Download Dr. Baxley`s Intro to Thermochem.

Dr. Baxley’s Intro to Thermo
Chapter 6 notes
Section 6.1-6.3: Potential and Kinetic Energy
Energy cannot be bottled or measured, it's more of an ability of an object to do something
(work, defined as the energy that can move an object against a force)
1. Potential energy is the term used for energy in "storage,"
 A rock could be pushed off a table and move towards earth, there is a natural attraction from gravity
 “Thomas” trains can move closer together, there is a natural attraction from the magnets
 A coil spring wants to push away when it is compressed, a natural repulsion of the metal coils
 All objects that have the potential for motion have potential energy
 The potential energy of an object can be converted to kinetic energy
 Energy is released any time two objects with an attractive force are allowed to come
together. Think about a rock falling to the Earth (because of the attractive force). Your foot is in the
way. Is energy released to your foot?
 To separate two objects with attractive forces, some E must be absorbed from the
surroundings. For kids‟ toys to rise above the ground to the shelf, they must absorb energy from the
surroundings (me). For a rock to rise above the ground, it absorbs energy from it‟s surroundings (you).
2. Kinetic Energy is the term used for the energy of motion, when something is moving
 objects in motion have KE, which has a formula of KE = 1/2 mv2 (m is mass, v=velocity)
 the larger the mass and/or velocity, the more KE an object has
 kinetic energy can be converted to PE, like lifting a book, pushing a car
 PE can be converted to KE, like dropping a book
 Temperature is a measure of the average KE of the particles that make up the object.
3. Chemical Energy is the potential energy difference between the way atoms are arranged in reactants and
products. Chemical energy can be converted to PE or KE when chemical bonds break and/or form.
(Questions to ponder: if the nucleus is all protons, all with a + charge, what keeps it all together? Don‟t +
charges repel each other), also, how do 2 nuclei come together and stick to form compounds? The electrons
also repel each other…more later in chemistry)
 We call the interaction that keeps atoms linked up in compounds a chemical bond
 A chemical bond (the stickiness between atoms) results from the attractive force between two ions
(ionic compounds), or by two nuclei sharing electrons (molecular compounds)
 Atoms coming together to form bonds is similar to any other 2 objects with attractive forces coming
together. As they come together, energy is released. It takes energy to break bonds, even the reactions
of ATP  ADP + Pi.
 Forming chemical bonds releases energy, THIS IS WHAT WE STUDY IN
Chapter 6
4. PE and KE can be converted to CE
 PE of water about to fall over a dam can be converted to KE, then to CE when you charge a battery with
electricity
 CE can be converted to KE when the battery produces a current and drives your CD player
5. Law of conservation of energy
 One natural law that has been discovered is that energy cannot be created or destroyed, it all ends up in
one form or another
 Energy of universe = a constant, it is never made or lost
 So when KE is converted to PE or CE, all of the original energy is still there
 Often, some energy seems lost, like when you push a shopping cart, but if you look carefully, you can
find the "lost" energy in the form of heat from friction
 HINT: whenever energy seems to be “lost” or gone, it‟s usually lost as heat to the surroundings
6. Units of energy (Look over Tables 6.1 and 6.2)
 The official unit of energy is the Joule (J), and is equal to the KE of a 2 kg object moving at 1 m/s (or
4.4 pounds moving at 2.25 mi/hr)
 1 J is also the energy needed to lift a 2 pound book 4 inches up off of the ground
 Often, energy is listed in kJ, or 103 J, since J is such a small amount of Energy
 The old unit for energy is calorie, which was enough to raise 1 mL of water 1 C
 1 calorie = 4.184 J exactly
 On food labels, you will see a number of Calories (upper case). This is the same energy unit, except
that a nutritional Calorie is 1 kcal, so a soda pop has maybe 240,000 calories (can you convert this to
kcal?) of energy that can be obtained from it.
7. Heat, temperature and energy
 Energy is defined as a property of matter that allows it to do work (the ability to do work)
 Work is defined as the energy used for movement
 Heat is defined as a transfer of energy, it is a property of matter that flows from a warm body to a cool
body, it is a form of energy
 Heat is transferred from a hot piece of glassware in the lab to your cool hand (ouch).
 Temperature is different from heat. Temp is a measure of the kinetic energy of matter, the more
movement, the higher the temperature
8. Examples and clarification of temperature and heat
 Consider 1 gallon of water and 1 cup of water, both can have the same temperature, say 30 C. If you
pour a cup of 30 C water on a 10 pound block of ice, not much will melt. But if you pour 1 gallon of
30 C water on the block, much more ice will melt. The water has the same starting temperature, but the
larger volume contains more heat energy, and the heat flows from the warm water to the ice.
 The flame of a match has about the same temperature as the flame near a bonfire, but the bonfire can
transfer more heat energy.
 The atoms or ions in all forms of matter are constantly in motion. When heat energy is added to matter,
the atoms or ions speed up (KE depends on velocity)
Kinetic Theory of matter
 All atoms/ions are in constant motion
 Adding heat energy increases their motion, but some atoms/molecules may receive more energy than
others
 The KE of matter is the average of the KE of all of the atoms/molecules or ions
 Don‟t forget that Temp is a measure of the average KE
 In gases and liquids, the atoms/molecules/ions are in constant, random motion, but nothing ever moves
very far due to all of the collisions
 Here is how heat is transferred: Fast moving particles collide with slower moving particles until the
average KE of the particles in each substance is identical. See figure below (different text)
from Brady, Russell, Holum, Chemistry, 2002
Section 6.4 Quantifying Heat and Work
 Equation 6.6 is important, so are the 3 definitions of heat capacity.
 Look over Examples 6.2 and 6.3. Similar examples will be worked in class.
 Quickly skim the Work: Pressure-Volume portion of section 6.4.
Section 6.5-6.6 Energy changes in Chemical Reactions
 Much of the energy that we use comes from fossil fuels
 combustion is chemical reaction of fuel and O2, converts chemical energy into KE (heat)
 CH4 + 2 O2  CO2 + 2 H2O + heat energy
 This reaction is termed exothermic, (heat releasing)
 Why is energy released? Each compound in the reactants and products has chemical bonds. When
chemical bonds are formed, energy is nearly always released, because the atoms
have attractive forces and are coming together.
 For the reaction of methane, the products have lower potential energy than the reactants
 High PE  Low PE, energy released (like the falling rock)
 It takes energy to break chemical bonds, (separating 2 objects with attractive forces) so if
a reaction has bonds breaking and forming, there is E gained and E absorbed
 In the case of exothermic reactions, the products have lower potential energy than the reactants, the
balance of the energy is released in the form of heat.
 Some reactions produce compounds with higher PE than the reactants. Where does the extra energy
come from? (The surroundings) These reactions are called endothermic (energy
absorbing). You did one of these in lab in week 5. The reactants combined to make products,
absorbing heat energy from the surroundings (and your hand, that‟s why the flask felt cold). The
products are higher in PE than the reactants.

Biology…biologists routinely „teach‟ students that the decomposition of ATP to ADP and phosphate
releases energy, creating two huge misconceptions. The first is that energy is released when bonds
break. Completely false, even for ATP. Breaking the phosphate bond of ATP requires
energy. The second falsehood is that the ATP molecule doesn‟t just break apart, leaving fragments and
excess energy that the cell would somehow use later. Nearly all useful ATP reactions result in a
transfer of the phosphate group to another molecule, so some new bonds are formed, and forming new
bonds releases energy. The products of ATP  ADP reaction have lower potential energy than the
reactants, and this is energy difference is what drives the reaction, and most of life as we know it.