Chemistry Ch. 3

Front 1

Extensive properties

Back 1

-properties that are proportional to the size of the system
-if a property doubles when the systems are combined, the property is extensive.

-the study of energy and its relationship to macroscopic properties in chemical systems.
-based on probabilities, and are only valid for systems composed of a large number of molecules.

-examples are moles and volume.

Front 2

Intensive properties

Back 2

-properties that are independent of the size of the system.
-if you combine 2 identical systems and a property is the same for both the single system and the combined system, the property is intensive.

Front 3

State Functions

Back 3

-Properties that describe the state of a system, but not necessarily how the state was formed or what reaction pathway brought a state into being.

Front 4

The two ways to transfer energy between systems

Back 4

1. Heat
2. Work

Front 5

Heat (q)

Back 5

The natural transfer of energy from a warmer body to a cooler body.

Front 6

Work

Back 6

-Any energy transfer that is not heat.

Front 7

3 heat forms

Back 7

1. Conduction
2. Convection
3. Radiation

Front 8

Conduction

Back 8

-the thermal energy transfer via molecular collisions.
-requires direct physical contact
-higher energy molecules from one system transfer some of their energy to the lower energy molecules of the other system via molecular collisions.
-temperature difference in thermal conductivity is like pressure difference in fluids or potential difference in electricity.

Front 9

Thermal conductivity

Back 9

-the ability of an object to conduct heat
-depends upon the objects composition and its temperature.
-the rate of heat flow is constant across any number of slabs between two reservoirs.
-conservation of E!!

Front 10

Convection

Back 10

-thermal energy transfer via fluid movements.
-differences in pressure or density drive warm fluid air in the direction of cooler fluid.

Front 11

Radiation

Back 11

-thermal energy transfer via electromagnetic waves.
-all objects above 0K radiate heat
-objects that radiate heat faster also absorb heat faster, which means that it come to equllibrium with its environment more quickly.

Front 12

PV work

Back 12

w = PdeltaV(constant pressure)

-work as it applies to a chemical system at rest.
-at constant pressure, work is equal to the product of the pressure and the change in volume.
-A system at rest with no gravitational potential energy and no kinetic energy may still be able to do PV work.
-PV work takes place when a gas expands against a force regardless of whether or not the pressure is constant.
-there must be a change in volume!

Front 13

Internal energy

Back 13

-the collective energy of molecules measured on a microscopic scale.
-all the possible forms of energy imaginable on a molecular scale.
-may also be referred to as 'thermal energy' or 'heat energy'.
-if we have a closed system with no electric or magnetic fields, the only energy change will be in internal energy.
-for a reaction within a system involving no change in volume, there is NO work done and the change in internal energy is equal to the heat.

Front 14

Temperature

Back 14

-A state function that describes changes in thermal energy.
-An increase in thermal energy is due to increases in internal energy.
-An increase in thermal energy increases temperature.
-In a fluid, temperature is directly proportional to the translational kinetic energy of its molecules.
-ideally, can be thought of as the thermal energy per molecule or mole of molecules.

Front 15

Average kinetic energy in a fluid

Back 15

-given by the equation KEavg = (3/2)kT

Front 16

Pressure of an ideal gas

Back 16

-the random translational kinetic energy per volume. An intensive function.
-the greater the random translational kinetic energy of gas molecules per volume, the greater the pressure.

Front 17

Enthalpy

Back 17

Equation (under constant pressure conditions):

deltaH = deltaU + PdeltaV
H = entropy
U = internal energy
P=pressure
V=volume

-a man-made property that accounts for the extra capacity to do PV work.
-a state function
-measured in units of energy (joules) but not conserved like energy.
- we are interested mainly with CHANGE in enthalpy.

Front 18

Standard state

Back 18

-an element in its standard state at 25C is arbitrarily assigned an enthalpy value of 0J/mol.
-from this, enthalpy values are assigned to compounds based upon the change in enthalpy when the are formed from raw elements in their standard states at 25C.

Front 19

Standard enthalpy of formation

Back 19

-the change in enthalpy for a reaction that creates one mole of that compound from its raw elements in their standard state.
-for reactions involving no change in pressure, the change in enthalpy is equal to the heat.

Front 20

Heat of reaction

Back 20

delta(enthalpy of reaction) = delta(standard enthalpy of formation of products) - delta(standard enthalpy of formation of reactants).

-Since in many reactions in the lab, enthalpy approximates heat, the change in enthalpy from reactants to products is often referred to as the heat of reaction.

Front 21

Hess's law

Back 21

The sum of the enthalpy changes for each step is equal to the total enthalpy change regardless of the path chose.

Front 22

Endothermic

Back 22

-a reaction is endothermic if it has a positive enthalpy change.

Front 23

Exothermic

Back 23

-a reaction is exothermic if it has a negative enthalpy change.

Front 24

Transition state

Back 24

-the point in a reaction in which old bonds are breaking and new bonds are forming.
-represented in the 'peak' of the energy hill.
-occurs during the reaction collision.

Front 25

Reaction intermediates

Back 25

-products of the first step in a two step reaction.

Front 26

Thermodynamics

Back 26

-the study of energy and its relationship to macroscopic properties in chemical systems.
-based on probabilities, and are only valid for systems composed of a large number of molecules.
-CANNOT be applied to microscopic phenomena (for the most part).

Front 27

Entropy (S)

Back 27

-nature's tendency to create the most probable situation that can occur within a system.
-nature likes to lower energy of a system when it is high relative to the energy of the surroundings, but that means that nature likes to raise energy of a system when it is low relative to the energy of the surroundings.
-Entropy, not energy, dictates the direction of a reaction.
-equilibrium is the point in a reaction where the universe has achieved maximum entropy.
-entropy increases with number, volume, and temperature.
-an extensive property.
-Units: J/K

Front 28

Second law of thermodynamics

Back 28

-states that the entropy of an isolated system will never decrease because the odds of a decrease in entropy are so low.
-if we consider the universe as a system:
-the sum of entropy changes of any system and its surroundings equals the entropy change of the universe, which must be equal to or greater than zero.

delta(S)system +delta(S)surroundings = delta(S)universe, which is greater than or equal to zero.

-this indicates that entropy can decrease only if at the same time, entropy of the surroundings increases by a greater or equal magnitude.

Front 29

Irreversible reaction.

Back 29

-If the activation energy of a chemical reaction is sufficiently high for the reverse direction, the probability that it will occur may be sufficiently high for the reverse direction, the probability that it will occur may be sufficiently low that chemists call the reaction irreversible.
-a thermodynamically reversible reaction is one that stays infinitely close to a state of equilibrium at all times.

Front 30

Third law of thermodynamics

Back 30

-assigns by convention zero entropy value to any pure substance (either an element or a compound) at absolute zero and in internal equilibrium.

Front 31

Thermodynamics

Back 31

-the study of energy and its relationship to macroscopic properties in chemical systems.
-based on probabilities, and are only valid for systems composed of a large number of molecules.
-CANNOT be applied to microscopic phenomena (for the most part).

Front 32

First law of thermodynamics

Back 32

-energy of the system and surroundings is always conserved.