Chemistry Exam 2!!

What are chemical reactions?

making/breaking bonds

What is the rate of reaction?

how fast products are made (appear) and reactants are used (disappear), the speed depends on nature of reactants (M/s)

What is the formula for rate of reaction?

^[x]/ ^t

What does it mean if the rate of reaction is negative?

rate of disappearance is greater than rate of formation

What does it mean if the rate of reaction is positive?

the rate of formation is greater than the rate of disappearance (this is most common)

What are the 4 factors that manipulate reaction rate?

physical state, concentration, temperature and the presence of a catalyst

How does physical state affect reaction rate?

For the rxn to occur, reactants must collide (physical contact)- they need proper orientation/correct energy, homogenous rxns (in same phase) are faster because collisions increase

How does the concentration of reactants [x] affect reaction rate?

faster rate if 1 or more reactants concentration is higher (bc it increases collisions)

How does temperature affect reaction rate?

if temperature increases, then KE increases and collisions increase

How do catalysts affect reaction rate?

catalysts are chemical reagents that increase the rate of reaction w/o being consumed (aka enzyme)

What does an average rate graph look like?

At zero = initial concentration, change in concentration over time, the curve is approximately linear, the larger the time interval, the more the rate deviates from instantaneous (as time goes on rate slows, because less reactant to interact with

How do you calculate the instantaneous rate from an average rate graph?

take the slope of line tangent to curve @ time

What is rate law?

k [A]^m [B]^n, k=rate constant, m&n=reaction orders, sum of exponents=overall reaction order

What are the units of K?

M-1 from the order times S-1

What is the method of official rates?

An experiment you do where you change the concentration of 1 substance, and compare 2 concentrations & determine how much the rate changes

What is the integrated rate law?

how rate of rxn changes @ temp. If concentrations are changed. It’s integrated into the equation to find the order

What is the first-order integrated rate law?

ln [A] = ln [A0]- KT

What is the second-order integrated rate law?

1/[A]= Kt+ 1\ [A0]

What is the zero-order integrated rate law?

[A]= -k +[A0]

What is half-life?

time required for 50% of initial concentration to be used up—→ all radioactive decay q’s= 1st order (concentration independent)

What is zero order?

decreases by ½ over time

Why do rxn rates increase as temperature increases?

Collision model-molecules must collide to react, so concentration increases —→ reaction rate increases, temperature increases —→ kinetic energy-speed increases so they collide more forcefully & more often, which increases rxn rate

What is typical for a bimolecular rxn?

10^9 collisions/sec—not every collision = formation otherwise rxns would be much faster

What are the other considerations in kinetics?

orientation, energy of molecules

How does orientation affect the kinetics of a reaction?

it’s very important- the atoms have to be facing the ones that’ll react to sucessfully form product (otherwise-fail)

How does the energy of molecules affect the kinetics of a reaction?

need a minium energy (activation energy -Ea) for each reaction-its endothermic specific for each rxn and determines the rate of the reaction

What does the arrhenius equations of reaction rate depend on?

fraction of molecules with Ea, # of collisions/sec, fraction of collisions w/ correct orientation

What is the arrhenius equation?

k= Ae^-Ea/RT

What is the graphic arrhenius equation?

ln (K) = ln (A) -Ea/RT

What is an energy profile?

graph we make to show energy of rxn (energy=y, reaction progress=x)

What is a transition state?

it is the activated complex (all parts ready to move)- draw the bonds breaking and being formed

What is the Arrhenius equation for calculation?

ln (k2/k1)= Ea/ R (1/T1-1/T2)

What is an elementary step?

a one step reaction

What is molecularity?

the # of molecules that participate in an elementary step (uni, bi, and ter)

What is a unimolecular reaction?

decomposition (usually)

Why is a 3+ molecular reaction unlikely?

because it is really hard to have so many molecules all interact @ right orientation and energy at once

What is a mechanism of reaction?

a multiple-step reaction, each reaction step is elementary and can be used to determine the rate law for each step (slower step=rate determining step-rds)

What is an intermediate?

molecule formed only for next step of rxn & does not show up in products (it’s usually really high energy)

What is a catalyst?

substance that affects the rate of rxn but is not consumed in reaction sequence-works by altering Ea/A of rxn—→ provides alternative mechanism for rxn w/ a lower Ea, it is consumed in early mechanism and made in later step`

What is a homogeneous example of a catalyst?

peroxide dissociating w/ light add HBR to speed (Ea= lower in graph)

What is a heterogeneous example of a catalyst?

solid in a gas rxn—→ changes shape of molecule (metal surface attracts h-h and ch2=chs and react then once ethane is mad it has less affinity for the surfact)-effecting frequency factor

What are catalytic converters?

honeycomb structure with rare metals embedded that attract pollutants made by burning gasoline (NO, CO, etc) and oxidize CO and reduces nitrogen oxide byproducts

What is equilibrium?

state of balance where concentration of reactants are constantly converted into products and vice versa- theres no change happening (dynamic situation—→ rxn never stops)

What is ratef and rate r?

forward rate and reverse rate

What does equilibrium look like on a graph?

straight line

How do you determine the rate law at equilibrium?

ratef = rate r so kf/kr = k (equilibrium constant- no units) - can’t be negative flip to 1/x

What does it mean when k > 10³?

position favors product formation (forward favored)

What does it mean when k < 10^-3?

position favors reactants (reverse favored)

How are Kc and Kp related?

Kp= Kc (RT) ^ n

what is ^n?

mol gas products-mol gas reactants

What is a heterogeneous system?

when soilds/pure water are in a reaction, they are not included in the equilibrium expression

How do you solve for equilibrium?

Use an ICE table (initial, change, equilibrium) to relate the steps