Download Ch 16 - Reaction Energy

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Ch 16.1 – Thermochemistry
Chapter 16 Notes
Standard 7.d.:
 Students know how to solve problems involving heat flow and temperature changes,
using known values of specific heat.
Objective:
 We will define heat and its units and then perform specific-heat calculations.

: the study of the transfer of energy as heat that occurs
during chemical reactions and changes in state.

: q, is energy transferred from one object to another because of a
temperature difference between them.
 Heat always flows from a
object to a
continue to flow until they are in
object and will
.

: one that absorbs heat from the surroundings (+q).

: one that releases heat to it’s surroundings (-q).

: the insulated device used to measure the absorption or
release of heat in chemical or physical processes.
Specific Heat
 Heat flow is measure in
(J).
 One joule of heat raises the temperature of 1g of pure water

: the amount of heat needed to increase the temp of 1 g of the
substance 1°C or 1 Kelvin.
 Water’s specific heat is
Calculating Specific Heat (C)
 Divide the heat input, q (Joules) by the temperature change, ΔT (°C or K) times the
mass of the substance, m (g).
C= q
=
heat
m x ΔT mass x change in temp
Example 1
 The temperature of a 95.4 g piece of copper increases from 25.0°C to 48.0°C when
the copper is absorbs 849J of heat. What is copper’s specific heat (C)?
Example 2
 How much heat, q, is required to raise the temperature of 400.0 g of silver 45°C?
Look at Table 17.1 on page 508 for specific heat of silver.
Enthalpy of Reaction

is the amount of energy absorbed by a system as
during a process at constant
.
o The enthalpy change is always the
between the enthalpies of the
products and the reactants and is called Enthalpy of Reaction.
∆H = Hproducts - Hreactants
 ∆H is
for an exothermic reaction because the system
 ∆H is
for an endothermic reaction because the system
heat.
heat.
Example 3
 Enthalpy of Reaction for the formation of water vapor.
o What we already know:
o This equation does not tell us that energy is
the reaction.
o Thermochemical Equation:
as heat during
o Writing with ∆H: 2H2(g) + O2(g)  2H2O(g) ∆H =
 Enthalpy of Reaction for the decomposition of water vapor.
o What we already know:
o This equation does not tell us that energy as heat is
reaction.
o Thermochemical Equation:
during the
o Writing with ∆H: 2H2O(g)  2H2(g) + O2(g) ∆H =
 Compounds that release a
are formed are very
amount of energy as heat when they
.
 Compounds that release a very
amount of energy as heat or
absorb a large amount of energy as heat when they are formed are sometimes
and may decompose or react violently.
Chapter 17 Notes
Ch 17.1 – The Reaction Process
Standard 8.a.:
 Students know the rate of reaction is the decrease in concentration of reactants or the
increase in concentration of products with time.
Objective:
 We will interpret chemical reactions and define activated complex. We will draw energy
diagrams.
Chemical Reactions

: Ea - The minimum energy that colliding particles must
have in order to react.

: an unstable arrangement of atoms that forms
momentarily at the peak of the activation-energy barrier.
 This is also called the
.
 Energy Diagrams
o ∆Eforward = energy of
– energy of
o ∆Ereverse = energy of
– energy of
o Ea = energy of activated complex – energy of
o Ea’ = energy of activated complex – energy of
o ∆Eforward is
for endothermic and
for exothermic
o ∆Ereverse is
for endothermic and
for exothermic
Example 1
 For the Energy Diagram below:
o Label the reactants, products, ∆E, Ea, and Ea’.
o Determine the value of ∆Eforward, ∆Ereverse, Ea, and Ea’
Energy (kJ/mol)
80
 ∆Eforward =
60
 ∆Ereverse =
40
 Ea =
20
0
Forward 
 Reverse
 Ea’ =
Example 2
Energy (kJ/mol)
 Draw and label an energy diagram for a reaction in which Ea = 125 kJ/mol and
Ea’ = 86 kJ/mol. Place the reactants at energy level zero.
o Label the reactants, products, ∆E, Ea, and Ea’.
o Determine the value of ∆Eforward and ∆Ereverse
 ∆Eforward =
140
120
100
80
60
40
20
0
 ∆Ereverse =
 Endothermic or Exothermic
Forward 
 Reverse
Ch 17.2 – Reaction Rate
Standards 8.b. and 8.c.:
 8.b. Students know how reaction rates depend on such factors as concentration,
temperature, and pressure and know the role a catalyst plays in increasing the rate.
Objective:
 We will discuss the factors that influence reaction rate and define a catalyst.
Rate Influencing Factors
 The
of a chemical reaction mainly depends upon temperature,
concentration, particle size, and the use of a catalyst.
 The nature of the
and their bonds is also a factor but not one
that can be easily changed so we will not talk about it.
Temperature
 Raising the temperature
the reaction and lowering the temperature
the reaction.
Concentration
 The
the concentration, the more likely collisions will take place,
which
the reaction rate.
Particle Size
 The
the particle size, the more
, which
the
reaction rate.
Catalyst
 Adding a catalyst will
the rate of reaction, in some cases, better
than increasing the
.

: a substance that interferes with the action of a catalyst.
– These will slow down or even
a reaction