chem1046 exam 2

chem1046 exam 2

0 order rate law

rate =k

zero order unit

M s^-1 or M/s

1ST Order rate law

rate= k[A]

1st order unit

s^-1 or 1/s

2nd order rate law

rate = k [A]^2
rate = k [A][B]

2nd order k units

M-1 s-1 or 1/Ms

3rd order rate law

rate = k [A]^3
rate = k [A]^2[B]
rate = k [A][B][C]

3rd order k units

M-2 s-1 or 1/M²s

How to find units

X=-(m+n-1)

Rate law

rate=k[A]^x[B]^y

expresses the relationship of the rate of a reaction to the rate
constant and the concentrations of the reactants raised to some powers

The rate law is defined in terms of reactant concentrations (not products).
x and y are the order of the reaction.
The power coefficients x and y are not the same as the stoichiometric coefficients a and b.
For rate laws (x and y) are determined experimentally.

Rate expression

From a chemical equation

Tells you how concentrations change

Rate depends on concentration

Rate law

Determined experimentally

How rate changes relative to concentration change

Reaction

How fast

Chemical kinetics

Understanding the rates of chemical reactions

Reaction rates

The change in amount of stuff (reactant or product) per time interval

Reaction rate units

M/s

rate equation

all rates are __

Positive

Look

Yay

Initial rate

The rate calculated at the start of the reaction

Instantaneous rate

The rate calculated at any specified moment during the reaction

factors affecting reaction rates

Nature of the reacting species

State of reactants (powder vs solid)

Temp (faster at higher temps)

Catalyst

Concentration of reactants (reg oxygen normal or liquid oxygen explosion)

Rate constant

Constant of the proportionality between the reaction rate and the Concentration of reactant

Rate law

Expresses the relationship of the rate of a reaction to the rate constant, and the concentrations of reactants raised to some powers

reaction order

Specify the relationship between the concentrations of reactants, A and B and the reaction rate

reaction rate F2 + 2ClO2 → 2FClO2

Rate = k[F2][ClO]^1

MUST BE DETERMINED Experimentally

Determine rate laws by inspection

Calculating rate constant

Find rate then plug in 1 experiment and do algebra

Not by inspection

Reaction order

Reaction is xth order in A

Reaction is yth order in B

Reaction is (x+y) order overall

Reaction orders

Integrated rate laws (concentration time equations)

Graphs of integrated rate laws

Experimental kinetics

Set up reaction

Measure concentration change over time

Graph the result in different ways

Find the graph that generates a straight line

Determine order rate law and k

Half lives

Amount of time required for an amount of substance to be reduced by 50%

half life equation 0th order

1st order half life

2nd order half life equation

Half life graphs

Collision theory

Molecules must collide

Have enough energy to make and break bonds

Have the correct orientation to react

Rate = #collisions / time

Lower temp

Slower molecules

Less collisions

Lower rate

Higher temp

Faster molecules

More collisions

Increased rate

Concentration collision theory

Rate doubles with concentration doubling

Kinetic energy < bond energy

Bounces

no reaction

Kinetic energy > bond energy

Collide bond breaks

Form two new molecules

Reaction occurs

Activation energy

Minimum energy that must be overcome for the reaction to occur

Reaction diagram

Exo vs Endo graphs

Reaction rate

Depends on kinetic energy and temperature

Reaction rate is temperature dependent

Arrhenius equation

Know

Transition state

Short-lived, high energy configuration of atoms at an energy maximum in a reaction energy diagram

Neither reactant nor product but a transition species with partial bonds

Cannot be isolated

Intermediate

A species that is produced in one step and consumed in a subsequent step

Appears in the steps, but does not appear in the overall reaction

Product for one step reactant for the next

Can sometimes be isolated

Each hill on a reaction diagram

Is each step

Elementary step / reaction

Explicit Representations of the change taking place

Reaction occurs as depicted with intermediates

Only one hill on the diagram

Molecularity

Number of molecules that come together in an elementary step

Reactants only add coefficients

Some of the stoichiometric Coefficient of the reactants

Only one hill on the reaction coordinate diagram

Rate law for elementary step

Per each elementary step

Elementary steps

Coefficients can be used for Rate coefficients !!! ONLY ELEMENTARY

Overall rate law for elementary steps

Determined experimentally by the slowest step!!

