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CHEM 113: EXAM 1
Reaction Rate
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the change in concentrations of reactants (or products) as a function of time
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Different reaction rates
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-in a faster reaction (higher rate) the reactant concentration decreases quickly, where as in a slow reaction (lower rate), it decreases slowly
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Concentration |
-molecules must collide to react
-reaction rate is pro to collision freq which is pro to concentration
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physical state
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-molecules must mix to collide
-when reactants are in different phases, stirring or grinding may be needed
-how well reactants mix
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Temperature
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-molecules must collide with enough energy
-cold= slows
heat= speeds up
-at a higher temp, collisions occur more frequently and more molecules react
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curved line
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rate is changing
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instantaneous rate
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the slope of a line tangent to the curve at any point gives the instantaneous rate at that time
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initail rate
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the moment when the reactants are mixed
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When does instantaneous rate occur?
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rate at time t is the slope of the tangent to a curve that plots concentration vs. time
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When does initial rate occur?
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t=0 occurs when reactions have just been mixed and before any product accumulates
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rate law
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based on an experiment
-expresses rate as a function of concentrations and temperature
-rate depends only on reactant concentrations and temp
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k
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-rate constant
-does not change as reaction proceeds
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m, n
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-reaction orders which represent the rxn order in respect to [A] and [B]
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two concepts of reactant concentration
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1. balancing A and B id not necessarily related to orders of m and n
2. rate law consists of rate, reactions orders, and rate constant (must be found by experiment)
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first order
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-if the rate doubles when [A] doubles, the rate depends on [A] raised to the first power
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second order
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-if the rate quadruples when [A] doubles
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zero order
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-if the rate does not change when [A] doubles, the rate DOES NOT depend on [A]
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Integrated rate laws
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-different forms of rate laws
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collision theory
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-atoms, molecules, or ions must collide to react
-collisions depend on the numbers of reactant particles, not their sum
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relationship between k and T
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k increases exponentially as T increases
-as T increases, the value of the negative exponent becomes smaller, which means that k becomes larger
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How does an increase in temp affect the rate of rxn
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-temperature increases number of collisions
-temperature increases energy of collisions
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What are two reasons for measuring initial rates in kinetics?
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-to focus on rxn rate
-to find factors that affect it
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units for k
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L2/mol2*min
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If [A] is doubled, the rate will
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double |
If [B] is halved, the rate will
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decrease by 4
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If [C] is tripled, the rate
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increases by 9
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effective collisions
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-those that actually lead to product because the atoms that become bonded in the product make contact
-must have enough energy and the appropiate molecular orientation
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reaction mechanism
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a sequence of single reaction steps that sum to the overall equation
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unimolecular reaction
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-one that involved the decomposition or rearrangement of a single particle
-ex: O3
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bimolecular reaction
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-one in which two particles (O3 and O) react.
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rate-determining step
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-limits how fast the reaction proceeds
-rate law of rate-determining step becomes the rate law for overall reaction
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reaction intermediate
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-substance formed and used up during the reaction
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three criteria for a valid mechanism
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1. elementary steps must add up to the overall balanced equation
2. elementary steps must be reasonable (uni or bi)
3. must correlate with rate law
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Heterogenous catalyst
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-heterogeneous catalyst provides alternate, lower energy, pathway for the formation of a product
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True about Catalysts
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-speed up forward and reverse of a reaction
-not consumed during a reaction
-not required for any chemical reaction to occur
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What changes with temperature
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-rate constant
-NOT activation energy, rate law, or transitional state
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Transitional state/ activated complex
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-an extremely unstable species that exists for a moment when the reacting system is at its highest energy
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Activation energy equation
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ln k2/k1=-Ea/R(i/t2-1/t1)
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Difference between reaction intermediate and transition state
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transition states are unstable and reaction intermediates have some stability
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average rate
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slope of the ling joining two points along the curve
-total change in concentration divided by total change in time
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integrated rate law first order
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ln [A]o/[A]t=kt
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integrated rate law second order reaction
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1/[A]t-1/[A]o=kt
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zero order integrated rate law
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[A]t-[A]o=-kt
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half life, first oder rxn
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t1/2=ln2/k[A]
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How does an increase in T affect the rate?
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-as temp increases the fraction of collisions exceeds activation energy thus the rxn rate increases
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How does a decrease in Ea effect the rate?
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A decrease in Ea lowers the energy threshold which increases collisions and increases the rate
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Is the rate overall lower, higher, or equal to the average rate of individual steps?
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-the rate of the rxn depends on the rate of the slowest step. The ate of the overall rxn will be lower than the avg becuase the avg includes higher rates
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Why is a bimolecular step more reasonable than a termolecular?
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-bi is more reasonable because the probabilty of three partciles colliding with one anoter is much less then the probibilty of two particles.
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