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
