Download Thermochemistry Intro

Thermochemistry
Introduction
Thermochemistry

The study of energy changes during
chemical reactions or changes of state.
First Law of Thermodynamics
Energy is neither created nor destroyed
during chemical or physical changes.
 Also called the Law of Conservation of
Energy

What is Energy?

Energy is
the ability
to do
work.
Some Types of Energy:

Kinetic
 Energy

of motion
Potential
 Energy
stored by object due to its position or
composition
 Chemical Potential Energy is stored in
chemical bonds
Energy We’re Going to Focus On

Chemical Energy
(energy in bonds)

Heat (energy due to
atomic/molecular
motion)
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.
Units for Energy:

calorie (cal)
 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 = 1.000 cal
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
England
Germany
Australia
New Zealand
China
Unit Conversion Problems

Convert 345.7 J into calories.
1.000cal
345.7 J 

4.184 J
Answer: 82.62 calories
Unit Conversion Problems

Convert 325 Calories to Joules
1000cal 4.184 J
325Cal 


1Cal
1cal
Answer: 1.36 x 106 J
Some Basic Definitions

System:
 The


Surroundings
part of the universe we’re focusing on
Often the materials involved in a chemical reaction
or phase change
Surroundings:
 Everything
Surroundings
else in the universe
Surroundings
Surroundings
System
Surroundings
Surroundings
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
System
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.
Surroundings
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
Surroundings
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.

Chemical (Potential) Energy

Think about this:
 Is
the breaking of bonds an endothermic or
exothermic process?
 Endothermic! It requires energy to occur.
Chemical (Potential) Energy
Breaking bonds is ALWAYS endothermic
(it always requires energy!)
 So, qsys is positive for breaking bonds.

Chemical (Potential) Energy

Think about this:
 Is
the formation of bonds endothermic or
exothermic?
 Exothermic! Energy is released when bonds
form.
Pow!!!
Chemical (Potential) Energy
Forming bonds is ALWAYS exothermic(it
always releases energy!)
 So, qsys is negative for forming bonds.

Pow!!!
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
broken.
because bonds are being
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.)
Your turn

HW: Thermochemistry Introduction ws