We need rto know rxns happening in the fast steps – have to do experiments ot find this out
• Find a friend, and each of you crumple up a piece of paper. Sitting facing one another, with knees almost touching, toss your pieces of paper together, trying to get them to hit while still in the air. Do this 10 times - how many successful "collisions" did you record?• Now, grab another friend with their own wad of paper. This time, a successful collision will only occur when all three pieces of paper hit together simultaneously - just two out of three wont do. Out of 10 tosses, how many are successful? Add one more friend and repeat the exercise. Any successful collisions, where all four pieces of paper collide at the same instant?
16.1 Rate Expression• Understand – Rate constant – Overall order – Order of reaction• Use experimental data to deduce rate expression.• Solve problems using the rate expression.
KINETICS GIVES US CLUES TO A REACTION MECHANISM THIS IS ALL EXPERIMENTALLY DETERMINED
DEFINITIONS• Rate is how fast concentration of reactant is falling ( or vice versa ) measured in – mol dm-3 s-1• Rate Constant (k) is only constant if T and Ea stays constant. A catalyst or higher T would increase k. Changes in concentration do not.
The Concentration Terms• Where rate = k[A]a [B]b [C]c• The powers are called the ‘order of reaction’. – If no effect on rate regardless of concentration we call it zero order. – If we double concentration of [A], and the rate doubles we say Rate = k[A] – If we double concentration of [A], and rate is quadrupled we say Rate = k[A]1 – If we double the concentration of [A}, and the rate is 8x faster we say Rate = k [A]2• Overall orders are sums of the individual orders.
Orders of Reaction• xA + yB + zC…… products• Rate = k [A]x [B]y [C]zthis is rate equation• How do we measure rate of reaction ? – How fast concentration of one of reactants is falling – Units
Zero Order• This means the concentration of that substance does not influence the rate• Rate = k[A]0
First Order• Whatever you do to the concentration, you do to the rate.• Double conc = double rate• Triple conc = triple rate• Etc• Rate = k[A]
Second Order• Whatever you do to the concentration you square the rate.• Rate = k[A]2
Notes• The coeffcients in balanced chemical equation are not the orders !• You cannot predict orders they must be found experimentally.• Orders give clues to mechanisms…….later
Finding Orders from Rate Experiments• Use blog to calculate experimental k and order
Finding Orders of Reaction Graphically• The real world is not so straight forward
Zero Order wrt [A]• Rate = constant, i.e. independent of value of [A].• If A is only substance taking part in the reaction then Rate = k• A horizontal straight line shows a zero order rxn.
First Order wrt [A]• Rate = k[A]• Rate constant is slope of the graph• What are units ?
Second Order wrt [A]• Rate = k[A]2• Plot rate against concentration of A2
Finding value for Rate Constant k• Rate is slope of the graph
Recognising Order from Graphs• http://www.chem.purdue.edu/gchelp/howtos olveit/Kinetics/IntegratedRateLaws.html
Using Half Life• For a first order reaction, half life is constant – independent of initial concentration.• ONLY FIRST ORDER REACTIONS HAVE CONSTANT HALF LIVES.• Sums 142
Activation Energy• Describe qualitatively the relationship between the rate constant (k) and temperature (T).• Simple but worth restating……..The rate of reaction is affected by Temperature ( and catalyst ).
Qualitative description• T appears in Ea/RT• If T increases then Ea/RT gets smaller• This is a negative terms so as T increases Ea/RT gets less negative• -Ea/RT is a power so as T is raised then e-Ea/RT gets larger.• So the rate constant gets larger as T gets larger.
Huh ?• For every rise in T of 10K there is a DOUBLING of reaction rate.• This is a very useful ( and oft tested ) rule of thumb
Example Calculations• 146 – 7.• Lab : determination of activation energy.
Reaction Mechanism• Explain that reactions can occur by more than one step and that thesloweststep determines the rate of reaction• This is called the RATE DETERMINING STEP.
The What ?• http://www.chemguide.co.uk/physical/basicra tes/ordermech.html• Review using ChemGuide and produce a 1 side A4 substituting your own examples to illustrate the link between order and reaction mechanism.