Exam 1: Rates and Thermo Flashcards
What is the Rate Law
Rate=k[A]^x[B]^y
It expresses the relationship between the rate and concentrations
Directly proportional to the concentration and collisions per second.
What is the rate constant
K
relates the rate of a reaction with the concentration. Remains the same for that reaction.
efficiency of the reaction, higher k=more efficient
k=pze^-Ea/RT
What is the reaction order
the exponent on the reactants. Indicates how the concentration of that substance will effect the rate (zero order no effect, first order doubles, second order increases twofold)
How do you find the overall order?
The sum of the orders in a reaction.
Integrated Rate Law- what is this used for
Allows us to predict concentrations of reactants and products over TIME.
This equation will differ depending on order
Shows most accurate data of [ ] at any given time
Explain what is meant by half life
The amount of time it takes for half of the reactant to be consumed.
First order: ln2/k=t1/2
Arrhenius equation
The relationship between temperature and rate constant.
k=Ae^(-Ea/RT)
-RT(ln k- ln A)=Ea
needs to be manipulated into slope intercept form to be able to graph. once in slope intercept form, we can use that equation to find Ea.
Activation Energy
Ea- or activation barrier, the minimum amount of energy required to initiate a chemical reaction. If there is not suffient energy, the reaction cannot occur.
Collision Model
More collisions means faster reaction, means higher rate. Increasing temp and [ ] will increase collisions, increase rate. “Measure of success rate” by measuring collisions, temperature, rate constant, Ea, and frequency factor.
Orientation factor
P-steric factor, The collision must occur at the correct orientation to allow for a reaction
Collision frequency
Z- the frequency of collisions occuring
Frequency Factor
A- derived from collision frequency and steric factor.
Reaction Mechanism
Sequence of elementary steps by which the reaction will occur
Elementary Step
One step in a series that shows the progress of a reaction
Reaction intermediate
cannot be part of the proposed rate law. It is made then used up.
Rate determining step
Always the slow step. This must match the proposed rate law.
Catalyst
The addition of a catalyst will bring down the Ea by setting proper orientation to allow for more collisions, thereby increasing the rate. Collisions that do occur will be more efficient at producing products
Rate expression
An equation that shows the dissapearance of the reactants and the appearance of the products:
Rate=-A/nT = -B/nT=C/nT
What is reaction rate
The increase (or decrease) in molar concentration of the product (or reactant) per unit of time
Average Rate
Rate of change over the course of entire reaction
Instantaneous rate
Rate of change over a small period of time. Represented by finding the slope at any point on a curve. (change in y/change in x) Equal to the tangent to the curve. More useful than the average rate because we can decipher order based on the graph line
What are some ways we can determine rates?
Spectrophotometrically, Chromatographically, Pressure Changes, Mass.
Why is it more useful to graph rate and concentration than to graph concentration and time?
[ ]/t will give you either a straight line with a constant curve (zero order) a curve( 1 or 2 order).
Rate/[ ] will provide a linear graph which we can use to determine order.
(Remember rate= [ ]/t, meaning rate is our slope)
Draw an energy diagram and label the Ea, transition state, reactants, and products in exo and endo reactions
What are the 4 factors affecting Rates? (chemical kinetics)
- Concentration of reactants.
- Presence of a catalyst
- Temperature
- Surface area (increase sa = increase concentration of material that can react)
What happens when you increase the temperature of a reaction?
A higher temperature will increase the kinetic energy of the molecules, thereby allowing an increase of molecules to exceed the activation barrier, and the reaction rate will increase.
Increase in temp=increase collisions=faster rate
Zero Order Reactions
Concentration does not dictate the rate. This is concentration independent.
Straight horizonal line if graphed rate/[ ]. Straight downward line if graphed [ ]/t
First order reaction
Straight line if graphed rate/[ ], rate is directly dependent on the reactant concentration. If [ ] doubles, rate will double
Second order reaction
graphed as a curved line. Second order means higher collisions
What is the method of initial rates?
The mathematical calculations and experimentally observed comparisons to determine order of reactants.
Concentration and Rates relationship.
Determines the order of the reactants in order to write the rate law.
The higher the reaction order the ________the reaction
More complex
Integrated Rate law
Rate vs Time relationship. Allows us to predict concentrations over time.
Zero: [A]t= -kt+[A]o
1st: ln[A]t= -kt+[A]o
2nd: 1/[A]t= kt+1/[A]o
Half life
First order equation: ln2/k=t1/2
or derived from the integrated rate law.
Effects of temperature on rate
Increase temp=increased rate.
Think of energy diagrams here: increased temp=lower ea=faster reaction
What are the three criteria for proposed rate in reaction mechanisms?
