1. Thermochemistry Introduction

2. Thermochemistry
The study of energy changes during chemical reactions or changes of state.

3. First Law of Thermodynamics
Energy is neither created nor destroyed during chemical or physical changes.
Also called the Law of Conservation of Energy

4. What is Energy?
Energy is the ability to do work.

5. Some Types of Energy: Kinetic Potential Energy of motion
Energy stored by object due to its position or composition
Chemical Potential Energy is stored in chemical bonds

6. Energy We’re Going to Focus On
Chemical Energy (energy in bonds)
Heat (energy due to atomic/molecular motion)

7. Heat (q)
The flow or transfer of energy due to a temperature difference.
Heat always flows from an object of higher temp to one of lower temp until both are the same temp. Why?
Heat flows from the hot pack to the woman’s cooler back.

8. Units for Energy: calorie (cal) Joule (J)
Amount of energy required to raise the temperature of 1g of water 1oC
1000 calorie = 1 kcal = 1 nutritional Calorie (Cal)
Joule (J)
4.184 J = cal

9. Energy in Food: Nutrition Labels The United States
This is in nutrition (food) calories, Cal. It is actually 453,000 cal, or 453 kcal.
1 Cal = 1000 cal

10. England

11. Germany

12. Australia

13. New Zealand

14. China

15. Unit Conversion Problems
Convert J into calories.
Answer: calories

16. Unit Conversion Problems
Convert 325 Calories to Joules
Answer: x 106 J

17. Some Basic Definitions
Surroundings
System:
The part of the universe we’re focusing on
Often the materials involved in a chemical reaction or phase change
Surroundings:
Everything else in the universe
Surroundings
Surroundings
System
System
System
Surroundings
Surroundings
Surroundings

18. Describing the Direction of Heat Flow:
Exothermic:
Heat flows out of the system and into the surroundings
The system loses energy
The surrounding gain energy
qsys is negative
qsurr is positive
We see an increase in temperature in the surroundings.
An exothermic reaction would feel hot to the touch because heat is being transferred to your hand.
System
System
System
System
System
System
Surroundings
Surroundings
Surroundings
Surroundings

19. Describing the Direction of Heat Flow:
Endothermic:
Energy flows into the system from the surroundings
The system gains energy
The surroundings lose energy
qsys is positive
qsurr is negative
We see a decrease in temperature of the surroundings
An endothermic reaction would feel cold to the touch because it is absorbing heat from your hand.
System
System
System
System
Surroundings
Surroundings

20. General Outcome: qsurr = -qsys
Signs are used to indicate the direction of energy flow.
According to the law of conservation of energy, the amount of heat lost or gained by the system MUST be exactly the same as the energy gained or lost by the surroundings. However, the signs are opposite.

21. Chemical (Potential) Energy
Think about this:
Is the breaking of bonds an endothermic or exothermic process?
Endothermic! It requires energy to occur.

22. Chemical (Potential) Energy
Breaking bonds is ALWAYS endothermic (it always requires energy!)
So, qsys is positive for breaking bonds.

23. Chemical (Potential) Energy
Think about this:
Is the formation of bonds endothermic or exothermic?
Exothermic! Energy is released when bonds form.
Pow!!!

24. Chemical (Potential) Energy
Forming bonds is ALWAYS exothermic(it always releases energy!)
So, qsys is negative for forming bonds.
Pow!!!

25. Your turn
Would you expect a synthesis reaction to typically be an endothermic or exothermic reaction?
Exothermic because bonds are being formed.
Would you expect a decomposition reaction to typically be an endothermic or exothermic reaction?
Endothermic because bonds are being broken.

26. Your turn Is the melting of ice endothermic or exothermic?
Endothermic because intermolecular forces are being broken.
Is the condensing of steam endothermic or exothermic?
Exothermic because intermolecular forces are being formed. (This is why burns from steam are so bad.)

27. Your turn
HW: Thermochemistry Introduction ws