Unit 3:
Thermochemistry
What is Cold?
https://www.youtube.com/watch?v=Akd7MMRKDwc
Basic Definitions
Thermochemistry: the study of energy changes
that accompany physical or chemical changes
of matter.
Thermal Energy: the energy available from a
substance as a result of the motion of its
molecules.
Basic Definitions
System: any sample under observation (could be the coffee in a
coffee cup)
Chemical System: the set of reactants and products that are being
studied (usually represented by a chemical equation)
Surroundings: all matter around the system being studied that is
capable of absorbing or releasing energy.
Types of Systems
OPEN SYSTEM:
matter
energy
A chemical system in which energy
and matter can move between the
system and its surroundings
Types of Systems
CLOSED SYSTEM:
One in which matter can not move in
or out but energy may.
y
g
r
e
en
matter
Types of Systems
ISOLATED SYSTEM:
A thermos is
almost an
isolated
system but it
does lose or
gain heat
over time….
energy matter
A system in which neither mass or
energy can move in or out; this would
be the ideal way to measure energy
changes in a chemical system.
Measuring Energy Changes
Calorimetry is the process through which energy
changes in a chemical system are measured.
The fancy calorimeter
The Bennies science lab
version
Calorimeters consist of 3 main parts:
•A well-insulated
reaction chamber
•Tight-fitting cover
with insulated
holes for a
thermometer
•Some mechanism
to stir contents.
When analyzing data from a coffee-cup
calorimeter, assume 3 things:
•Any thermal E transferred from
calorimeter to the outside
environment is negligible.
•Any thermal E absorbed by the
calorimeter itself is negligible.
•All dilute, aqueous solutions
have the same density (1.0g/ml)
and specific heat capacity of
water (4.18 j/g·˚C)
Measuring Energy Changes
Hard to measure thermal energy changes but you
can measure:
•changes in T, Volume of or mass contained in the
system
•pressure of system on surroundings and
vice-versa
Temperature – the measure of
the average kinetic energy of all
the particles of a sample of matter.
Factors Affecting Energy Changes
The factors that affect energy change in a
chemical system are:
•q = quantity of heat transferred - in Joules (J)
•m = mass (in g)
•c = specific heat capacity of a substance.
•∆ T = temperature change (∆T in ˚C)
Specific heat capacity
The amount of energy required to raise the T of 1 gram of
a substance by 1C. (units = J/g·°C) See pg. 799 for some
Pure Substances.
Factors affecting energy changes
Heat change in a chemical system (enthalpy)
can be calculated using the formula:
q=mc∆T
∆T – the change in T from the beginning to the
end (Tfinal – Tinitial)
Endothermic vs Exothermic
Energy changes are classified into endothermic or
exothermic based on how energy flows between the
chemical system and its surroundings.
•If energy is lost to the surroundings, the change is
EXOTHERMIC.
•q is negative for the system: System transfers
thermal energy to surroundings.
Endothermic vs Exothermic
•If energy is absorbed from its surroundings, the
change is ENDOTHERMIC.
•q is positive for the system
• System absorbs thermal energy from
surroundings.
More on this tomorrow…
Enthalpy Change
•symbol: ΔH
•the difference in enthalpies of reactants
and products during a change
•determined from the energy changes of
the surroundings
For a given reaction, the enthalpy change, ΔH,
is given by:
ΔH = Hproducts - Hreactants
Examples and Communication
1. A student has 125.0 mL of water at 20.0˚C, then places a sample of silver at
125˚C into the calorimeter. The final temperature of the water is 22.5 ˚C.
Calculate the quantity of thermal energy absorbed by the water.
2. What quantity of energy is transferred to cool 25.0g of water from 100.0˚C
to 20.0˚C?