unit 7 study guide Flashcards
thermodynamics laws
0) If two things are the same temperature as a third, they are also the same temp.
1) Energy is Conserved, ΔEuniverse = 0
2) Energy spreads out, ΔSuniverse > 0
3) A perfect crystal at Absolute Zero
has zero entropy, S = 0.
enthalpy
state function
H = E + PV
entropy
entropy (S) – the degree of disorder.
The “order” of a system is just as important
S > 0 means more disorder.
gibbs free energy
To simplify various calculations and comparisons, and to make full use of the 2nd Law of Thermodynamics, chemists define another term, Gibbs Free Energy:
G = H – TS
Where H = enthalpy, T = temperature, and S = entropy. G is a state function because H, T, and S are state functions.
what reactions are spontaneous without help and what causes rxn to be spontaneous
Exothermic reactions usually take place spontaneously (without help)
High energy -low energy
Order -disorder
These natural processes together drive reactions to continue spontaneously
whats inequality equation for exothermic rxn
ΔH<0
inequality equation for edothermic rxn
ΔH>0
hess’ law
Hrxn° = nHf°,products – mHf°,reactants
where n and m are the stoichiometric coefficients.
breaking bonds is what type of reacrion
endothermic
work
w = F × d
pv work
most common
p/FA
Gases can do work through expansion or compression against a constant external pressure. Work done by gases is also sometimes called pressure-volume or PV work for reasons that will hopefully become more clear in this section! Let’s consider gas contained in a piston.
Pressure–volume work. Pressure–volume work (or PV work) occurs when the volume V of a system changes. PV work is often measured in units of litre-atmospheres where 1L.
specific heat
q=mcat (mc delta t)
Where q is the heat gained by the system, c is the specific heat of the substance, m is the mass, and ΔT is the change in temperature (in °C or K). Every substance has a different specific heat capacity, which is the amount of heat required to raise one gram of it 1 °C.
kinetics factors
The speed of a reaction depends on:
1) Temperature
2) Concentration
3) Surface area (for solids)
4) Catalysts / inhibitors
temp in kinetics
) Temperature: The hotter a reaction is, the more reactants have enough energy to react. (Hotter = faster reaction)
concentration in kinetics
The more concentrated a solution is, the more often reactants collide. (More concentrated = faster reaction)
surface area in kinetics
(for solids): Finer powders allow more reactants to collide. (More surface area = faster reaction)
catalysts/inhibitors in kinetics of rxn
Catalysts can speed a reaction by aligning reactants or by changing the reaction mechanism.
kinetic molecular theory
A theory of the thermodynamic behavior of matter, especially the relationships among pressure, volume, and temperature in gases, based on the dependence of temperature on the kinetic energy of the rapidly moving particles of a substance.
The kinetic theory describes a gas as a large number of submicroscopic particles (atoms or molecules), all of which are in constant rapid motion that has randomness arising from their many collisions with each other and with the walls of the container.
rate law
The rate law of a given reaction is a concise mathematical formula that describes the instantaneous rate of that reaction under various conditions.
The rate law of a reaction can only be determined experimentally!
rate law gen form
The general form of a rate law is:
Rate = k [A]^x [B]^y [C]^z
k is the rate constant
(depends on temperature and state of reagents)
A, B, and C are reactants, (catalysts, or inhibitors)
x, y, and z describe how the rate depends on the concentration of A, B, and C
order of reaction
In chemical kinetics, the order of reaction with respect to a given substance (such as reactant, catalyst or product) is defined as the index, or exponent, to which its concentration term in the rate equation is raised. … Reaction orders can be determined only by experiment.
rate constant
The general form of a rate law is:
Rate = k [A]x [B]y [C]z
k is the rate constant
equilibrium
Equilibrium is the state at which forward and reverse rxns occur at the same rate, so the concentration of all species remains constant.
Given enough time, all reactions will eventually reach equilibrium. However, many reactions never reach equilibrium (e.g. rusting metal, living systems, etc.).
equilibrium constant
Keq > 1 products are favored
Keq»_space; 1 almost entirely products
Keq < 1 reactants are favored
Keq «_space;1 almost entirely reactants
Le Châtelier’s Principle
Any reaction at equilibrium that is disturbed by a change in concentration, pressure or temperature will shift its equilibrium position to counteract the change.
heterogenous equilibrium
The concentrations of reactants and products are typically expressed as “activities” for equilibriums. The activity of a chemical is the ratio of its concentration or pressure and its standard value.
1) The equilibrium constant is unitless
2) Heterogenous chemicals (e.g. solids in an aqueous rxn) have an activity of 1
3) The rxn solvent has an activity of 1 if other species have low concentrations
what kind of reactions are spontaneous only at low temperatures
deltaG= deltaH<0 and delta S<0
what kind of reactions are spontaneous only at high temperatures
deltaG= deltaH>0 and deltaS>0
if you add a catalyst to a reaction, where will the equilibrium go
products
if you increase the temperature of a reaction, where will the equilibrium go?
products
if you decrease the volume of the reaction vessel, where will the equilibrium go?
the reactants