Rate limiting step (rate determining step)

The slowest step of a multi step reaction

The rate limiting step is the one with the highest activation energy

Dictates the overall rate and rate law for a multi step reaction

As Ea increases , k and rate decrease

For Arrhenius equation

Overall rate law (of a multistep reaction) can only contain

Reactants not intermediates

If intermediate is in an elementary rate determining (slow) step you must

find which step is At equilibrium, find the forward and reverse rates

then set them equal to each other

Replace what you get for the intermediate into the final rate law

Proposing a reaction mechanism

Catalyst

A substance that increases the rate of a chemical reaction without being consumed

Catalysts

Takes part in the reaction

Lowers the Ea by changing the mechanism of the reaction

Is not consumed

Speeds up both the forward and reverse reactions

Does not increase the yield of product but gets the products more quickly

Catalyzed vs un catalyzed

How does a catalyst lower the activation energy

It changes the mechanism of the reaction

Identify a catalyst

Not actually being consumed

Not in overall reaction

Homo vs hetero catalysts

Homo - same phase as reactants

Hetero- different phase than reactants

Bio catalysts

Enzymes- biological molecules (proteins) The act as homogeneous catalyst in the Aqueous solution of cells

Ex : lock and key catalysts

Only one shape fits in active site of enzyme

Chemical equilibrium

A reversible process where the reaction rates in both directions are equal so that the system gives the appearance of having a static composition

Reaction must be

Reversible

The system must be closed

The rate of forward and reverse reactions are equal

A chemical change is occurring, but Concentrations of the reactants and products remain constant

equilibrium does not mean

The concentrations of reactants and products are equal

The molecules are unchanging

Example of equilibrium graph

Equilibrium graphs

Why a system won’t see equilibrium

Not enough reactant/product

Not enough time

Not a closed system

Remember about chemical equilibrium

Equilibrium does not mean that concentrations are all equal

It does mean that the rate of Forward and reverse reactions are equal

Every equilibrium has an equilibrium constant

Equilibrium rate

Rate A→ B = Rate B → A

Equilibrium constant formula

Equilibrium constant has ___ units

No

If kfwd > Krev

Mostly products

Kfwd<krev

Reactant formation favored

Homo equilibrium

All reacting species are in the same phase

Hetero equilibrium

Reacting species in different phases

Concentration of pure liquids and solids are ____ included in the expression for the equilibrium constant

never

The equilibrium expression for a reaction written in the reverse direction is the

Reciprocal of the one for the Forward reaction

If the coefficients in a balanced equation are multiplied by a factor of x, the equilibrium expression is raised to the xth power

If two or more reactions are added to give another the equilibrium constant for the overall reaction is

the product of the equilibrium constants of the equations added

Reaction quotient (Q)

Measures the relative amounts of products and reactants present during a reaction at a particular point in time

Reaction quotient

K is constant at a given temperature Q..

Changes depending on the reaction progression

Q vs K

Q VS K

Q= K

Equilibrium

Q<K

Too many reactants Not enough products

Reaction proceeds to the right

Generate more Products Consume reactants

Q> K

Too many products, not enough reactants

The reaction proceeds to the left

Will generate more reactants, consume products

Solving for Q

First find K

K = B/ A

1.2/2.8 = .43

Find Q

Count b and a to do so

Find which one matches K

Answer is B

Le chateliers principle

When I chemical system at equilibrium is disturbed, it returns to equilibrium by undergoing net reaction that reduces the effect of the disturbance

Le chateliers principle In action.