Download Thermochemistry - Waterford Public Schools

Energy and Chemical
Reactions
Ms. Grobsky
Introduction to Thermodynamics
 So far, we have discussed the various types of chemical
reactions, the driving forces behind them, and how to
quantitatively predict amounts of reactants and products
using stoichiometry
 In order to properly understand chemistry, we must
understand the energy changes that accompany these
chemical reactions
 The study of thermodynamics is concerned with the
question:
Can a Reaction Occur?
System and Surroundings
 Unlike matter, energy does not have mass and cannot be
held in our hands
 However, its effects can be observed and measured
 When discussing energy, one must assign the following
labels:
 System
 Area of the universe we are focusing on (i.e., the experiment)
 Surroundings
 Everything else outside the system
SURROUNDINGS
SYSTEM
Exchange:
Open
Closed
Isolated
Mass & Energy
Energy
Nothing
So, What is Energy?
 Defined as the capacity to do work or to produce
heat

Units are calorie (cal) or Joule (J)
 Before we can make use of this definition, we must
understand the concepts of work and heat
 Work is the energy used to cause an object to move against a
force
 Heat is the energy used to cause the temperature of an object
to increase
 Before we examine these definitions more closely, let’s
first consider the ways in which matter can possess
energy and how that energy can be transferred from one
piece of matter to another
Types of Energy
•
There are many different forms of energy:
•
Thermal energy is the energy associated with the random motion
of atoms and molecules
•
Kinetic energy is the energy due to motion
•
Potential energy is the energy available by virtue of an object’s
position
•
Chemical energy is the energy stored within the bonds of chemical
substances
• Type of potential energy
•
All forms of energy are able to interconvert!
Example of Energy Conversion –
Kinetic Energy and Potential Energy
Total Internal Energy =
E
Ball thrown
upwards slows &
loses kinetic
energy but gains
potential energy
Kinetic Energy
=
EK
+
+
Potential Energy
EP
The reverse
happens as it
falls back to the
ground
• Energy can also be transferred back and forth
between a system and its surroundings in the
forms of work and heat
Why Can Energy Interconvert?
 All of these conversions and transfers proceed in
accord with one of the most important
observations in science:
Energy can neither be created
not destroyed – it is
conserved!
 This important observation is known as the First
Law of Thermodynamics
What Does Energy Have to Do
with Chemical Reactions?
Introducing Thermochemistry!
Tying in the Citric Acid/Sodium
Bicarbonate Lab
 During both parts of the lab, you felt striking temperature
differences between yourself (the surroundings) and the
baggie (the system)
 Well, what do you think caused this temperature
difference?
 It was due to the exchange of heat!
 What exactly is heat?
 It is the exchange of thermal energy between two systems at
different temperatures that are in contact with one another
 Heat flows from a warmer object to a cooler object
 Heat is represented by the letter “q”
 But, temperature is NOT a measure of thermal energy
 It just reflects the kinetic energy (motion) of the particles
Energy and Physical/Chemical
Changes
Come to find out, many physical changes
involve release and absorption of energy in
the form of heat
Dissolving of salts
Changes in state
Compression/expansion of gases
Energy Changes in Chemical Reactions
• Almost every chemical change either releases or
absorbs energy in the form as heat as chemical
bonds are broken and formed during the course of a
reaction
• Energy is consumed when bonds are broken
• Energy is released as new bonds are formed
• Net total energy change for a reaction will either be
a negative value with the release of heat
(exothermic) or a positive value with the
absorption of heat (endothermic)
• Thermochemistry is the study of heat exchange in
chemical reactions
Heat Exchange between System and the
Surroundings
• An exothermic process is any process in which energy (heat) flows
out of the system into the surroundings
• “q” is negative
• Energy of reactants is greater than that of the products
• Energy can be thought of as a product in a chemical reaction
because it is “given off”
H2O (g)
H2O (l) + Energy
• An endothermic process is any process in which energy (heat) flows
into the system from the surroundings
• “q” is positive
• Energy of products is greater than that of the reactants
• Energy can be thought of as a reactant in a chemical reaction
because it is being absorbed
Energy + 2HgO (s)
2Hg (l) + O2 (g)
Endothermic vs. Exothermic
Reactions