Exam 2
Which of the following is (are) unaffected by reversing the direction in which a chemical equation is written?
A) The equilibrium concentrations
The average rate of a reaction is the rate of reaction at any given time.
A) True
If a reaction is exothermic, then the activation energy for the forward reaction is ____ the activation energy for the reverse reaction.
B) less than
Consider the following reaction: 4HCl (g) + O2 (g) <-> 2H2O (g) + 2Cl (g) with deltaH = -114.4 kJ One way to decrease the amount of chlorine present at equilibrium is
B) raising the temperature.
For which of the following reactions is Kc equal to Kp?
C) PCl3 (g) + 3NH3 (g) <-> 3 HCl (g) + P(NH2)3 (g)
At equilibrium
C) the forward and reverse rates are equal.
Strong acids:
HClO4 H2SO4 HI HBr HCl HNO3
Ionization Constants of Conjugate Acid-Base Pairs
KaKb = Kw
Comparing concentration and pressure
Kp = Kc (RT) ^n
In pure water or any aqueous solution the equilibrium constant of water is:
Kw = [H3O+] [OH-] = 1.00 x 10^-14
Strong bases:
OH-
Bases produce
OH- ions in water
average rate
[Br2] final - [Br2] initial/t final - t initial
percent ionization
[H+]/[HA]0 x 100%
Equilibrium
a state in which there are no observable changes as time goes by
Equilibrium constant (K)
a temperature dependent constant that represents the ratio of reactant and product concentration at equilibrium K = [products]^x/[reactants]^x
Effect of Concentration
an increase/decrease in concentration of any of the components of a reaction will shift the reaction to lower/raise that concentration
first order
as the reactant concentration doubles the rate doubles rate = k[A]^1
second order
as the reactant concentration doubles the rate quadruples rate = k[A]^2
Quadratic Equation
ax^2 + bx + c = 0
Bases can
be neutral or negatively charged
reaction quotient (Qc)
calculated by substituting the initial concentrations of the reactants and products into the equilibrium constant (Kc) expression
Rate of creation of product:
change in molarity of product/change in time
Strong acids and bases
completely dissociate
unimolecular reaction
elementary step with 1 molecule
bimolecular reaction
elementary step with 2 molecules
termolecular reaction
elementary step with 3 molecules
rate law
expresses the relationship of the rate of a reaction to the rate constant and the initial concentrations of the reactants raised to some power Rate = k[A]^x[B]^y Rate = reaction time k = rate constant [A] and [B] = concentration of reagents sum of all exponents (x + y) = reaction order
Effect of temperature
for endothermic or exothermic reactions heat can be written as a reactant or product, thus changing the temperature has the same impact as changing the concentration of any other reactant or product.
reaction mechanism
how do the atoms move during the reaction, are there multiple steps?
reaction rate
how fast does a chemical reaction occur?
Temperature dependence of the rate constant
k = A x e^(-Ea/RT)
Strong acids have
larger K values because they are product favored
Bronsed-Lowry bases
must have a lone pair of electrons to accept the H+
pH Scale Calculations:
pH = -log [H3O+] pOH = -log [OH-] [H3O+] = 10^-pH [OH-] = 10^-pOH Kw = 1.00 x 10^-14 pH + pOH = 14.00
Weak acids and bases
partially dissociate
Effect of Pressure and Volume
pressure changes only impact the system if one or more chemicals is in the gaseous form
instantaneous rate
rate for specific instance in time
zero order
reactant concentration does not effect the reaction rate rate = k[A]^0
When the concentration of A or B increased
shifts to products
When the concentration of C is continually decreased
shifts to products
When the concentration of C is decreased
shifts to products
When the concentration of A of B is decreased
shifts to reagents
When the concentration of C is increased
shifts to reagents
simple rate laws
show how the rate depends on the initial reactant(s) concentration(s)
rate-determining step
slowest step in the sequence of steps leading to product formation
intermediate
species that appear in a reaction mechanism but not in the overall balanced equation
Bases
substance that can accept the H+ ion from an acid; proton acceptors
catalyst
substance that increases the rate of a chemical reaction without itself being consumed; lower the activation energy
Acids
substances that can donate a H+ ion to another molecule or ion; proton donors
activation energy (Ea)
the minimum amount of energy required to initiate a chemical reaction
molecularity of a reaction
the number of molecules reacting in an elementary step
elementary steps/elementary reactions
the overall progress of a chemical reaction can be represented at the molecular level by a series
half life (t 1/2)
the time required for the concentration of a reactant to decrease to half of its initial concentration
integrated rate laws
used to determine the reactant concentration at any time
ICE Table
using the initial concentrations and the equilibrium constant to predict the concentrations at equilibrium
Le Chatelier's Principle
when a stress is applied to a system at equilibrium, the equilibrium shifts to relieve the stress. Stresses include changing concentration, pressure, temperature, or volume.
rate laws:
- are always determined experimentally - is always defined in terms of reactant concentrations - is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation
Rate of consumption of reactant:
- change in molarity of reactant/change in time
Rate of chemical reaction depends on many factors:
- initial concentration of reagents - time - temperature - collision frequency - orientation - activation energy - reaction mechanism/multiple steps
Methods for increasing the reaction rate:
- reactant concentration - temperature - catalyst
Chemical equilibrium:
- the rates of the forward and reverse reactions are equal - the concentrations of the reactants and products remain constant
Adding a catalyst:
-does not change K -does not shift the position of an equilibrium system -system will reach equilibrium sooner -lowers Ea for both forward and revers reactions
Diprotic and Triprotic acids:
-lose one proton at a time -separate K expressions for each proton transfer -the Ka for removing the 2nd or 3rd protons is smaller than the previous Ka
Writing plausible reaction mechanisms:
-the sum of the elementary steps must give the overall balanced equation for the reaction -the rate-determining step should predict the same rate law that is determined experimentally