Chapter 16 Reaction Energy
Thermochemistry
 I can define temperature and state
the units in which it is measured
 I can define heat and state its units.
I can perform specific-heat
calculations.
What is thermochemistry?
…. the study of the transfer of
energy as heat that accompany
chemical reactions and physical
changes.
www.scilinks.org
Topic: Heat/Temperature
Code: HC60726
Temperature and Heat
Temperature is a measure of
the average kinetic energy of
the particles in a sample of
matter.
Temperature is measured
with a thermometer.
 Celsius
 Freezing\melting
 Kelvin
 Freezing\melting
0⁰C
 Boiling 100⁰C
273K
 Boiling 373K
 ⁰C = K - 273
 K = 273 + ⁰C
Fahrenheit vs. Celsius
Celsius vs. Kelvin
The amount of energy
transferred as heat is usually
measured in joules, J.
A joule is the SI unit of heat as
well as all other forms of energy.
J = N x m = kg x m2
s2
Calorimeter –
Devise used to
measure the
energy
absorbed or
released as heat
in a chemical or
physical
change.
Simple Coffee Cup Calorimeter
Heat – The energy transferred
between samples of matter because of
a difference in their temperatures.
Energy transfers
from warmer
objects to cooler
objects.
SPECIFIC HEAT
… the amount of energy required
to raise the temperature of one
gram of a substance by one
Celsius degree 1⁰C) or one Kelvin
(1K).
So… if you have
a high specific
heat value, what
does that mean
by definition?
http://www.chem1.com/acad/webtext/chem
bond/cb01.html#SEC1
cp =
q
q = cp x m x T
m x T
cp = specific heat at constant pressure
q = heat energy lost or gained
m = mass of the sample
T = difference between the initial and
final temperatures
Sample problem A, page 533
A 4.0 g sample of glass was heated from 274 K
to 314 K, a temperature increase of 40.0 K , and
was found to have absorbed 32 J of energy as
heat energy.
a.What is the specific heat of this type of glass?
b.How much energy will the same glass sample
gain when it is heated from 314 K to 344 K?
Practice, page 534
1.Determine the specific heat of a material if a
35 g sample absorbed 96 J as it was heated
from 293 K to 313 K.
2.If 980 J of energy are added to 6.2 g of a
substance at 291 K. The final temperature is
311K. What is the specific heat of this
substance.
Additional practice
go.hrw.com keyword HC6NRGX
Enthalpy of Reaction
Enthalpy change is the amount of
energy absorbed by a system as
heat during a process at constant
pressure. H, change in enthalpy.
Enthalpy for a reaction
H = H products – H reactants
Enthalpy of reaction…. is the
quantity of energy transferred as
heat during a chemical reaction.
Exothermic - energy is released
(given off) feels warm
Endothermic – energy is absorbed
(taken in) feels cool
Thermochemical Equation … an
equation that includes the quantity
of energy released or absorbed as
heat during the reaction as written.
2H2 (g) + O2 (g)  2H2O(g) + 483.6 kJ
In an exothermic chemical reaction the
enthalpy change is negative meaning energy is
released from the system as heat
In an endothermic chemical reaction the
enthalpy change is positive meaning energy is
absorbed into the system as heat
Driving Force of Reactions
Learning Target:
 I can explain the relationship
between enthalpy change and the
tendency of a reaction to occur.
 I can explain the relationship
between entropy change and the
tendency of a reaction to occur.
Driving Force of Reactions
The change in energy of a reaction
system is one of two factors that
allow chemists to predict whether a
reaction will occur spontaneously
and to explain how it occurs.
The randomness of the particles in a
system is the second factor
affecting whether a reaction will
occur spontaneously.
Entropy
Entropy, S, is the tendency toward
randomness.
It is defined as a measure of the degree
of randomness of the particles such as
molecules, in a system
www.scilinks.org
Topic: Entropy Code: HC60523
Solid – particles are fixed in position –
degree of randomness is low – low entropy
Liquid – particles can move – system
more random – higher entropy
Gas – particles moving rapidly – most
random – highest entropy