3.6 - Rates of reaction Flashcards
Define rate of reaction
how fast reactant used up/product formed
State the equation for rate of reaction and name the units
change in conc/time
units: moldm-3s-1
Name the conditions for a reaction to occur
reacting particles must collide with:
- correct orientation
- sufficient energy
How does increasing concentration affect rate?
- more particles in same volume
- more frequent collisions per unit of time
- increased rate
How does increasing pressure affect rate (gases)?
- molecules closer together
- more frequent collisions per unit of time
- increased rate
Describe the effect of decreasing pressure on rate
- decreases rate
- decreased conc of molecules/less molecules per unit volume
- less frequent collisions per unit of time
How can reaction progress be monitored?
- removal of reactant
- formation of product
Define catalyst
- substance that changes rate of a chemical reaction without undergoing any permanent change
- provides alternative reaction pathway w/ lower Ea
Define homogeneous catalyst
same physical state as reactants
Define heterogeneous catalyst
different physical state to reactants
When may it be difficult to classify a catalyst into homogeneous or heterogeneous?
when reactants have different physical states
State the benefits of catalysts
- lower temps/pressures can be used
- less energy/electricity required
- lower energy costs
- less fossil fuels combusted so less environmental damage via CO2 emissions
What does sustainability require?
- high atom economies
- few pollutants
Name the features of Boltmann distribution
- curve starts at origin (no molecules have 0 energy)
- area under curve = total no. of molecules
- doesn’t meet x-axis (no maximum energy)
What is the effect of increasing temperature on Boltmann distribution
- peak lower and shifted to right
- more molecules have energy greater than Ea
- greater proportion of collisions exceeding Ea as molecules have more energy and move faster means increased rate
What is the effect of catalyst on Boltmann distribution
- lower Ea (Ec) to left on graph
- greater proportion of molecules have energy
greater than Ec - greater proportion of collisions exceeding Ea mean increased rate
Describe the meaning of the orders of reaction
0 order: changing conc has no effect on rate
1st order: if conc x2, rate x2
2nd order: if conc x2, rate x4
How to find the overall order of reaction?
sum of orders for each reactant
Define half life
time taken for half of a reactant to be used up
Define rate determining step
slowest step
How do half lives vary depending on orders?
0: half lives decrease
1st: half lives constant
2nd: half lives increase
State the equation to determine k using half lives
k = ln2/half life
NB: only for 1st order
Why use an excess of 1 reactant in a rates experiment?
to keep its concentration constant so it’s effectively 0 order
State the equation to find Ea and/or how to determine it graphically
- Ea = -R x gradient
- plot graph using ln k and 1/T
- find gradient
Describe how to find the pre-exponential factor (A)
e to power of y-intercept
Describe how to determine the order of reaction from a graph
- find half life duration
- draw 2 tangents (2nd point should be half of first value) and find gradients
- 1st gradient should be 2x 2nd gradient
Why would a reactant NOT be a catalyst?
- used up in reaction
- appears in overall equation
Why would an absorbance reading for e.g iodine decrease in a rate experiment?
iodine is orange and its concentration is decreasing
Describe the effect of increasing temperature on rate constant (k)
increases
Why is a mechanism consistent with a rate equation?
steps _ and _ together give reactants in overall equation (name these)
Suggest 2 reasons why it is unlikely a reaction could take place in 1 step
- stoichometry in rate equation not same as in overall equation
- collision is unlikely with more than 2 species
