Download Thermochemistry ch 16 energy diagrams phase

ENERGY AND CHEMICAL CHANGE
CH. 16
•Energy is the ability to do work or
produce heat and is found in two
forms
•potential energy is energy due to the
composition of the substance
•kinetic energy is the energy of motion
(1/2 mv2)
Law of Conservation of Energy
• Law of conservation of energy:
Energy can be converted from one
form to another, but cannot be
created nor destroyed
Energy and units
• Chemical potential energy often releases heat or
needs heat
• Review- kinetic energy is the energy of motion
• Measuring in heat is done using units called
Joules.
• One Joule is = 0.2390 calories
• One calorie = 4.184 Joules
• One Calorie = 1000 calories (note the capital C) =
1 kcal
Units and math
• Measuring in heat is done using units called
Joules.
• One Joule is = 0.2390 calories
• One calorie = 4.184 Joules
• One Calorie = 1000 calories (note the capital
C) = 1 kcal
• How many Joules are in a bowl of breakfast
cereal and milk which contain 340 Calories?
Specific Heat
• Specific heat is the amount of energy
required to raise the temperature of one
gram of a substance one degree Celsius
• specific heat is measured in J/goC
• specific varies based on the substance that is
being measured
• water has one of the highest specific heats
• metals have lower specific heats (it heats up
quickly)
Energy Diagram for Water
100 oC __
D
C
0oC __ B
-20 oC
A
Energy increase ____________________>>>
E
Key to energy diagram for water
•
•
•
•
•
A = Ice heating ( 1 state of matter) S only
B = Ice melting (2 states of matter – S and L
C = Water heating ( 1 state of matter) L only
D = Water boiling (2 states of matter) L and V
E = Water vapor boiling (1 state of matter) V
only
Calculations for specific heat can only be used
when the temperature changes (A, C, E)
Specific Heat
• To calculate heat absorbed or released by a
substance:
• q = m cp DT
•
•
•
•
q = heat absorbed or released (heat = temp)
cp = the specific heat of the substance
m = mass of the sample in grams
DT = change in temperature in oC or Tfinal - Tinitial
Specific Heat Math
• If the temperature of 34.4 g of ethanol has
been raised from 25.0oC to 78.8oC how much
heat has been absorbed by the ethanol?
• What if this sample were cooled from 78.8oC
to 25.0oC?
Energy diagram for exothermic
reaction
Reactants
Activation energy – Minimum amount of energy
_ _ _ _ _ _ _ _ required to start the reaction
DH is negative
DH – The change in energy from reactants to products
_ _ _ _ _ _ _ _ Products
Time (increasing)
Energy Diagram of an endothermic
reaction
An endothermic reactions
has a higher
activation energy than an
exothermic
reaction
_ _ _ _ _ _ _ _ _products
Activation energy
+DH
Reactants _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Energy increases
DH is positive
The effect of a catalyst on a
chemical reaction
A catalyst lowers the activation energy
and shortens the path of a reaction
A catalyst speeds the rate of reaction