Transcript Unit 11 PowerPoint

Key Terms
 Average kinetic energy - Energy associated with the movement of
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matter and mass
Bond energy - The amount of energy it takes to break one mole of
bonds
Calorimetry - measurement of quantities of heat
Chemical energy - the energy in a substance that can be released by a
chemical reaction
Collision theory - explains how chemical reactions occur and why
reaction rates differ for different reactions
Delta H (ΔH) - a measure of the energy associated with a system
Endothermic - Requiring a net input of heat for its formation
Enthalpy - a measure of the energy associated with a system
Key Terms
 Exothermic - Accompanied by the release of heat
 Heat - a form of energy that is transferred by a difference in
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temperature
Molecular movement - how molecules move and is measured by
temperature
Potential energy diagram - plots the change in potential energy that
occurs during a chemical reaction
Specific heat - The heat required to raise the temperature of the unit
mass of a given substance by a given amount
Temperature - The measure of molecular motion or the degree of heat
of a substance
Thermal energy - The total potential and kinetic energy associated
with the random motions of the molecules of a material
Types of Energy
 Mechanical
 Kinetic – based on particle motion
 Potential – based on height and acceleration due to gravity
 Chemical
 Kinetic – based on particle motion to define temperature
Equation: KE=1/2 MV2
 Potential – energy stored in chemical bonds
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 Electrical – based on the flow of electrons from an area of high charge to an area of low
charge
 Nuclear – based on the release of energy stored in the nucleus
 Thermal – based on the transfer of heat
Energy Transfer
 Energy always flows from a position of high energy to a
position of low energy
 There are multiple ways that energy is transferred
 Potential to Kinetic as objects fall
 Kinetic to potential as objects rise
 Electrical
 Chemical as reactions occur
 Thermal from hot to cold (energy cannot be transferred
from cold to hot)
Chemical Energy Transfer
 The chemical energy transferred in chemical reactions is related
to the difference in potential energy of the reactants and the
products
 The potential energy of chemical compounds is stored in the
compounds when they are formed – the heat of formation (Hfo)
 We standardize the heats of formation based on the difference
found when the compounds are formed from elements.
 This means that the heat of formation of any element always
equals zero
Enthalpy & Types of Reactions
 When thermal energy leaves the reaction, the reaction is called
exothermic
 ∆HRXN is negative – thermal energy is leaving the system
 Potential energy of products is less than the potential energy of
the reactants
 It is the excess potential energy of the reactants that is leaving as
thermal energy
 When thermal energy comes into the reaction, the reaction is called
endothermic
 ∆HRXN is positive – thermal energy is entering the system
 Potential energy of products is greater than the potential energy
of the reactants
 It is the thermal energy entering the system that provides the
additional potential energy needed to form the products.
Reaction Coordinate Diagrams for
Endothermic & Exothermic Reactions
Energy of Reactants < Energy of Products
Energy of Products < Energy of Reactants
Endothermic
Exothermic
ΔHRXN = ΔHProducts - ΔHReactants
Heat of Reaction - Enthalpy
 The heat transfer in a chemical reaction is defined as enthalpy which
represents the change in the energy stored in the chemical bonds
 The Enthalpy is equal to the overall change in chemical potential
energy: ∆HRXN
 Enthalpy = Potential Energy of Products – Potential Energy of
Reactants
 ∆HRXN = ∑ ∆Hf0 (products) - ∑ ∆Hf0 (reactants)
Law of Conservation of Energy
 Energy may not be created or destroyed
 Energy may change form
 Therefore, the sum of all of the energies in a system is a constant
 Enthalpy = Potential Energy of Products – Potential Energy of
Reactants
 ∆HRXN = ∑ ∆Hf0 (products) - ∑ ∆Hf0 (reactants)
 ∆HRXN is positive, the reaction is endothermic
 ∆HRXN is negative, the reaction is exothermic
Specific Heat & Calorimetry
 The heat of reaction is measured through a process of
determining the effect on the environment.
 Assuming no loss of energy, the energy out of one system
is equal to the gain in energy of another system.
 Calorimetry is the process of quantifying this energy
 Calorimetry is dependent on knowing the effect of
temperature change on the environment – specific heat
Specific Heat
 Specific Heat (cp) - The amount of thermal energy (heat)
required to change the temperature of one gram of matter, one
degree Celsius.
 The use of specific heat allows for determining the amount of
heat lost or gained.
 Heat gained or lost = (mass) (specific heat) (change in temperature)
q = m cp ΔT
Heating Curve of Water
Image used courtesy of http://ch301.cm.utexas.edu/thermo/selector.php?name=heat-curves
Calorimetry
 Assuming no loss of heat, the thermal energy out of one
system is equal to the gain in thermal energy of another
system.
qin = - qout
 If qout = ∆HRXN , then
qin = - ∆HRXN
 If enthalpy is negative, temperature increases (exothermic)
 If enthalpy is positive, temperature decreases (endothermic)