Reaction Kinetics And Energetics Experiments Flashcards
What are the 4 physical methods to determine the rate and thus order of reaction?
- Volume of gas collected over time
- Measuring mass of reaction mixture
- Measuring conductivity of solution over time (when the reactants and products have different number of moles
- Measuring the colour intensity of a solution over time (for reactions which produce a distinct colour change, eg de colourisation of Br2
When should you use iodine clock, and when should you use sampling + titration? (Hint: difference in nature of reaction, and what you’re trying to find)
Iodine clock (discontinuous) is for when reaction produces a sudden visual change. For reactions with no visible change, sampling + titration can be used to find out amount of reactant/product (and thus concentration) which remains in the solution at any given point in time
I think sampling and titration is used to determine rate of reaction for ONE reaction (you draw conc time graph) —> can determine whether its first order/second order wrt limiting reagent through graph gradients.
Describe the mode of action of a hetero catalyst (ie describe adsorption) —> think marshmallow stuck on sticky tape
- Reactant molecules are strongly adsorbed onto catalyst surface, forming weak attractive forces with catalyst surface
- This weakens the covalent bonds between reactant molecules, lowering the activation energy of the reaction
- Adsorption also brings reactant molecules closer together, and correctly orientated for reaction
- After forming product molecule, it desorbs away from the catalyst surface, freeing up active sites again.
What is rate of reaction?
Rate of reaction is the change in concentration of a reactant or a product per unit time.
Explain why relative rate of reaction is taken instead of specific rate of reaction for clock experiments.
Relative rate of reaction measures how fast a reaction proceeded in one experiment compared to other experiments to form the same amount of product/use up the same amount of reactant. Specific ROR cannot be calculated for clock expts because the amt of product formed or reactant used in the reaction was never measured.
Relative rate = 1/time taken. Why?
Since a fixed amount of product is formed in each experiment, rate = change in [product] / time can be seen as constant/time across all expts. Hence rate is proportional to 1/t.
Describe what are discontinuous experiments (pg 17 handbook)
Discontinuous experiments refer to a series of experiments that are performed using the same procedure, by varying the initial concentration of one reactant and keeping all other variables of the experiment constant.
Note: initial concentration of reactant is NOT the given one they tell you. Need to account for dilution too
Explain why total volume of reaction mixture must be kept constant (COMMONLY TESTED). Use equations to prove your point
The total volume of the reaction mixture is typically kept constant (via water in aqueous solutions). When volume of reaction mixture is kept constant, concentration of a reactant is proportional to its volume.
CoVo = CdVd.
For X, a reactant,
[X] =([Xgiven] x Vx) / Vtotal
Since [Xgiven] and Vtotal are constants, [X] only dependent on Vx, concentration proportional to volume of reactant.
For clock experiments involving the formation of a fixed amount of product OR the removal of a FIXED. Amount of reactant, Define relative rate of reaction
Define the relative rate of reaction for clock experiments involving the disappearance of a coloured reactant
Relative rate = 1/time taken. Why?
Since a fixed amount of product is formed in each experiment, rate = change in [product] / time can be seen as constant/time across all expts. Hence rate is proportional to 1/t.
Relative rate for disappearance of coloured reactant = Vcoloured reactant / time taken for it to disappear
Time taken for reactant to disappear can be the same across two experiments. Does it mean rate is same? Not if the initial volume of reactant that needs to disappear in the same time is different.
[iodine clock expt] Why does reaction mixture remain colourless when S2O32-ions are present in the solution?
Reaction between I2 and thiosulfate ions occurs much faster than the formation of I2 from I- ions. This causes I2 to be consumed much faster than it is generated, hence reaction remains colourless.
Describe the planning procedure for continuous experiment quenching, sampling and titration method
For determining order of reaction for the reaction A+B -> C, with the second titration reaction being between B and another reactant of known concentration, D.
- Fill a burette with D (for the second reaction).
- Using a measuring cylinder, add ___cm3 of A to a 250cm3 conical flask.
- Using a second measuring cylinder, measure out __cm3 of B. Pour B rapidly into conical flask containing A and start the stopwatch. Swirl the reaction mixture to ensure even mixing
(If got catalyst, add __cm3 of H2SO4 into a second conical flask. H2SO4 also is a quenching agent) - Using a dropping pipette, transfer 10.0cm3 of reaction mixture into a 10cm3 measuring cylinder.
- Immediately transfer it into the conical flask containing the H2SO4. QUENCHING OCCURS. Read and record the time of transfer to the nearest second
- Immediately titrate D into the conical flask. End point is reached when a permanent pale pink colour is observed in the conical flask. Record initial and final burette readings and calculate the titre volume used.
- Repeat steps 4-6 (including the addition of catalyst if applicable) till five aliquots have been titrated against D.
NOTE: read and record time of transfer only at the step where quenching occurs. Not the transfer from the reaction mixture using the dropping pipette. In the 10cm3 measuring cylinder, A and B still reacting
Differentiate between the purposes of the 2 collection of gas methods: downward displacement of water and use of gas syringe
Downward displacement of water only for insoluble gases like H2 or gases which do not dissolve easily in water like CO2 and O2
Explain the difference in the maximum temperature change when a weak acid is reacted with a base vs a strong acid reacting with a base
Weak acid partially dissociates in water to give H+ ions. Some of the energy evolved in neutralisation is used to completely dissociate the weak acid molecules, resulting in a less exothermic enthalpy change of neutralisation and a smaller temperature change
[calorimetry] Explain why the wick doesn’t burn when a fuel is burnt and produces a flame.
The vapourising alcohol around the wick cools the exposed wick and prevents it from burning
[calorimetry] what are some precautions to take when setting up calorimetry experiments for combustion reactions?
Ensure wick is soaked in the fuel.
Adjust wick such that it is 2cm above the cap of the spirit burner
Arrange draught shields around the apparatus to minimise heat loss to surroundings
After wick is lighted, ensure that flame is centred under the copper can
[thermometric titration] Why does using a measuring cylinder to add a reactant to the styrofoam cup increase accuracy of results as compared to using a burette?
Measuring cylinder allows for faster transfer of the reactant into the styrofoam cup, which will reduce heat loss to surroundings
