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SCH 4U1 Mr. Dvorsky Friday November 15
Reaction Mechanisms SCH 4U1 Mr. Dvorsky Friday November 15
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What is a reaction mechanism?
In any chemical change, some bonds are broken and new ones are made. Quite often, these changes are too complicated to happen in one simple stage. Instead, the reaction may involve a series of small changes one after the other.
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A reaction mechanism describes the one or more steps involved in the reaction in a way which makes it clear exactly how the various bonds are broken and made.
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In other words, it is believed that most chemical reactions actually occur as a sequence of elementary steps. The overall sequence is called the reaction mechanism.
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Rate-determining Step
The overall rate of a reaction (the one which you would measure if you did some experiments) is controlled by the rate of the slowest step. The slow step of a reaction is known as the rate determining step
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Let us take the following example:
4 HBr(g) + O2(g) 2H2O(g) + 2Br2(g) This rxn won’t occur in one step. -collision theory: four molecules of HBr and one molecule of O2 would have to collide in proper orientation and with enough energy to break old bonds and form new ones. -it is hard enough for just two molecules to collide!
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Example continued: 4 HBr(g) + O2(g) 2H2O(g) + 2Br2(g)
Experimental evidence shows adding O2 increases the rate as we would expect. Since four molecules of HBr react for every O2 -we would expect [HBr] to have a larger effect on rate. Not the case. Experiments give the following rate law expression: r=k[HBr][O2]
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Example continued: 4 HBr(g) + O2(g) 2H2O(g) + 2Br2(g)
Experimental evidence shows adding O2 increases the rate as we would expect. Since four molecules of HBr react for every O2 -we would expect [HBr] to have a larger effect on rate. Not the case. Experiments give the following rate law expression: r=k[HBr][O2]
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Rule: the slowest or rate-determining step must be consistent with the rate equation
In this case: r=k[HBr][O2]
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IN ADDITION: there is a direct link with equation coefficient and exponents in the rate equation. So for r=k[HBr]1[O2] 1 [so one molecule of each must collide] The rate determining step is 1 HBr + 1 O2 -> reaction intermediate [this also says one molecule each must collide]
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The take home message of all this
If the experimentally determined rate equation is: rate = [molecule X]m[molecule Y]n Then the rate determining step will be: mX + nY -> products or reaction intermediates
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A couple of other rules before we can get started in building our own reaction mechanisms:
Each step must be elementary; that is to say no more than three reactants (usually 1-2 reactants). The elementary steps must add up to the overall equation.
Chapter 17 - Chemical Kinetics
Chemical Kinetics Reaction rate - the change in concentration of reactant or product per unit time.
Collision Theory For a rx to occur reacting molecules must collide so 1 - The old bonds “loosen up”, or start to break apart 2 - The correct orientation.
Chapter 14 Chemical Kinetics.
The collision theory states that for a chemical reaction to occur the reacting particles must collide with one another. The rate of the reaction depends.
Collision Theory. Reaction Coordinate Diagrams Multistep Reactions.
Chapter 15 Kinetics. Kinetics Deals with the rate of chemical reactions Deals with the rate of chemical reactions Reaction mechanism – steps that a reaction.
Reaction Mechanism The reaction mechanism is the series of elementary steps by which a chemical reaction occurs. The sum of the elementary steps must.
Chemistry 1011 Slot 51 Chemistry 1011 TOPIC Rate of Reaction TEXT REFERENCE Masterton and Hurley Chapter 11.
Chapter 14. Physical state of reactants: Reactants must come in contact with one another in order for a reaction to occur. Concentration of reactants:
1 Reaction Mechanism The series of steps by which a chemical reaction occurs. A chemical equation does not tell us how reactants become products - it is.
From the Arrhenius equation we have: 301. From the Arrhenius equation we have: 302.
Reaction Mechanisms Overall Reaction: A → Z
Reaction Mechanisms. A balanced equation for a chemical reaction indicates the substances present at the start of the reaction and those produced as the.
Collision Theory & Reaction Mechanisms
How fast chemical reactions proceed How chemical reactions occur.
Chemical Kinetics Chemical Kinetics or Rates of reaction.
Reaction mechanisms and catalysts
Chemical Kinetics The speed with which chemical reactions occur depends on external conditions The area of chemistry concerned with the speed at which.
Rates of Reaction. Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how does a reaction proceed (reaction mechanism) and how.
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