5.1.1 How Fast? Flashcards
Rate of reaction definition
Change in concentration of a substance in unit time
Unit: mol dm^-3s^-1
On a concentration time graph what does the gradient represent
Rate of reaction
How do you calculate rate from a graph
Draw a tangent
What is the rate constant
K
How do you work out the total order of a reaction
Add all the individual orders
What do square brackets mean in a rate equation
Concentration
Zero order
Concentration has no effect on rate of reaction
Rate= k
First order
Rate of reactiln directly proprotional to concentration of A
rate= k[A]
Second order
Rate of reaction is proportional to concentration of A squared
rate= k[A]^2
Rate concentration for zero order
Line is horizontal
Parallel to the x axis
Rate on y, conc on x axis
Rate concentration for first order
Straight line y=x
/ shape
Rate on y, conc on x axis
Rate concentration second order
Curved, starts at x and increases exponentially
Rate on y, conc on x axis
What is continuous rate data
Data from one experiment where concentration of a substance is followed throughout the experiment
How is continuous rate data processed
Plotting data
Calculating successive half lives
How to tell if a reaction is first or second order from a graph
Half lives are constant when first order
Rate constant k
Units of k depend on overall order of the reaction
Work it out from rate equation
K is independent of concentration and time
Only changes with temperature
Calculating a value of k using intial rate data
Use one of the experiments and put the values for each concentration into your calculated equation
Rearrange to find k
Effect of temperature on rate constant
Increasing temperature increases rate constant
Relationship is given by arrhenius equation
Arrhenius equation to find k
k=Ae^-Ea/RT
Rearranged arrhenius
lnk = constant - Ea/RT
plot graph ln rate or lnk on y axis
Then 1/T on x axis
Ea = gradient of line x R
Concentration time graph zero order
\
conc on y
Time on x
Concentration time graph 1st order
\ but curved slightly
conc on y
Time on x
Concentration time graph 2nd order
\ quite curved
conc on y
time on x
Arrhenius equation units
Ea= kJ/mol R= kJ/mol/ K T= K
How to measure rates of reaction
Measure change in volume of gas
Measure change in mass
Titrating samples of reaction mixture with acid, alkali, sodium thiosulfate
Colorimetry
Measure change in electrical conductivity
Measure of optical activity
Measure change in volume of gas
Works if theres a change in number of moles of gas in the reaction
Gas syringe
Measure change of mass
Works if there’s a gas produced which is allowed to escape
Works better with heavy gases such as CO2
Titrating samples of reaction mixture with acid, alkali, sodium thiosulfate
- Small samples removed from reaction mixture
- Quench which stops reaction (dilute with water, cooling, add a reagent with one of the reactants)
- Titrate with a suitable reagent
Colorimetry
If one of the reactants or products is coloured then colorimetry can be used to measure the change in colour of the reacting mixtures
Measuring change in electrical conductivity
Can be used if theres a change in the number of ions in reaction mixture
Measure optical activity
If theres a change in the optical activity through the reaction this could be followed in a polarimeter
Slowest step in the reaction
Rate determining step
Which step controls the rate of reaction
Rate determining step
Number of moles of each substance of molecules in slowest step is the same as what
The order of reaction for each substance
0 moles of A in slow step means A is zero order
