Rate of reaction:
How fast a reaction is e.g. how quickly reactants are turned into products.
Quantified rate of reaction. Rate[A][B]
The time taken for the concentration of a reactant to decrease by half.
Rate determining step:
The slowest step in a multi-step reaction. ALWAYS 1ST STEP IN OCR.
The order of a reaction in respect to a reactant is the index to which its concentration term in the rate equation is raised.
Overall order of reaction:
All orders in the rate equation added together.
Rate= change in concentration/ change in time
- Draw a tangent from where your chosen time meets the line.
- Concentration minus concentration of where your line meets the axes.
- Time minus time where your line meets the axes.
- UNITS= MOLDM-3 SEC-1
Half-life for 1st order
- Gradient is rate.
- 1st order reactions- half life is constant!
- Time at initial concentration- time at half of initial concentration.
CAN BE DRAWN USING CONTINUOUS MEASUREMENTS
Drawing/sketching concentration time graphs
- ensure all reactants plateau at the same point.
- straight line down- as rate doesnt change with concentration
- half life decreases
- goes down relatively steeply.
- half life is constant
- Decreases more steeply and will decrease by double the amount of any 1st order reactants.
- Plateaus at the same TIME value as 1st order.
- half life increases
WAYS TO MEASURE RATE: Acid +Base
pH probe during reaction
- 10^(-pH) will give you H+ concentration.
- [HA]= [H+]
- change in concentration of HA/ change in time.
- Gives you mols of fluid used until equilibira point
- convert mols to concentrations.
WAYS TO MEASURE RATE: involving gas
Measure change in volume
- volume produced= vol/24 = mol
- convert mol to conc
Measure change in mass of reactants (as gas is produced)
- should decrease
- giving you reacted mols
WAYS TO MEASURE RATE: when there's visual change
- Measure mass of precipitate
- Use colorimeter to measure colour change
Conc-time graphs: Half-life: zero order
- Gradient is the same as rate
- So straight negative correlation.
- HALF LIFE DECREASES.
HALF-LIFE: first order
The time take for the concentration of a reactant to decrease by half.
First order half lives examples:
- Radioactive chemicals
- Removal of drugs from the body
- Half life is constant for 1st order
- Curve will steep decrease and plateau.
Conc-time graphs: Half life: 2nd order
- Very steep decrease
- HALF LIFE INCREASES.
ORDERS: assumptions + determination
- Only 2 step reactions
- The first step is the rate determining step
- Order is determined by the molar ratio in the rate-determining step (NOT overall equation!!)
- line is straight but does not meet y axis
- concentration doesnt effect rate
- line is straight positive correlation
- concentration is proportional to rate
- conc doubles; rate doubles
- line increases steeply
- concentration is proportional to rate-squared.
REQUIRES SEPARATE EXPERIMENTS WITH DIFFERENT CONCENTRATIONS of reactants to DRAW.
Determing order from experiments PHRASE
- Between experiements ___ and ____
- The concentration of A__
- increases by a factor of ___
- and the initial rate of reaction increases by a factor of _____
- Therefore the order of reaction in respect to _A is ______.
DO PAST PAPER QUESTIONS M8.
Determining rate from experimental data
DETERMINING RATE FROM DATA
- Find Orders
- Check headings and use 'x10^-y' values
When using data where two reactants change in concentration
- Use one order you've already worked out
- Times factor of concentration change by order worked out.
- Use this value to times the rate of smaller rate.
- Divide larger rate by this value.
- Compare to factor of change in concentration of unknown reactants.
Temperature and K
Temperature increase will increase the rate constant and therefore the rate of reaction.
- More heat
- Particles have more energy
- more successful collisions
- leading to more reaction