Topic 16: Kinetics II Flashcards
Define rate of reaction
Change of concentration/amount of reactant or product, per unit of time
Amount of product used / time
Define rate equation
Links the rate with concentrations of substances
Rate = K[A]^a[B]^b
K = rate constant
AB = concentration of substances
ab = orders of reaction
Define order with respect to a substance in a rate equation
How the concentration of the substance affects the rate
0 order - a change in the conc. has no effect on rate
1 order - change in conc. has a proportional change on rate
2 order - change in conc. has a squared proportional change on rate
—> e.g. if [A] doubles then rate quadruples
Define overall order of reaction
Define rate constant
K - allows us to equate rate with concentration
only fixed at a particular temperature
If temp changes, K changes
Larger value of K, larger rate of reaction but conc. of substances remain the same
Define half-life
The time it takes for half the reactant to be used up
Define rate-determining step
The slowest step in a multi-step reaction
Define activation energy
Define heterogeneous/homogeneous catalyst
How can rate be measured by electrical conductivity?
Electrical conductivity - may be a change in the number of ions during reaction
How can rate be measured by Mass lost?
Mass lost - place reaction on balance and measure the mass lost (as a gas)
How can rate be measured by volume of gas produced?
Volume of gas produced - using a gas syringe measured over certain time period. (if mixed in wrong proportions, too much gas can be produced)
How can rate be measured by pH change?
Change in pH - use pH meter to measure pH at regular intervals (acid-base)
How can rate be measured by Titration?
Titration - change in concentration, by taking aliquots at certain time intervals.
How can rate be measured by Colourimetry?
If there is a colour change, measure absorbance of light —> e.g. propanone + iodine (brown to colourless solution, as products are all colourless)
—> Must plot a calibration curve of absorbance on Y by concentration on X.
—> used to find conc. of Iodine.
Why does the reaction need to be stopped in a Titration when measuring rate?
When we take the aliquot, we must stop the original reaction, otherwise it would carry on and the conc. would change as we do the titration
What is quenching / how is it done?
Stopping a reaction by adding a chemical quenching agent
What is the Initial Rates Method used for?
A way to calculate the rate equation for a reaction
How do you do an Initial rates method experiment?
- Repeat the experiment several times, changing the concentrations of the reactants one at a time
- Calculate the initial rate for each experiment
- Record concentrations of each reactant and their initial rate to look at the relationship between them, to calculate the orders.
Why is a ‘Clock reaction’ used?
Simplify the initial rate method
It times how long it takes for a reaction to occur
What 3 assumptions are made during the clock reaction?
- Temperature remains constant
- Concentration of reactants does not significantly change
- Reaction has not proceeded too far when the end point is seen
Explain the Iodine-clock reaction
Small amount of Sodium thiosulfate and starch are added to excess H2O2 and iodide ions in acidic solution (H+ ions)
The sodium thiosulfate reacts instantaneously with any iodine which is formed (from the reaction between H202 + I- —> H20 + I2)
Initially, all the iodine is used up in the reaction between sodium thiosulfate and iodine. BUT once all the sodium thiosulfate is reacted, the Iodine produced stays in solution, causing the starch indicator to suddenly turn blue-black.
When is the end-point of the Iodine clock reaction?
When the indicator turns blue-black.
What changes the time taken for the end point of the iodine clock reaction?
Varying concentrations of Iodine, or Hydrogen peroxide concentration
How is a rate-concentration graph created?
By knowing the rate:
Once the rate is calculated from the gradient at various points on a concentration-time graph
How can half-life be calculated?
Concentration-time graph
1. Find the first conc. e.g. 10 and the second concentration e.g. 5
2. Follow the lines across to the curve, and then down to the corresponding times.
3. Calculate the time difference, and this shows the time it takes for half the reactant to be used
Explain the half-life on a concentration-time graph for 0 order
The difference between times isn’t the same each time
the rate on a straight line graph is constant, therefore changing conc. doesnt change the rate
HALF-LIFE DECREASES
Explain the half-life on a concentration-time graph for 1 order
Half-life is constant
The rate changes in equal amounts
Explain the half-life on a concentration-time graph for 2 order
Half-life increases
Rate changes in unequal amounts
Reactants that appear in the rate equation….
Reactants that appear in the rate equation will affect the rate of reaction
Therefore these reactants must appear in the rate determining step
Substances not in the rate equation…
Wont be in the rate determining step
Where can we find the rate-determining step?
From the rate equation
How can you find the reaction mechanism?
From the rate determining step.
Which mechanism do halogenoalkanes react by?
Nucleophilic substitution
What is a nucleophile?
An electron pair donor
What property of the haloalkane allows it to be attacked by a nucleophile?
The C - Br (or any halogen) bond is polar.
This is because Br is more electronegative than C, so pulls the bonding pair of electrons towards itself.
This results in a more + C and more - Br
What conditions are required for the reaction of haloalkanes and hydroxide ions?
- Warm NaOH (aq)
- Reflux - allows this to be heated without losing reactants or products
What is the general equation for the overall reaction of the haloalkane and hydroxide ions?
R-X + NaOH —> ROH + NaX
Haloalkane + sodium hydroxide –> alcohol + Sodium compound
What does the SN1 reaction tell us?
There is one molecule in the rate-determining step
it is a two step reaction
What type of haloalkane reacts via SN1?
secondary and tertiary
What does the SN2 reaction tell us?
There are two molecules in the rate-determining step
It is a one-step reaction (with an intermediate/transition stage)
What type of haloalkane reacts via SN2?
primary and secondary
What does the Arrhenius equation link?
Activation energy and temperature to the rate constant
What is the relationship between activation energy and the rate constant?
as the activation energy decreases, the rate constant increases
Explain the relationship between activation energy and rate constant
As the activation energy decreases, more particles have enough energy to react and collide, therefore the rate of reaction increases
What is the relationship between temperature and the rate constant?
As the temperature increases, the rate constant increases
Explain the relationship between temperature and rate constant
As the temp increases, particles have more kinetic energy and are more likely to collide, therefore rate of reaction increases
How do you rearrange the Arrhenius equation to calculate Ea?
Ea = (lnA −lnK) × RT
What equation is used to plot a graph of lnK against 1/t?
lnK = lnA - Ea/RT
