Objectives:
Energy Notes
Thermodynamics Problems
1.
2.
Types of Heat Transfer

Thermal energy (heat) is transferred in
three ways:
1.
Conduction
Convection
Radiation
2.
3.
Conduction
All atoms are vibrating (moving), which
means they have kinetic energy. Hot
atoms have more Ek. When hot atoms
bump into cold atoms they transfer some
energy.
Conduction
Closer atoms mean more collisions.
So solids tend to transfer heat better than
liquids or gases.
Gases tend to make good insulators.
Convection

Convection transfers heat through
moving currents in gases or liquids.
Convection
Radiation
Radiation transfers heat through
electromagnetic radiation; occurs
even in a vacuum (empty space).
1. Radiation transfers heat in all
directions—even down. Convection
currents always rise.
2. Radiation requires no contact—
convection and conduction require
touching.
3. Radiation can go through transparent
materials (barriers) like glass.

Energy
The Nature of Energy
Energy is the ability to do work.
There are two types of energy:
Kinetic energy – energy of motion
Potential energy – energy of position
For example: take a rollercoaster:
Energy
Law of Conservation of Energy
- energy can neither be created nor
destroyed, it can be converted (potential
energy to kinetic energy etc…)
Chemical energy stored in a substance
because of its composition.
Most common form of energy transfer is heat
Energy
Heat (q) - energy that is transferred as a
result of a temperature difference.
Transfer always occurs from a warmer
object to a cooler object.
For example: pouring hot coffee into a cup
Energy
SI unit of heat and energy is the Joule
Calorie amount of heat required to raise the
temperature of 1 gram of water 1C.
Conversion units:
1 calorie =4.184 joules
dietary Calories (uppercase C)
on the back of food packages
are actually kilocalories
(1 Cal or 1 kcal = 1000 cal)
Practice Problems

What is the equivalent in joules of 126
Calories?

Convert 455 kilojoules to kilocalories?
Specific Heat
Specific Heat amount of heat needed to
raise the temperature of one gram of a
substance by one Celsius degree. (unit of
specific heat = J/g C )every substance
has different specific heat, water in the
liquid phase has a specific heat of 4.184
J/g C (see Table 16-2) note that water in
the gas and solid phases have a different
value.
Specific Heat
Amount of energy transferred between two objects can be calculated using
the following equation:
Q = mcΔT OR mcΔT = mcΔT

Q is the amount of heat needed to change the temperature of a
substance

m is the mass of the heated substance

c is the specific heat capacity

ΔT (pronounced delta T) is the temperature difference; the difference in
temperature before and after you applied the heat

Practice PROBLEMS
Practice Problems

The temperature of an unknown metal with
a mass of 10.0g changed from a 50.4C to
25.0C with the release of 114J of heat.
Calculate the specific heat of the metal
and use the table to identify it.
Practice Problems

How much heat is required to warm 122g
of water by 23.0C?
Objectives
Homework Questions
 More Heat Notes
 Time to work on Problems

Practice Problems

If 355 g water at 65.5C loses 9750 J of
heat, what is the final temperature of the
water.

The temperature of a sample of water
increases from 20.0C to 46.6C as it
absorbs 5650 J of heat. What is the mass?

If 150.0 grams of iron at 95.0C, is placed
in and insulated container with 500.0
grams of water at 25.0C, what is the final
temp?
Measuring Heat
Energy changes during chemical and
physical processes can be accurately
measured using an insulated device called
a Calorimeter. It is used to determine the
specific heat of an unknown metal or food.
Calorimeter
For example: Suppose you put a125 g of
water into a foam-cup calorimeter and find
that its initial temperature is 25.6C. Then
you heat a 50.0 g sample of the unknown
metal to a temperature of 115.0C and put
the metal sample into the water. What will
happen?
Calorimeter
The metal will Lose heat and the water will
Gain heat until they both reach a final
temperature of 29.3C.
Assuming no heat is lost to the surroundings,
the heat Gained by the water should equal
the heat Lost by the metal.
Calculate the specific heat of the metal?
Types of Heat transfer
Endothermic reaction – energy is absorbed.
In a chemical reaction, heat is a reactant
and the value of q is positive. So in the
above example, what absorbed the
energy?
Exothermic reaction – energy is released. In
a chemical reaction, heat is a product and
the value of q is negative. So in the above
example, what released energy?
Thermochemical Equations

Thermochemical Equations – Balance
chemical equations that includes the
physical states of all the reactants and
products and the energy changes,
expressed as the change in enthalpy, H
Example:
Equation 1:
4 Fe(s) + 3O2(g)  2Fe2O3(s) ∆H = -1625KJ
Energy is Released
Equation 2:
NH4NO3(s)  NH4+(aq) + NO3-(aq) ∆H = 27KJ
Energy is Absorbed
In the thermo equation the sign of H tells
you rather the equation is endothermic,
absorbs energy, or exothermic, produces
energy.
 Equation 1 is an exothermic reaction,
because H is neg.
 Equation 2 is an endothermic reaction,
because H is pos.

Reaction Diagrams

H reaction = H products + H reactants

Using the graphs lets calc the H for each: