3.1.9- Rate equations (PAPER 2) Flashcards
What are the 4 factors that affect rate?
Temperature- higher temp means particles have more kinetic energy so vibrate more= more successful collisions above EA= higher ROR
SA- More particles on surface so more exposed particles increasing ROR
Catalyst- Higher ROR by providing alternate pathway with lower activation energy
Conc/pressure- more particles so more frequent successful collisions, increased likelihood that reactant particles collide.
Draw a graph of a product against a reactant- conc on y axis, time on x axis.
Product has upwards curve, reactant has downwards curve- they cross once.
Curve inwards/ outwards
Explain the shape of this graph
ROR the fastest at the beginning as higher likelihood of successful collisions as more particles at beginning, slope of reaction decreases with time as conc of reactants decrease.
What is rate of reaction?
The amount of reactant used/product formed over time.
What does rate of reaction depend on?
The slowest step in the reaction mechanism- the rate determining step.
How to find rate of a reaction?
Change in conc or moles of product/ time
How to find rate using a gradient?
Draw a tangent to the curve, change in y/ change in x
What is the rate expression?
Rate (mol dm−³ s−¹) = k [A]m [B]n
[A]= concentration of substance UNITS of moldm-3
m and n= orders of reaction
How to find total order for a reaction?
Sum of the separate orders- m+n
How to know which substance affects rate?
Species that appear in chemical equation but not rate expression do not affect rate.
A species that is not in chemical equation can be in rate expression eg catalyst
Which factors increase the rate constant k?
Increasing the temperature
Adding a catalyst
Larger value of k indicates faster reaction, reaction with large activation energy has small rate constant so slow reaction.
What is an order of reaction?
Relationship between the rate of a chemical reaction and the concentration of the species taking part in it
What does it mean if a reactant is in 0 order?
A change in concentration of this reactant has no overall effect on the rate.
Draw the graph of a reaction in 0 order: rate on y axis, conc on bottom
Conc of this species does not impact rate
Shown as horizontal straight line from middle y axis to right.
Rate= k
What does it mean if a reactant is in 1st order?
Changes in concentration of this reactant have a proportional change on the rate- eg if [A] doubles, rate doubles.
Draw the graph of a reaction in 1st order: rate on y axis, conc on bottom
Conc of species and rate are directly proportional
Diagonal straight line from origin to right
Rate= k[A]
What does it mean if a reactant is in 2nd order?
Changes in concentration of this reactant have a squared proportional change on the rate- eg if [A] doubles, rate quadruples
Draw the graph of a reaction in 2nd order: rate on y axis, conc on bottom
Rate is proportional to conc squared
Curved line from origin reaching to left
Rate = k[A]2
What are the shapes of a 0, 1st and 2nd order reaction graphs with conc on y axis and time on x axis?
0 order- diagonal going down from left to right
1st order- curly line from left to right like half a U
2nd order- steeper and smaller curly line from left to right
How to find initial rate using a graph?
Take the gradient of the tangent at 0 minutes- at the very start of the reaction.
Change in y/ change in x
How do we use initial rates to work out the rate equation for a reaction?
Repeat the experiment several times changing the concentrations of A, B and C one at a time in each experiment to find effect of changing conc of each one on rate.
Calculate initial rate for each experiment
Record concentrations of reactants and their initial rates- work out order with respect to each reactant by looking where the other reactant (s) remain constant.
Work out the rate equation of this reaction.
2NO (g) + Cl2 (g) —–> 2NOCl (g)
Initial [NO] Initial [Cl2] Initial rate
1 0.20 0.10 0.63
2 0.20 0.30 1.92
3 0.80 0.10 2.58
4 0.50 0.50 ?
Where [NO] is changing and [Cl2] is constant:
1&3 [NO] x4 from 0.20 to 0.80, rate has also x4 from 0.63 to 2.58 1ST ORDER
Where [Cl2] is changing and [NO] is constant:
1&2 [Cl2] x3 from 0.10 to 0.30, rate has also x3 from 0.63 to 1.92 ALSO 1ST ORDER
Rate equation= k[NO][Cl2]
Calculate the missing data from this table
Calculate k using any experiment
experiment 1-
k= rate/[NO][Cl2]
k= 0.63/ (0.20x0.10)
k= 31.5
rate = k [NO][Cl2]
= 31.5 x 0.50 x 0.50= 7.88 moldm-3 s-1
What are the units of initial rate, conc and k in this reaction?
Initial rate= moldm-3 s-1
Conc= moldm-3
k= moldm-3 s-1/ moldm-3 x moldm-3
k= mol-1dm3 s-1
