Transcript Heat and Energy Changes - stpats-sch4u-sem1-2013

UNIT 3: ENERGY CHANGES & RATES
OF REACTION
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; the sum of the kinetic energies
of all of the particles of a system (can’t
measure this but can measure heat released
or absorbed).
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:
A chemical system in which
energy and matter can move
between the system and its
surroundings (e.gg a reaction
taking place in a beaker or a
test tube).
TYPES OF SYSTEMS
CLOSED SYSTEM:
One in which matter can not move in or out but
energy may.
(e.g. once the lid is on and when heated, the steam
cannot leave (matter) but the heat is passed
through the pot)

TYPES OF SYSTEMS CONT’D
ISOLATED SYSTEM:
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.

This is difficult to do, so to measure thermal energy
changes, we use calorimeters to be isolated
systems even though they are really closed
systems
A thermos is almost an
isolated system but it
does lose or gain heat
over time….
MEASURING ENERGY CHANGES
Calorimetry is the process through which
energy changes in a chemical system are
measured.
The fancy calorimeter
The Pats science lab version
MEASURING ENERGY CHANGES
Hard to measure thermal energy changes but you
can measure: changes in T, Volume of or mass
contained in the system and 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.
So to indirectly measure thermal energy, we
measure heat (Q) – how much is leaving or
entering a system
FACTORS AFFECTING ENERGY CHANGES
The factors that affect energy change in a chemical
system are mass (m in g), temperature change
(∆T in C) and the specific heat capacity of a
substance (c).
Specific heat capacity
The amount of energy required to raise the T of 1
gram of a substance by 1oC. (units = J/g/C)
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)
The type of substance is taken care of by the c
value which is the specific heat
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.
 In
an exothermic change,
the value of Q will be
negative and the temperature
of the system will decrease.
ENDOTHERMIC VS. EXOTHERMIC
If energy is absorbed from its surroundings, the
change is ENDOTHERMIC.
 In an endothermic change, the value of Q will be
positive and the temperature of the system will
increase.
