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
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
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)
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!
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^
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
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!
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.
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.