chem unit 3 rates

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chem unit 3 rates
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2012-10-15 20:09:46
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rates of reaction
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  1. chemical kinetics
    the study of ways to make chemical reactions go faster or slower (deals with rates of reactions)
  2. rate of reaction
    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
  3. average rate of reaction
    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)
  4. find average rate of reation 
    slope of secant (a line between two points on a curve) on concentration vs elapsed time
  5. instantaneous rate of reaction
    the speed at which a reaction is proceeding at a particular point in time
  6. finding instantaneous rate of reaction
    find slope of tangent (concentration of reactant vs elapsed time)
  7. what does evidence show for most reactions 
    the concentration changes are more rapid near the beginning of the reaction and the rate decreases with the time elapsed 
  8. measuring reaction rates-produce gas
    collect gas and measure volume/pressure as reaction proceeds (faster reaction =greater change in volume/pressure)
  9. measuring reaction rates-- ions
    the conductivity of the solution changes as the reaction progresses (measure conductivity)
  10. measuring reaction rates--change colour
    measure colour intensity using a spectrophotometer
  11. what are the five factors that affect the rate of reaction
    • nature of reactants
    • concentration of reactants
    • temperature
    • presence of a catalyst
    • surface area
  12. chemical nature of reactants
    • in homogeneous/ aqueous solutions most reactions of monatomic ions are cast (Ag+ or Cl+) are fast
    • reactions of molecular substances are often slower
  13. concentration
    initial concentration of a reactant is increased then the reaction rate increases (generally)
  14. temperature
    when temperature of system increases the reaction rate also increases
  15. presence of a catalyst
    catalyst--> a substance that alters the rate of a chemical reaction without itself being permanently changed 

    catalyst=inrease in rate reaction
  16. surface area
    in heterogenous systems, an increase in reactant surface area increases rate of reaction 
  17. rate laws and order of reaction are determined ...
    empirically
  18. the rate of a reaction is ________ proportional to _____
    exponentially, the product of te initial concentrations of the reactants
  19. rate law
    • 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
  20. rate law equation
    r=k[x]m[y]n
  21. what does k rep in rate law equation
    • constant
    • the proportionality constant in the rate law equation
  22. order of reaction
    the exponent value that describes the initial concentration dependence of a particular reactant

    the lower the order the more likely the mechanism
  23. the overall order of reaction
    the sum of the exponents in the rate law equation
  24. when finding a rate equation one tip is to 
    look for pairs of data in which the initial concentration of only one reactant changes
  25. relating reaction to time
    • average rate is inversely related to elapsed time 
    • rav~ 1/delta time

    • if reactant a is consumed then 
    • rav~[a]then
    • 1/delta time ~ [a]n
  26. zeroth order
    straight horizontal line
  27. first order
    straight diagonal line
  28. reaction with order greater than one
    curve (increasing)
  29. chemical reactions can only occur
    if enough energy is provided to breake chemical bonds within molecules (source=kinetic energy of themolecules)
  30. concepts of the collision theory
    • -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
  31. collision theory and rate of reaction formula
    rate=frequency of collisions x fraction that are effective

    • rate =1000/1 x 1/100
    •      = 10 reactions
  32. activation energy
    the minimum increase in potential energy of a system required for molecules to react
  33. factors that affect collision frequency (number of collisions)
    • surface area
    • temperature
    • concentration (more collisions per unit of time)
    • order of reaction --lower order = less particles required --> more collisions
  34. factors that affect fraction effectiveness
    • 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)
  35. effective collison reaction happens 
    • 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
  36. activated complex
    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
  37. reaction mechanisms
    2 particle collisions are more likely to gappen than 3 particle collisions
  38. elementary step
    • a step in a reaction mechanism that only involces one two or three particle collisions 
    • ex A+B-->AB
  39. reaction mechanism definition
    a series of elementary steps that makes up an overall reaction
  40. rate determining/ limiting step
    the slowest step in a reaction mechanism
  41. reactin intermediates
    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)
  42. rules when proposing a mechanism
    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
  43. maxwell boltzman distribution
    molecules do not move at the same speed
  44. threshold energy
    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
  45. surface area hererogenous mixture reactants can collide only
    at the surface where substances are in contact inc SA inc rate
  46. theroetical effect of temp
    for any given energy a much larger fraction of molecules has the required kinetic energy at a higher temperature
  47. theoretical effect of catalyst
    • 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
  48. heterogeneous catalyst
    a catalyst in a reaction in which the reactants and the catalyst are in different physical states
  49. homogeneous catalyst
    a catalyst in a reaction in which the reactants and the catalyst are in the same physical state
  50. reaction rate as the reactants get used up
    the reaction rate slows
  51. rate is directly proportional to mole.....
    • RATIO 
    • 2NH3 --> N+3H2

    • rate NH3 = 2
    • rate H2    =3
  52. rate dependence on concentration is
    instantaneous

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