- Adding up elementary steps must equal the overall reaction
- Proposed rate law must equal the observed rate law. (observed can only be found experimentally)
- Must trap out the intermediate, intermediate is not found in the proposed rate law
Which step in a reaction mechanism is the rate determining step?
Always the slow one. A reaction can only go as fast as its slowest reaction
Predicted rate law for the RDS will have to equal the observed rate law.
What are the Laws of Thermodynamics?
- Energy cannot be created or destroyed, but can be transferred
- For a proces to be spontaneous, the entropy of the universe must be positive.
- Entropy of a perfect crystalline substance is zero at absolute 0 (0 Kelvin). As temp increases, motion increases.
What is meant by a “spontaneous” reaction?
A reaction that occurs under specified set of conditions. (up to mother nature, without our intervention)
Define Entropy
State function that measures the amount of disorder in a system, or how dispersed the energy is.
Define enthalpy
The heat exchanged between the system and the surroundings
Gibbs Free energy
The sign determines the reactions spontaneity. Relationship between entropy of the universe and the temperature.
-Gsys= spontaneous
DeltaGsys= -T(deltaSuniv)
DeltaGsys= deltaH - T(delta Ssys)
2nd law of thermodynamics
Suniv= Ssys + Ssurr
Suniv must be + to be spontaneous.
Finding S univ with temperature to determine spont
Suniv= Ssys + -Hsys/T
What states increase entropy
Solid<Liquid< Gas
How to predict increase in entropy
- determine state changes
- # of molecules increase in product
- molecular weight-larger mass=greater entropy
What is meant by equilibrium
G=0
chemical potential or kinetic energy is 0. The most stable it can be. It is the bridge between spontaneous and non spont.
The energy forward matches the energy reverse
If H is and S is, then G is
1: - +
2: + -
3: + +
4: - -
1: spontaneous
2: nonspontaneous
3: Spontaneous at high temps
4: Spontaneous at low temps
How to find G
If at stp and values are given:
Grxn= G[products] - G[reactants]
If stp and H and S are given: Grxn= Hrxn - TSrxn
What does stp mean
Standard Temperature and pressure, 25 C, 1 atm pressure (and 1 M where applicable)
In Lab, how did we determine the rates for each sample?
By using the general rate expression
[ ] /t
taking the concentration of potassium permanganate and dividing by the average time in seconds.
In Lab, why did we use the concentration of potassium permanganate for the rate, and no the oxylic acid?
Because the potassium permanganate was purple in color we could visual time when it had completely reacted. The oxylic acid is clear so we would not be able to see a change.
In lab, how did we determine the final rate law equation?
solving for order of each substance then plugging in to solve for k
What is the purpose of the Arrhenius equation?
To find Ea. In lab, we can already find k at different temps. This equation allows us to see the effect of T on Ea.
Arrhenius said we need energy to make or break bonds.
Give examples of a spontaneous reaction
Ice melting at room temp
Sodium metal reacting violently with water
A ball rolling downhill
Rusting of iron at room temp
Water freezing at -10C
Give examples of a nonspontaneous process
Water freezing at room temp
Sodium hydroxide reacting with hydrogen gas
ball rolling uphill
converting rust back to iron at room temp
Ice melting at -10 C
What are the units for entropy?
Why do we care?
J/K mol
We care because gibbs and enthalpy are both in kj. Must remember to convert for accurate numbers. Remember Kelvin as well. (+273)
What is the significance of the Gibbs sign?
Tells us the direction of a reaction and allows us to determine spontaneity.
Equation to find the temperature at which reactions will become spontaneous
G(or 0)= H/S
Derived from H-T(S). Add T(S), divide by S, set T to 0 (since negative G becomes spontaneous)
Instantaneous rate
a specific point on the curve, a more accurate slope is given with this than with the average rate.
This is the tangent line we will “eyeball”
If you increase the activation barrier, what does that affect in terms of time and rate
Increasing the Ea will increase the time, decreasing the rate.
Decreasing the Ea will decrease the time, increasing the rate.
Example: turning granite into diamond. Takes a long long long time, therefore the Ea must be huge
What is the goal in understanding rates? Why do we care?
So we can understand the behavior of a chemical reaction.
Observed rate law
can only be determined by experimenting and testing method of initial rates in lab.
Define Thermodynamics
The study of energy transfer (not always heat)
Keq
equilibrium constant
Ratio between reactant and product
negative enthalpy means:
the reaction is exothermic-decrease, or expels energy
positive enthalpy would be endothermic- increase or takes in energy
Define Electronegativity and state the trend
the ability of an atom to attract electrons.
up and to the right is more electronegative. Most EN element is Florine.
What is the density of water
1 g/ml
Soluble salts vs insoluble salts. list the ones to know
