P R A I R I E S C H O O L
C H E M I S T R Y
S P R I N G 2 0 1 5

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 + O
2
 CO
2
10
C + O
2
Reactants

C O
2
Products
395kJ

3

2 2
CaO + CO
2
176 kJ
CaCO
3
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 o C.

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 o C (abbreviated “C”)

This is found in a chart in your books

Heat Capacity and Specific Heat
14

For water, C = 4.18 J/(g o C), and also
C = 1.00 cal/(g o C)

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 o C) or cal/(g o C)

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

C
In terms of bonds
19
O
O
C
O
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




An equation that includes energy is called a thermochemical equation
CH
4
+ 2O
2
 CO
2
1 mole of CH
4
+ 2H
2
O + 802.2 kJ releases 802.2 kJ of energy.
When you make 802.2 kJ you also make 2 moles of water

CH
4
+ 2 O
2

CO
2
+ 2 H
2
O + 802.2 kJ
21

If 10. 3 grams of CH
4 are burned completely, how much heat will be produced?
10. 3 g CH
4
1 mol CH
4
16.05 g CH
4
802.2 kJ
1 mol CH
4
= 514 kJ

CH
4
+ 2 O
2

CO
2
+ 2 H
2
O + 802.2 kJ
22

How many grams of O
2 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