Transcript Measuring and Using Energy Changes

JOHNMAR S. DELIGERO
Chemistry/Biology 12 (Nova Scotia Curriculum)
Sino-Canadian Program
Henan Experimental High School
Zhengzhou Henan, China
http://www.bananateachersworld.wikispaces.com
Official Reference Textbook: McGraw-Hill Ryerson Chemistry
Why
is
it
important to
know how to
determine the
energy changes
associated with
chemical and
physical
changes?
- Engineers need to know how much energy is
released from different fuels when they design an
engine and decide between different fuels.
- Firefighters need to know how much heat can be
given off by the combustion of different materials
so they can decide on the best way to fight a
specific fire.
- Manufacturers of hot packs need to know how
much heat is released by a given exothermic
process so that their pack will warm but not harm
the user.
How do you determine the heat absorbed or released by
chemical and physical processes?
In this section:
1. You will learn some ways to determine the enthalpy
changes of various processes by experiment, based on the
heat they release or absorb.
2. You will apply what you have learned by performing your
own heat experiments.
3. You will also learn how to use tabulated values to
determine enthalpies of physical and chemical processes.
4. Finally, you will examine the efficiency and environmental
impact of traditional and alternative energy sources.
- Much of the technology in our lives is designed to stop the flow of
kinetic energy as heat.
- Your home is insulated to prevent heat loss in the winter and heat
gain in the summer. If you take hot soup to school for your lunch,
you probably use a Thermos™ to prevent heat loss to the
environment.
- Whenever there is a temperature difference between two objects,
kinetic energy is transferred as heat from the hotter object to the
colder object.
- You measure the heat being transferred in a reaction or other
process by monitoring temperature change.
- Therefore, you must minimize any heat transfer between the
system and portions of the surroundings whose temperature
change you are not measuring.
- To measure the heat flow in a process, you need an
isolated system, such as a Thermos™.
- An isolated system stops matter and energy from
flowing into or out of the system.
- You also need a known amount of a substance,
usually water.
- The water absorbs the heat that is released by the
process, or the water releases heat if the process is
endothermic.
- To determine the heat flow, you can measure the
temperature change of the water. With its large
specific heat capacity (4.184 J/ g·°C) and its broad
temperature range (0°C to 100°C), liquid water can
absorb and release a lot of heat.
- Water, a thermometer, and an isolated system are
the basic components of a calorimeter. A
calorimeter is a device that is used to measure
changes in kinetic energy. The technological process
of measuring changes in kinetic energy is called
calorimetry.
a. In a coffee-cup calorimeter, a known mass of water
is inside the coffee cup.
b. The water surrounds, and is in direct contact with,
the process that produces the energy change.
c. The initial temperature of the water is measured.
d. Then the process takes place and the final
temperature of the water is measured.
e. The water is stirred to maintain even energy
distribution, and the system is kept at a constant
pressure.
f. This type of calorimeter can measure heat changes
during processes such as dissolving, neutralization,
heating, and cooling.
g. The law of conservation of energy states that energy
can be changed into different forms, but it cannot
be created or destroyed.
h. This law allows you to calculate the energy change
in a calorimetry experiment.
However, you need to make the assumptions:
a. The system is isolated. (No heat is exchanged with
the surroundings outside the calorimeter.)
a. The amount of heat that is exchanged with the
calorimeter itself is small enough to be ignored.
a. If something dissolves or reacts in the calorimeter
water, the solution still retains the properties of
water. (For example, density and specific heat
capacity remain the same.)
Once you make those assumptions, the following equation
applies:
qsystem = −qsurroundings
- The system is the chemical or physical process you are
studying.
- The surroundings consist of the water or solution in the
calorimeter.
- When a process causes an energy change in a calorimeter,
the change in temperature is measured by a thermometer
in the water.
- If you know the mass of the water and its specific heat
capacity, you can calculate the change in kinetic energy
caused by the process using the equation q = m• c • ΔT.
When all the materials in the calorimeter (all its components)
have the same final temperature, the system is said to be at
thermal equilibrium.
a. A constant-pressure calorimeter measures the
change in enthalpy of a reaction occurring in
solution during which the atmospheric pressure
remains constant.
b. A coffee-cup calorimeter is well-suited to
determining the enthalpy changes of reactions in
dilute aqueous solutions.
c. The water in the calorimeter absorbs (or provides)
the energy that is released (or absorbed) by a
chemical reaction.
d. When carrying out an experiment in a dilute
solution, the solution itself absorbs or releases
the energy.
e. You can calculate the amount of energy that is
absorbed or released by the solution using q =
m• c • ΔT.
f. The mass, m, is the mass of the solution.
Let’s do…
- Some substances dissolve exothermically,
substances dissolve endothermically.
and
some
- A coffee-cup calorimeter to determine the molar enthalpy of
solution for two different substances.
Let’s do…
- Compare molar enthalpies of combustion for short, straightchain alkanes.
- In your everyday life, you may have encountered another
type of hydrocarbon: paraffins.
- Paraffins are longchain hydrocarbons. They are semisolid or
solid at room temperature.
- One type of paraffin has been a household item for
centuries—paraffin wax, C25H52(s), better known as candle
wax.
- Like other hydrocarbons, the paraffin wax in candles
undergoes combustion when burned. It releases thermal
energy in the process.
Let’s do…
A calorimeter could be more flameresistant
than
a
coffee-cup
calorimeter, but still some heat is
transferred to the air and the metal
containers.
To measure precisely and accurately
the enthalpy changes of combustion
reactions, chemists use a calorimeter
called a bomb calorimeter.
A bomb calorimeter measures
enthalpy changes during combustion
reactions at a constant volume.
- The bomb calorimeter works on the same general
principle as the polystyrene calorimeter.
- The reaction, however, takes place inside an inner
metal chamber, called a “bomb.”
- This “bomb” contains pure oxygen. The reactants
are ignited using an electric coil.
- A known quantity of water surrounds the bomb
and absorbs the energy that is released by the
reaction.
- A bomb calorimeter has many more parts than a
polystyrene calorimeter.
- All of these parts can absorb or release small
quantities of energy. Therefore, you cannot assume
that the heat lost to the calorimeter is small enough
to be negligible.
- To obtain precise heat measurements, you must
know or find out the heat capacity of the bomb
calorimeter.
- The heat capacity of a calorimeter takes into
account the heat that all parts of the calorimeter
can lose or gain.
- A bomb calorimeter is calibrated for a constant mass of water.
- Since the mass of the other parts remains constant, there is
no need for mass units in the heat capacity value.
- The manufacturer usually includes the heat capacity value in
the instructions for the calorimeter.