56 terms

Laws of Thermodynamics and Changes in Matter

What causes energy transfer?
Molecular collisions; as the faster moving molecules collide with the slower moving ones, they transfer some of their energy which changes speed of both molecules
First law of thermodynamics
Energy can be neither created nor destroyed (it is a universal constant)
Second law of thermodynamics
If a process is favored in one direction, it can't be favored in the reverse; An increase in entropy promotes favored reactions
State functions
△H, △S, △G
What do state function to depend on?
Change between the initial and final states of a system (not on the process by which change occurs)
Why would the addition of a catalyst to a reaction have no effect on the overall energy or entropy change of reaction?
State functions are independent of the reaction pathway
Standard state conditions
1 atm, liquids & solids pure, 1M, Energy formation of Element in natural state is zero, 25°C
What indicates that △H, △S, etc are under standard conditions?
△H°, △S°, △G°
Heat of formation, △H°f
Change in energy that takes place when one mole of a compound is formed from its component pure elements
△H°f for a pure element is defined as ____
If △H°f for compound is negative, energy is______ [released/aborbed] when compound is formed from pure elements & product is ____ [more/less] stable than its constituent elements, so process is _______ [endo/exothermic]
Released, more, exothermic
If △H°f for compound is positive, energy is______ [released/aborbed] when compound is formed from pure elements & product is ____ [more/less] stable than its constituent elements, so process is _______ [endo/exothermic]
absorbed, less, exothermic
{one with △H°f} △H° =
Σ △H°f products -- Σ △H°f reactants
Bond energy
Energy required to break a bond
Bond energy is always a positive number, a negative number, or depends?
Positive number
{one with bond energy } △H° =
Σ Bond energies of bonds broken - Σ Bond energies of bonds formed
What is the number of bonds broken and formed effected by?
Number of that particular type of bond within a molecule and how many of those molecules there are in a balanced equation
Find the enthalpy change under standard conditions for t2H2 + O2 = 2H2O IF: H-H has a bond energy of 436; O-O has a bond energy of 499; O-H has a bond energy of 463
(H° = ΣBond energies of bonds broken - Σ Bond energies of bonds formed)
{1} △H°= [(2)(H-H) + (1)(O-O)] - [(4)(O-H)]
{2} △H° = [(2)(436 kJ) + (1)(499 kJ)] - [(4)(463 kJ)]
{3} △H° = (1,371 kJ) - (1,852 kJ)
{4} △H°= -481 kJ/mol
Hess's law
If a reaction can be described as a series of steps, then △H for the overall reaction is this sum of the △H values for all the steps
The three rules of Hess's Law
{1} if you flip equation, flip sign on enthalpy value {2} if multiply or divide equation by integer, also multiply/divide enthalpy value by same integer {3} sum of enthalpy values equals enthalpy value for all equations when summed up
Vapor pressure
Pressure exerted by molecules that have enough energy to break away from the surface and become gaseous as they escape
As temp increases, vapor pressure of a liquid will ____
Solid to liquid
Liquid to solid
Liquid to gas
Gas to liquid
Solid to gas
Gas to solid
Heat of fusion
Energy that must be put into a solid to melt or heat given off by a substance when it freezes
Heat of vaporization
Energy that must be put into a liquid to turn it into a gas; heat given off by a substance when it condenses
When A substance is changing phases, what happens to the temp?
Remains constant
When pressure is increased, a normal substance will change from ____ to ____, but water will change from _____ to ______
Liquid to solid & solid to liquid
Why does water change from liquid to solid when pressure is increased?
Hydrogen bonds form a lattice structure when it freezes; forces molecules to remain farther apart
Heat capacity (Cp)
Measure of how much the temperature of an object is raised when it absorbs heat
What can an object with a large heat capacity do?
Absorb a lot of heat without undergoing much of a change in temperature
Specific heat
Amount of heat required to raise the temperature of 1 g of a substance 1°C
Measurement of heat changes during chemical reactions
Specific heat equation
q (heat added in J or cal) = m (in kg or g) x c (specific heat) x △T (K or °C)
Entropy, S, of a system
Measure of the randomness or disorder of the system
The greater the disorder of a system, the _____ its entropy
All substances that will be encountered will have some _____ [Positive/negative] value for entropy
Gibbs free energy, or simply free energy, G, of a process
Measure of whether or not the process will proceed without the input of outside energy
A process that occurs without outside energy input is said to be thermodynamically ______[unfavor/unfavored]
If △G is negative,
Reaction is thermodynamically favored
If △G is positive,
Reaction is thermodynamically unfavored
If △G = 0,
Reaction is at equilibrium
Thermodynamically favored processes must result in ____ [Increasing/decreasing] enthalpy or _____ [Increasing/decreasing] entropy or ____
Decreasing, increasing, & both
Equation that relates favorability (G), enthalpy (H) and entropy (S)
△G° = △H° - T△S° (T must be in Kelvin)
-△H, +△S, -△G, low & high temp
Always favored
+△H, -△S, +△G, low & high temp
Never favored
+△H, +△S, +△G, low temp
Not favored
+△H, +△S, +△G, high temp
-△H, -△S, -△G, low temp
-△H, -△S, +△G, high temp
Not favored
When will a redox reaction be favored?
If its potential has a positive value
Equation that shows the relationship between reaction potential and free energy for a redox reaction
△G°=-nF(faradays constant: 96500)E°(stand. Reaction potential)