Unit 1: Section 7 - Rate Equations and Kp Flashcards
What is the definition of rate of reaction?
- Change in concentration (of any reactant or product) per unit time
- State what is being monitored (usually production of a product)
At a given instant, how could you calculate the rate of reaction?
Rate of reaction = change in concentration / change in time
How could you measure the rate of reaction experimentally (different methods)?
- Use a colorimeter if there is a colour change
- If a gas is evolved, use a gas syringe to collect volume of gas evolved, or measure the change in mass of the reaction mixture
How can you determine the rate constant and rate expression for a reaction?
Only experimentally
What affects the value of the rate constant for a given reaction?
Temperature, nothing else
Write a generic expression and state what each term means?
Rate = k[A]^x[B]^y
k - rate constant for the reaction
[A] and [B] are concentrations of species A and B respectively
x and y are the orders of reaction with respect to A and B
Do species need to be in the chemical equation to be in the rate expression?
No - species in the chemical equation may be excluded and species not in the chemical equation, e.g. catalysts, may be included
Define the term order of a reaction with respect to a given product
The power to which a species’ concentration is raised in the rate equation
How do you find the overall order of reaction?
The sum of the orders of reaction of all species in the rate expression
How would you calculate the units of the rate constant?
- Units of rate are moldm^-3s^-1 and units of concentration are moldm^-3
- Rearrange rate equation to get k=
- Sub in units and cancel them out
How would you draw a rate concentration graph?
- Plot [A] against time, draw tangents at different values
- Draw a secondary graph of rate against [A]
What does a rate concentration graph for a zero order reactant look like?
A straight horizontal line
What does a rate concentration graph look like for a first order reactant?
A straight line graph y=x
What does a rate concentration graph from a second order plus reactant look like?
A curve starting at (0,0) and increasing exponentially
How could you confirm that a reactant was second order using graphs?
Graph of rate against concentration squared would be a straight line y=x through the origin
How could you find the rate expression using the initial rate method?
- Do a series of experiments, during which you vary concentrations, so the concentration of just one reactant changes over time
- Plot a graph of concentration against time for each reactant and use a tangent at t=0 to find the initial rate of reaction
- Compare rates and concentrations between each experiment to find order reactants and overall rate equation
What must you add to react with the I2 as it is produced for an iodine clock reaction?
Known moles of sodium thiosulfate and a little starch
Reacts with I2 in 1:2 ratio
I2 + 2S2O3 ^-2 -> S4O6 ^-2 + 2I-
When does the starch turn a blue-black colour in an iodine clock reaction and why?
When all of the Na2S2O3 has been used up and so I2 is produced, which reacts with starch, leading to a blue-black colour
How can you calculate the rate of reaction from the data from an iodine clock reaction?
Record time taken for colour change to occur
Use rate = 1/t
This is effectively the initial rate
What is the effect of a 10K temperature increase on the rate of reaction, roughly?
Doubles rate of reaction
What is true of the half life of a first order reactant (concentration against time graph)?
Half life is constant
What does each term in the Arrhenius equation mean?
k - rate constant for reaction (same units as A)
A - Arrhenius constant (same units as k)
e - mathematical quantity
R - gas constant
T - temperature in Kelvin
Ea - activation energy for reaction in Joules
How can you convert the Arrhenius equation into a useful form for plotting a graph?
- Graph of lnk against 1/T is a straight line
- gradient - Ea/R
- y intercept - lnA
What is the rate determining step?
The slowest step in a reaction mechanism, which determines the overall rate of reaction
How does the rate determining step link to the species involved in the rate expression?
- Any species involved in the rate determining step appear in the rate expression
- Species only involved after the rate determining step do not appear in the rate expression
What is the definition of the rate constant?
The constant of proportionality in a rate equation, the value of which is dependent on temperature and activation energy of the reaction
How does the temperature affect k?
As temperature increases, concentration stays the same, so k increases
Why does doubling the temperature have a greater effect on the rate of reaction than doubling the concentration?
- Reaction occurs when molecules have E≥Ea
- Doubling temperature causes many more molecules to have this Ea
- Whereas doubling the concentration of a species only doubles the frequency of the collisions
What 2 things mean you can ignore the order of a reactant, why?
- If they have a significantly larger concentration than the other reactant - concentration change becomes negligible so has no effect on the rate
- The concentration remains almost the same
What is the definition of the overall order of a reaction?
The sum of powers/indices (to which the concentrations are raised in the rate equation)
What is partial pressure?
Each gas’s contribution to the total pressure
How would you calculate the partial pressure of a gas?
Partial pressure p = mole fraction x total pressure
What is the mole fraction?
Mole fraction of gas X = number of moles of gas X in the mixture / total number of moles of gas in the mixture
A reaction is represented by aA(g) + bB(g) ⇌ cC(g) + dD(g), what is Kp for the system?
Kp = pC^cpD^d/pA^apB^b
where pA = partial pressure of A and a = number of moles of A
How do you calculate the units for Kp?
Write out the units for the partial pressures in the same arrangement as the Kp equation and cancel out/multiply together
Usually in Pa, kPa, atm etc
DO NOT CHANGE UNITS
What is the effect of increasing temperature on Kp for an endothermic reaction?
Equilibrium shifts to the right, so partial pressures of products increase, so Kp increases
What is the effect of increasing the overall pressure on Kp for this reaction?
Pressure doesn’t affect Kp as, if moles are not the same on each side, either top or bottom of Kp expression will have a total pressure term that does not cancel
What will the kinetic effect of increasing the temperature and pressure for any reaction?
Increasing both will increase the rate of reaction as:
Temperature - many more particles habe energy greater than or equal to the activation enegry -> more successful collisions per second
Pressure - more particles in the same volume -> more successful collisions per second
