1. Chapter 9 - Thermochemistry Heat and Chemical Change
Prairie School
Physics
Spring 2015

2. The Flow of Energy - Heat
OBJECTIVES:
Explain the relationship between energy and heat.

3. The Flow of Energy - Heat
OBJECTIVES:
Distinguish between heat capacity and specific heat.

4. Energy - capacity for doing work or supplying heat
Energy and Heat
Thermochemistry - concerned with heat changes that occur during chemical reactions
Energy - capacity for doing work or supplying heat

5. Energy and Heat
Heat - represented by “q”, is energy that transfers from one object to another, because of a temperature difference between them.
How does heat travel?

6. Exothermic and Endothermic Processes
In studying heat changes, think of defining these two parts:
the system
the surroundings

7. 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?

8. 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

9. 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

10. C + O2 ® CO2
+ 395 kJ
Energy
Reactants
Products
C + O2
395kJ
C O2

11. CaCO3 + 176 kJ ® CaO + CO2 CaCO3 ® CaO + CO2 Energy Reactants Products

12. Heat Capacity and Specific Heat
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.

13. Heat Capacity and Specific Heat
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

14. Heat Capacity and Specific Heat
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!

15. Heat Capacity and Specific Heat
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)

16. Calorimetry
Calorimetry - the accurate and precise measurement of heat change for chemical and physical processes.

17. Calorimetry
For systems at constant pressure, the heat content is the same as a property called Enthalpy (H) of the system

18. Changes in enthalpy = H
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

19. 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= 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

20. 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

21. 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