chem unit 3 rates

the study of ways to make chemical reactions go faster or slower (deals with rates of reactions)

the speed at which a chemical change occurs, generally expressed as change in concentration per unit time (mol/ [L.s])

formula: average concentration rate=change inconcentration/elapsed time

the speed at which a reaction proceeds over a period of time (often measured as change in concentration of a reactant or product of time)

slope of secant (a line between two points on a curve) on concentration vs elapsed time

the speed at which a reaction is proceeding at a particular point in time

find slope of tangent (concentration of reactant vs elapsed time)

the concentration changes are more rapid near the beginning of the reaction and the rate decreases with the time elapsed 

collect gas and measure volume/pressure as reaction proceeds (faster reaction =greater change in volume/pressure)

the conductivity of the solution changes as the reaction progresses (measure conductivity)

measure colour intensity using a spectrophotometer

• nature of reactants
• concentration of reactants
• temperature
• presence of a catalyst
• surface area

• in homogeneous/ aqueous solutions most reactions of monatomic ions are cast (Ag⁺ or Cl⁺) are fast
• reactions of molecular substances are often slower

initial concentration of a reactant is increased then the reaction rate increases (generally)

when temperature of system increases the reaction rate also increases

catalyst--> a substance that alters the rate of a chemical reaction without itself being permanently changed 

catalyst=inrease in rate reaction

in heterogenous systems, an increase in reactant surface area increases rate of reaction 

empirically

exponentially, the product of te initial concentrations of the reactants

• the rate r will always be proportional to the product of the reactants where these concentrations are raised to some exponential values 
• r~[x]^m[y]ⁿ

m and n can only be dertermined empirically they can equal any real number

r=k[x]m[y]n

• constant
• the proportionality constant in the rate law equation

the exponent value that describes the initial concentration dependence of a particular reactant

the lower the order the more likely the mechanism

the sum of the exponents in the rate law equation

look for pairs of data in which the initial concentration of only one reactant changes

• average rate is inversely related to elapsed time 
• r_av~ 1/delta time

• if reactant a is consumed then 
• r_av~[a]ⁿ then
• 1/delta time ~ [a]ⁿ

straight horizontal line

straight diagonal line

curve (increasing)

if enough energy is provided to breake chemical bonds within molecules (source=kinetic energy of themolecules)

• -a chemical system consist of particles that are in constant random motion at different speeds. the average kinetic energy is proportional to the temperature of the sample
• -a chemical reaction must involve collisions of particles with eachother or the walls of the container
• -an effective collision is one that has sufficient energy and correct orientation of colliding partivles so that bonds can break and new bonds can be formed
• -ineffective collisions involve partivles that rebound from the collision essentially unchanged in cature
• -rate of reaction depends of frequency of collisions and the fraction of effective collisions

rate=frequency of collisions x fraction that are effective

• rate =1000/1 x 1/100
•      = 10 reactions

the minimum increase in potential energy of a system required for molecules to react

• surface area
• temperature
• concentration (more collisions per unit of time)
• order of reaction --lower order = less particles required --> more collisions

• nature of reactant
• catalyst 
• (changes the size of the activation energy barrier making it easier /more difficult to react)
• temperature (higher energy = easier to break bonds)

• reactions requre initial input activation energy to form unstable activated complex
• -for an effecvie collision the particles must have enough energy in the form of speed (kinetic energy)
• -temperature tell s us the probability of having enough speed or energy
• -higher temperature =higher change for enough speed=more likely effective collision

an unstable chemical species containing partially broken and partially formed bonds representing the max potential energy point in the change also known as transition state

2 particle collisions are more likely to gappen than 3 particle collisions

• a step in a reaction mechanism that only involces one two or three particle collisions 
• ex A+B-->AB

a series of elementary steps that makes up an overall reaction

the slowest step in a reaction mechanism

molecules formed as short lived products in reaction mechanisms (substances formed in a reaction but immediately react again they are not present in the final reaction)

each step must be elementary involving no more that three reactant molecules

the rate determining step. slowest must be consistant with rate equation

elementary steps must add up to verall equation

the coefficient of the ractant must be the same as exponent in the rate equation

molecules do not move at the same speed

minimum kinetic energy needed to conver kinetic energy to activation energy (potential energy) during the formation of the activated complex

reactions that occur quickly have lower avtication energy

at the surface where substances are in contact inc SA inc rate

for any given energy a much larger fraction of molecules has the required kinetic energy at a higher temperature

• catalyst accelerate a reaction by providing alternate lower energy pathwa from reactants to products they allow reactions to ovvur by a different mechanism 
• -diff intermediate steps
• -same product
• new mechanism has lower activation energy a greater fraction of molecules posses the minimum required energy and reaction rate inc

a catalyst in a reaction in which the reactants and the catalyst are in different physical states

a catalyst in a reaction in which the reactants and the catalyst are in the same physical state

the reaction rate slows

• RATIO 
• 2NH_{3 -->} N₂ +3H₂

• rate NH3 = 2
• rate H2    =3

instantaneous