Download Chapter Six Outline

A.P. Chapter Six Outline
Energy and Chemical Reactions
I.
II.
The Nature of Energy
 Energy: the capacity to do work
A. Energy is classified as kinetic or potential
1. Kinetic energy is energy that something has because it is moving
a) mechanical energy
b) thermal energy
c) electrical energy
 Ek= ½ mv2 m = mass v = velocity
2. Potential energy is energy that something has as a result of its position
and some force that is capable of changing that position.
a) gravitational energy
b) electrostatic energy
c) chemical potential energy
 Ep= mgh g = 9.8m/s2 h = height
B. Energy Units
1. Joule (J) 1J = 1 kg m2/s2
2. Calorie (c) 1 cal = 4.184 J
1 calorie will raise 1 gram of water 1 degree celcius
Conservation of Energy
 Law of Conservation of Energy: energy can be neither created nor
destroyed. This is the first law of thermodynamics.
 Work is a process that transfers energy to an object
A. Kinetic-Molecular Theory: all matter consists of nanoscale particles that
are in constant motion. Therefore, all matter has thermal energy. For a
given sample, the quantity of thermal energy is greater the higher the
temperature is.
B. Heat refers to the energy transfer that occurs when two samples of matter
at different temperatures come into contact. Energy always transfers from
the hotter to the cooler until both are at the same temperature. This is
called thermal equilibrium.
C. Systems: the area you are focusing on is referred to as a system, and
everything that can exchange energy with the system is called its
surroundings.
 The internal energy of a system is the sum of the individual
energies (kinetic and potential) of all nanoscale particles (atoms,
molecules, and ions) in the system.
 The total internal energy of a sample of matter depends on
temperature, the type of particles, and the number of particles in
the sample.
D. Calculating Thermodynamic Changes
 If E = a systems internal energy, then the change is calculated as
Efinal – Einitial, = ∆E. A positive value indicates an increase in energy and a
negative value indicates a decrease in energy. This also indicates the
direction of the energy change: a negative sign indicates that energy is
transferred out of the system whereas a positive sign indicates energy is
transferred into the system.
E. Conservation of Energy and Chemical Reactions
 The law of conservation of energy can be written as
∆Esystem = qsystem + wsystem where q represents the quantity of energy
transferred by heating the system, and w represents the quantity of energy
transferred by doing work on the system.
 If energy is transferred into the system by heating then q is positive; if
energy is transferred into the system because the surroundings do work
on the system then w is positive.
 The magnitudes of q and w indicate the quantities of energy
transferred, and the signs of q and w indicate the direction in which the
energy is transferred.
III.
Heat Capacity
 The heat capacity of a sample of matter is the quantity of energy required
to increase the temperature of that sample by one degree.
 Specific heat capacity is defined as the quantity of energy needed to
increase the temperature of one gram of a substance by one degree
Celsius.
 Specific heat capacity = the quantity of energy transferred by
heating/(sample mass x temperature change)
 c = q/(m x ∆t)
 Molar heat capacity, cm , is the quantity of energy that must be transferred
to increase the temperature of one mole of a substance by one degree
Celsius.
IV.
Energy and Enthalpy
 Energy changes also accompany physical or chemical changes, even
though there may be no change in temperature.
 Changes of state, or phase changes, are physical processes where one state
of matter is transformed into another, for example melting a solid.
 Heat of fusion: the quantity of thermal energy that must be transferred to
a solid as it melts.
 Heat of vaporization: the quantity of energy that must be transferred to
convert a liquid to a vapor.
 Endothermic processes: energy transfers in
 Exothermic processes: energy transfers out (exits)
A. Enthalpy: Heat Transfer at Constant Pressure
 Work is done when a force moves an object through some distance
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If a process is carried out in a closed container and the heat transfer
is measured, then ∆E has been determined.
The quantity of thermal energy transferred into a system at
constant pressure, qp, is called the enthalpy change of the system.
Enthalpy change is symbolized by ∆H
∆H = qp
∆E = qp + wexpansion = ∆H + wexpansion
Whenever heat transfer is measured at constant pressure, it is ∆H
that is determined.
Both energy and enthalpy are state functions, properties whose
values are invariably the same if a system is in the same state. A
systems state is defined by its temperature, pressure, volume, mass,
and composition.
V.
Thermochemical Equations
 A thermochemical equation is a balanced chemical equation together with
the corresponding value of the enthalpy change.
 Standard enthalpy change, ∆Hº, is defined as the enthalpy change at the
standard pressure of 1 bar and a specified temperature, usually 25ºC.
VI.
Enthalpy Changes for Chemical Reactions
 A temperature change is one piece of evidence that a chemical reaction
has taken place. The large energy transfers that occur during a chemical
reaction result from the breaking and making of chemical bonds as
reactants are converted into products.
 The sign of ∆H indicates the direction of energy transfer
 The magnitude of ∆H depends on the states of matter of reactants and
products
 The balanced equation represents moles of reactants and products
 The quantity of energy transferred is proportional to the quantity of
reaction that occurs
VII. Where Does the Energy Come From?
A. Bond Enthalpies
 Separating two atoms that are bonded together requires a transfer of
energy into the system.]
 The enthalpy change that occurs when two bonded atoms are separates is
called the bond enthalpy, or bond energy.
 Bond enthalpies are always positive, and bond breaking is always
endothermic, whereas the formation of bonds from separated atoms is
always exothermic.
VIII.
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Measuring Enthalpy Changes: Calorimetry
A calorimeter is a device that measures heat transfer, often a “bomb”
calorimeter.
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X.
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XI.
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XII.
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qreaction + qbomb + qwater = 0
-qreaction = qbomb + qwater
The heat transfer out of the system (-qreaction) equals the heat transfer into the
bomb and water.
Hess’s Law
If the equation for a reaction is the sum of the equation for two or more other
reactions, then ∆H for the first reaction must be the sum of the ∆H values of
the other reactions
Standard Molar Enthalpies of Formation
The standard molar enthalpy of formation, ∆Hºf, is the standard enthalpy
change for formation of one mole of a compound from its elements in their
standard states.
The standard state of an element or a compound is the physical state in which
it exists at 1 bar and at a specified temperature, normally 25ºC.
Standard enthalpies of formation for the elements in their standard states are
zero.
Chemical Fuels for Home and Industry
A chemical fuel is any substance that will react exothermically with
atmospheric oxygen and is available at reasonable cost and reasonable
quantity.
Two important criteria for a fuel are the fuel value, which is the quantity of
energy released when 1g of fuel is burned to form carbon dioxide and water,
and the energy density, which is the quantity of energy released per unit
volume of the fuel.
Foods: Fuel for our Bodies
Foods are similar to fuels in that the caloric value of fats and carbohydrates
correspond to the standard enthalpy of combustion per gram of the food.