 mole of CH4 releases 802.2 kJ of energy.
 When you make 802.2 kJ you also make 2 moles of
water
CH4 + 2 O2  CO2 + 2 H2O + 802.2 kJ
21
 If 10. 3 grams of CH4 are burned completely,
how much heat will be produced?
10. 3 g CH4
1 mol CH4
16.05 g CH4
802.2 kJ
1 mol CH4
= 514 kJ
CH4 + 2 O2  CO2 + 2 H2O + 802.2 kJ
22
 How many grams of O2 would be
required to produce 23 kJ of heat?
 How many grams of water would be
produced with 506 kJ of heat?
Homework
 Read Section 9.1 and complete the section review
(problems 1-8) due Wednesday
This Week…
 Tomorrow: Activity over Thermodynamics
 Wednesday: Lab over Thermodynamics
 Thursday: Finish lab, work on chemistry
demonstrations
Born Haber Cycle
 A particular set of equations known as the Born-
Haber Cycle demonstrate how chemists are able to
use the first law of thermodynamics to find an
unknown energy value that is impossible to get from
chemical reactions in a lab.


Lattice energy
Election affinity
Born-Haber Cycle
 There is more than one path to the formation of a
substance to a particular state.
 Because of this, we can calculate unknown values
using known values
 Each Physical or chemical change has…



A chemical equation
A definition about the type of energy change
A ΔH (enthalpy value), expressed in kJ
Directions
 1. Cut out the cards for names, equations, definitions,
and symbols with energy values
 Arrange them with their correct type of reaction and
enthalpy value
 Glue to a piece of paper, to make a poster of each
reaction.
 Find the unknown lattice energy for the three
reactions (show your work)
Some terms to help…
 Sublimation: solid to gas formation
 Ionization energy: breaking an election from an




element (free electron in products)
½ bond energy: 1/2F2F
Election affinity: Amount of energy needed to gain
an election (electron on reactant side)
Lattice energy: energy required for ions to combine
together (strength of ionic bond)
Standard enthalpy of formation: energy required for
reaction to be complete, using the ½ bond energy.
Chapter 9: Calories and Finding your energy
needs
 The energy content of food is found from burning the
dry food sample in a bomb calorimeter.
 Calories can be converted into kJ


1 Cal= 4.184 kJ
Using this conversion, you can find the amount of J your body
takes in by looking at the calories

310 Calorie burger=1297kJ of energy
 You get energy from carbohydrates, fats, and
proteins.

Also get nutrients from vitamins, water and minerals
You can also look at a food label…
 Carbohydrates: 4 calories=17kJ/g
 Proteins: 4 calories=17kJ/g
 Fat: 9 calories=38 kJ/g
 Look at the food label and add the values for each of
the components above to find total kJ of energy
produced.
 Fats have more energy per gram than carbohydrates
and proteins.

Energy storing compounds in the body
What are your energy needs?
 Your daily energy needs are calculated by your basal
metabolic rate, your activity level, and your thermal
effect of food.
 Basal metabolic rate: the energy needed to sustain
life.



Not constant: varies with age, gender, stress level, general
health.
When you measure this, it needs to be taken before eating any
food.
This is also called the resting metabolic rate