Transcript Potential energy - Midland Park School District

Ch. 16: Energy and
Chemical Change
16.1 Energy
Objectives
Explain what energy is and distinguish
between potential and kinetic energy.
Relate chemical potential energy to the
heat lost or gained in chemical
reactions.
Calculate the amount of heat absorbed
or released by a substance as its
temperature changes.
What is Energy?
Energy is the ability to do work or
produce heat.
Energy exists in 2 basic forms:
Potential - energy due to the composition
or the position of an object
 Kinetic - energy of motion

Chemical Energy
Chemical systems contain both kinetic
energy and potential energy.
The kinetic energy of a substance is
directly related to the constant random
motion of its atoms or molecules and is
proportional to temperature.
 Potential energy of a substance depends
upon its composition - the #’s & kinds of
atoms, how they’re bonded and arranged.

Chemical potential
energy
The energy stored in a substance
because of its composition is called
chemical potential energy.
Fuels, like octane, store large amounts
of energy in the bonds between atoms.
When burned, the potential energy of
octane is converted to kinetic energy
(the pistons in your car move).
Energy
The Law of Conservation of Energy
states that in any chemical reaction or
physical process, energy can be
converted from one form to another, but
it is neither created nor destroyed.
In energy conversions, the total amount
of energy present remains constant.
Heat
Represented by the symbol “q”, heat is
the energy that flows from warmer to
cooler objects.
We recognize heat changes through
temperature changes.
Heat is measured in various ways.
Units of Heat
In the metric system, a calorie (cal) is
the amount of heat required to raise the
temperature of one gram of pure water
by one degree Celsius.
Energy released from food is measured
in Calories.
1 Calorie = 1000 cal = 1 kcal
In the SI system
The SI unit of heat & energy is the joule (J).


1 J = 0.2390 cal
1 cal = 4.184 J

1000 J = 1 kJ
1000 cal = 1 kcal

1 kcal = 4.184 kJ

Converting Units of
Energy
4.00 Cal x 1000 cal = 4,000 cal x 4.184 J = 16,700 J
1 Cal
1 cal
Practice Problems
A granola bar contains 142 Cal. Convert
this to calories.
An exothermic reaction releases 86.5 kJ.
How many kilocalories are released?
If an endothermic process absorbs 256
J, how many kcal are absorbed?
Specific Heat
The specific heat of any substance is
the amount of heat required to raise the
temperature of 1 g of the substance by
1 0C.
Recall that one calorie or 4.184 J is
required to raise the temperature of one
gram of pure water by 1 degree Celsius.
The specific heat (C) of water, then, is
1cal or 4.184 J.
g 0C
g 0C
Specific Heat
Because different
substances have
different compositions,
each substance has its
own specific heat.
The specific heat of a
substance is a measure
of how efficiently that
substance absorbs heat
(without a temperature
change).
Specific Heat
This means, if the
temperature of water (l) is
to rise 1 0C, each gram
must absorb 1 cal.
Each gram of gold, on the
other hand, needs 0.03 cal
to cause a 1 0C increase in
temperature.
Water has a high specific
heat; metals have a low
specific heat.
Pg. 492 text
Heat
The heat absorbed or released by a
substance during a temperature change
(no phase changes!) depends on
the specific heat of the substance
 the mass of the substance
 the amount by which the temperature
changes

Q = mCT
q or Q = heat absorbed or released
C = specific heat of the substance
m = the mass of the sample in grams
T is the change in temperature in
degrees Celsius
T is the difference between the final
temperature and the initial temperature or,
Tf - Ti
 This means that if the temperature
INCREASES, T is POSITIVE and Q is
positive; if the temperature DECREASES,
T is NEGATIVE and Q is negative.

Practice Problems
How many joules of heat will 2.25 x 107 g of
water absorb if its temperature increases
from 10 0C to 32 0C?
The temperature of a sample of iron with a
mass of 10.0 g changed from 50.4 0C to 25.0
0C with the release of 114 J of heat. What is
the specific heat of iron?
What mass of granite will absorb 2.6 x 108 J
of energy when its temperature rises 22 C0?
The specific heat of granite is 0.0803 J/g 0C.