1what is the definition of the following:
- rate of reaction
change in concentration of a substance in unit time (mol dm-3 s-1)
2list the techniques used to measure the rate of reaction
-titration
-colorimetry
-mass change
-volume of gas evolved
-optical activity
- electrical Conductivity
3how do you use the following method to measure the rate of reaction:
-volume of gas evolved
- works if there is a change in the number of moles of gas in the reaction
-using a gas syringe
- Direct/continous data
HCOOH + Br2 --> 2H+ + 2Br- + CO2
4how do you use the following method to measure the rate of reaction:
-change of mass
-works if there is a gas produced which is allowed to escape, works better with heavy gases
-Direct/ continous Data
HCOOH + Br2 --> 2H+ + 2Br- + CO2
5how do you use the following method to measure the rate of reaction:
-titration
1- remove a small sample from the reaction mixture
2- quench(stop) the sample (by cooling or adding acidase)
3- titrate the samples to find the concentration
-accurate for concentration
HCOOCH3 + NaOH --> HCOONa + CH3OH
6how do you use the following method to measure the rate of reaction:
-colorimetry
-works when the reactant/product is colored
- measures the absorbance of light by a solution
- non destructive/ fast
- the darker the color the higher the concentration
7how do you use the following method to measure the rate of reaction:
-change in electrical conductivity
can be used if there is a change in the number of ions (formation of ions)
8how do you use the following method to measure the rate of reaction:
-measurement of optical activity
- change in the optical activity can be followed in a polarimeter
9which method is used to find the rate of reaction between iodine and propanone
CH3COCH3+I2→CH3COCH2I + (H+) +(I−)
1- remove sample using a pippete
2- quench the sample by adding excess NaHCO3 to neutralise the acid catalyst
3- titrate the sample with sodium thiosulphate using a starch catalyst
10If a reaction has a zero-order reactant, what does this imply about the reaction mechanism?
It means the mechanism must have at least two steps, because the rate-determining step does not involve the zero-order reactant.
i.e.
step 1: The zero-order reactant is involved in a fast step
step 2: The rate-determining step (slow step) does not include the zero-order but the reactants that make it up
11In the acid-catalysed iodination of propanone, what does the rate-determining step involve, and what happens to iodine?
The rate-determining step involves one propanone molecule and one H⁺ ion reacting to form an intermediate
-The iodine (I₂) takes part in a faster step after the intermediate is formed.
12What is an SN2 reaction, and which type of haloalkanes usually react by this mechanism?
- one step nucleophilic subsitution
-2 species involved in the RDS
-primary haloalkanes react via the SN2 mechanisim
13What is an SN1 reaction, and which type of haloalkanes usually react by this mechanism?
- 2 step nucleophilic subsitution
-1 species involved in the RDS (haloalkane)
-tertiary haloalkanes react via the SN1 mechanisim
14In hydrolysis of haloalkanes, why is water often used instead of hydroxide ions?
Water acts as a weaker nucleophile, allowing the rate to be measured more slowly and clearly
15What is the Arrhenius equation?
The Arrhenius equation shows how the rate constant (k) changes with temperature:
k = A * e^(-Ea / (R * T))
*EA units are always Joules*
16How can the Arrhenius equation be rearranged in logarithmic form?
The Arrhenius equation can be rearranged as:
ln k = ln A − (Ea / (R * T))
*EA units are always Joules*
17How do you calculate the activation energy (Ea) graphically using the Arrhenius equation?
-Plot 1/T on the x-axis and ln k (or ln rate) on the y-axis.
-The line has a negative gradient:
-gradient = −Ea / R
-The line slopes downwards, not upwards.
18What is a heterogeneous catalyst?
A heterogeneous catalyst is in a different phase from the reactants, usually a solid when the reactants are gaseous or in solution.
19Where does a reaction occur in a heterogeneous catalytic process?
The reaction occurs at the surface of the catalyst, at active sites where reactants adsorb.
20What are active sites on a heterogeneous catalyst?
Active sites are the places on the catalyst surface where reactant molecules adsorb, their bonds weaken and then the products desorb
21Does increasing pressure affect a heterogeneous catalyst? Why or why not?
No, increasing pressure does not affect the catalyst because the active sites are already saturated with reactant molecules.