5.1.1 How Fast Flashcards
Define rate of reaction
Change in concentration of a reactant or a product per unit time
At a given instant, how could you calculate the rate of a reaction?
Rate of reaction = change in conc of reactants or products / time
If the order is zero with respect to a reactant, what does that mean?
Changing the conc of the reactant has no effect on the rate
What is the effect on rate in a first order reaction?
Rate is directly proportional to the concentration
What is the effect on rate in a second order reaction?
Change in rate = change in conc squared
Write a generic rate equation and state what each term means
Rate = k [A]a [B]b
k=rate constant
[A] and [B] are concenrations
a and b are the orders of reaction
Do zero order reactants appear on rate equation? Why?
No
because they dont have an effect on the rate
How is the overall order of a reaction calculated?
Sum of individual orders
How would you calculate the units of the rate constant?
Units of rate are mol dm-3 s-1
units of conc are mol dm-3
Rearrange rate equation to get k =
sub in units and cancel them out
When the overall order is 3, what would be the units for rate constant?
dm6 mol-2 s-1
How could you measure the rate of reaction experimentally (different methods)
Use a colorimeter at suitable intervals if there is a colour change
If gas is involved, use a gas syringe to collect volume of gas evolved or measure the change in mass of the reaction mixture
What does half life mean?
The time taken for concentration of a reactant to decrease by half
What is the symbol for half life?
t 1/2
What is the relationship between first order reactions and half life?
First order reactants have constant half lifes
What is the equation that is used to determine rate constant using half life in a first order reaction?
k = ln2 /t 1/2
What is the relationship between rate and time?
Rate proportional 1/t
How to determine the rate constant from a rate concentration graph of first order?
k = rate / conc
What is rate determining step?
The slowest step in a reaction with multiple steps
How does the rate determining step link to the species involved in the rate equation?
Any species involved in the rate determining step appear in the rate equation. Species only involved after the rate determining step do not appear in the rate equation
For a reactant in the rate equation, which indicates how many molecules of that reactant ae involved in the rate determinigng step?
The order of the reactant
The rate equation of a reaction is rate = k[NO]^2.
How many molecules of NO will be present in the rate dtermining equation
2
What affects the vale f the rate constant for a given reaction
Temp only
What is the effect of 10 degrees temp increase on the rate roughly?
Doubles rate of reaction
What is the Arrhenius equation?
What does each term mean?
k = Ae -Ea/RT
k=rate constant A= no. collisions between reactant molecules e R= gas constant T= temp in kelvin Ea = activation energy in joules
How can you convert the Arrhenius equation into a useful form for plotting a graph
ln k = -Ea/ RT +lnA
Graph of ln k against 1/T is a straight line
gradient = -Ea/R
y intercept = ln A
