KINETICS

aqa- simple rate equations+ determination of rate equation

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  • Created by: Michelle
  • Created on: 13-09-09 20:11

The Rate of chemical reactions

Main factors which affect rate=

1.Temp---2.Concentration---- 3.Pressure----- 4.Surface area---- 5. A Catalyst

RATE OF REACTION= CHANGE IN CONCENTRATION

TIME

THE CHANGE IN CONCENTRATION CAN BE MEASURED BY:

  • Volume of gas evolved
  • Changes in pH
  • Heat produced
  • Changes in pressure (reactions involving gases)
  • Appearance or disappearance of colour
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Rate Equation

A + B -----> Products

***Rate Equation- describes how rate of a reaction varies with the concentrations of reactants at a given temp***

Rate = k [A]^m[B]^n

k= a constant for a particular reaction at a given temp (rate constant)

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Order of reaction

***ORDER OF REACTION= The power to which the concentratons of the reactants are raised in the rate equation***

A + B ----> PRODUCTS

Zero order- if doubling [A]= no effect on rate then reaction is said to be zero order w.r.t A.

First Order- if doubling [A] doubles the rate reaction is said to be 1st order w.r.t A

Second order- if doubling [A] increases the rate fourfold then reaction said to be second order w.r.t. A E.g- rate by 2 + a by 2

To get the overall order you add all the power together!

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Measuring rates of reaction

*** RATE OF THE REACTION= The change in concentration (of any of the reactants or products) with time.***

Rate is measured in mol/dm^3 s^1

The rate decreases as time proceeds because- reactants get used up, fewer collisions e.t.cThe rate at a point in time is found at the gradient of tangent(http://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/Triangle-right.svg/460px-Triangle-right.svg.png)

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Initial Rate

Initial Rate- The rate at the start of a reaction

= the gradient of a concentrations- time graph at time=0

'Initial rates' are preferred because at the start of a reaction All variables are known by design!

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obtaining conc- time data to measure rate (rate mo

Measure, using an 'appropriate method', the concentration of a chosen reactant or product at known time intervals.

PROPERTY OF REACTION SUITABLE METHOD

  • gas evolved measure loss- balance/ volume- gas syringe
  • H+ or OH- measure pH- pH meter
  • coloured reagent used/product formed Colorimeter
  • overall change in num of ions Conductivity meter

Monitoring can be ---1.Continous- single reaction mixture used

2. For initial rate only- several diff reation mixtures used.

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Extra notes

EFFECT OF TEMP ON K- small changes in temp produce large changes in reaction rates- for every 10K rise in temp, the rate of a reaction doubles.

THE RATE DETERMINING STEP- 'reaction mechanism' is the separate steps that lead from reactants to products. In multistep reaction steps nearly always follow after each other so products from 1 stap are starting materials of the next. So the rate of the slowest step may govern the rate of the whole process.

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