Rates of Reaction Flashcards
Rate of reaction graph
At start, rate at its fastest due to more reactants, so more collisions per second with activation energy.
Rate slows down as more reactants converted to products.
Rate stops when all reactants converted to product.
Rate of reaction definition
The change in concentration/change in time
Order definition
The power to which the concentration of a reactant is raised in the rate equation
Units of concentration
concentration of A is written as: [A]
mol dm−³
Units of rate
mol dm−³ s−¹
What does the order of reaction mean
zero order
1st order
2nd order
The rate of reaction changes as the concentration changes.
zero order- no matter what happens to concentration, rate doesn’t change
1st order- whatever you do to concentration, rate changes by same factor
2nd order- whatever you do to concentration, rate changes by factor squared
Half-life definition
Half-life in first order reaction
The time taken for the concentration of a reactant to halve. Half-life of 1st order called exponential decay.
Length of half-life is constant
Half lives of different order graphs
zero order- decreasing half life
first order- same half life
second order- increasing half life
Concentration-time graphs each order gradient
Rate = gradient
Zero- straight line, negative gradient, decreasing half-life
First- Downward curve, decreasing gradient, constant half-life
Second- Steeper downward curve, decreasing gradient, increading half-life.
Methos to determine k from a first order concentration-time graph
Either from the: rate or the half-life
- Rearrange rate equation so k is subject. Use gradient as rate and concentration tangent is drawn at for the concentration.
- k= ln2 divided by half life
Rate-concentration graph:
- what diff order graphs look like
- how to find rate constant (k) from each one
zero order- straight line, no gradient
first order- straight line, +ve gradient, through origin
Second order- curve with increasing gradient
Rate constant:
zero order- y-intercept
first order- gradient
second order- plot a second graph of rate against concentration squared, find gradient
Initial Rate of reaction definition
The change in concentration of reactant/product per unit time at start of reaction
Method to obtain initial rate of reaction
Assumptions
Errors of this experiment
Improvements
Clock method (time for visual change to form)
Assumptions:
- Change in reactant concentration is insignificant
- temperature doesn’t change
- end-point reached before reaction has proceeded too far
Errors:
- Inaccurate timing of colour change- find average value found by 2 students
- adding starch slightly increases volume, which affects concentrations of reactants
Improvements:
- Dry equiptment
- Ensure constant temp using water bath
- Human error could measure time wrong, so round to nearest second
Iodine clock reaction
The reaction being monitored is:
H2O2 + 2I− + 2H+ → I2 + 2H2O
- Add sodium thiosulfate solution and starch to excess hydrogen peroxide, iodide ions in acid solution.
- The sodium thiosulfate reacts instantaneously with any iodine formed.
I2 + 2 S2O3 2− → 2I− + S4O6 2-
As the concentration of KI increases, the greater the concentration and rate of I2 formed.
Since the reaction rate is faster, sodium thiosulfate will be used up faster, so that iodine formed will stay in solution and cause a colour change.
Rate determining step definition
The slowest step in the reaction mechanism of a multistep reaction
