Rates -4
- Created by: Shannon
- Created on: 02-04-15 13:32
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- Rates
- Rate determing step
- The overall rate of a multi-step reaction is determined by the slowest step
- Only molecules involved in the rate determining step appear in the rate equation
- Rate of reaction
- The rate of a chemical reaction is the change in concentration over a given time
- The initial rate of a reaction is when the reactant are first mixed
- Increased temperature will increase the rate of a reaction
- Rate equation
- Links the rate of a reaction to the concentration of the reactants
- Done by either investigating increase or decrease in concentration of a reactant
- e.g r =k[A][B]2
- r = rate of reaction (moldm-3s-1)
- k = rate constant
- [ ] = concentration (moldm-3)
- Links the rate of a reaction to the concentration of the reactants
- Orders of reaction
- Orders tell you how much the concentration of a reactant affects the rate
- Order can be zero if the rate is unaffected
- First order
- Rate is directly proportional to concentration of reactant e.g as [A] doubles so does the rate
- Second order
- Rate is proportional to the square of the concentration of the reactant e.g tripling [A] would increase the rate 3x
- The overall order of the reaction is found by adding up the individual orders
- Orders tell you how much the concentration of a reactant affects the rate
- Measuring the rate of a chemical reaction
- Mass change - due to gas evolving
- Colorimetry
- Temperature change
- Volume of gas given off
- pH change
- Titration
- Conductimetric analysis
- Half life
- The time for the concentration of a reaction to half
- The half life of a 1st order reaction is constant
- The half life of a 2nd order reaction increases as conc falls
- Reaction mechanisms: hydrolysis of halogenoalkanes
- SN2
- Favoured by primary haloalkanes
- 2 species involved in rate determining step, 1 step mechanism
- Nucleophile attacks from opposite side of halogen atom
- Unstable transition complex formed
- SN1
- Favoured by tertiary haloalkanes because of steric hinderance
- 1 species involved in rate determining step, 2 step mechanism
- In SN1 reaction, a stable carbocation intermediate is formed - stable due to inductive effect
- SN2
- Rate determing step
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