1. Unit 5 ENERGY 5.4 What are endothermic and exothermic reactions?
April 1, 2011
AIM: How do we identify a substance to be exothermic or endothermic?
q = mcΔT
1. What much heat is released when 10 g of water are cooled from 20˚C to 10˚C?
2. What much heat is absorbed when 10 g of water are heated from 10˚C to 20˚C?
3. What is the difference between these two problems above?
DO NOW:
q=(10g)(4.18J/g•oC)(-10oC) = -418J
q=(10g)(4.18J/g• oC)(10oC) = 418J
One has negative heat and the other has positive heat

2. Heat of fusion and Heat of vaporization
REVIEW
Heat of fusion and Heat of vaporization
Explain the following in a coherent, well-written paragraph
1. Explain why we use the q=Hf and q=Hv
2. How we decide whether to use constant for the heat of fusion OR the heat of vaporization
3. Explain why we can’t use Table B for any other substance besides water.
4. Explain why temperature does not change during a phase change but heat is required for the phase change.

3. ENDOTHERMIC melting: vaporization: sublimation:
take in energy (heat)
heat is absorbed
heat is a positive number
ENDOTHERMIC REACTION
SAMPLE IS HEATED
melting:
vaporization:
sublimation:
solid  liquid (ice cream melting)
liquid  gas (boiling water)
solid  gas

4. EXOTHERMIC freezing: liquid  solid (making ice cubes) v condensation:
release energy (heat)
heat exits
heat is a negative number
EXOTHERMIC REACTION
SAMPLE IS COOLED
freezing:
liquid  solid (making ice cubes) v
condensation:
gas  liquid (rain)
gas  solid
deposition:

5. Let’s try some
q = mcΔT
1. What is the total number of joules of heat energy are absorbed when 100g of water are heated from 20˚C to 30˚C?
1. asking me to find out HOW MANY J  q
2. pick out important information
100g of water
how much stuff  m
temperature change  ΔT
30oC to 20oC
Specific heat of water  c
4.18 J/g • oC

6. q = mcΔT q = (100g) x (4.18J) x (30oC -20oC) q = 418J x oC (10oC)
3. plug in
q = mcΔT
q = (100g) x
(4.18J) x
g• oC
(30oC -20oC)
q = 418J x oC
(10oC)
Positive so:
ENDOTHERMIC
q = 4180 J

7. INDEPENENT PRACTICE
COMPLETE FOR HOMEWORK IF YOU ARE NOT DONE IN CLASS.
ASSIGNMENT IS TO BE COMPLETED BY TOMORROW!!!