RP7 - Measuring rate of a reaction Flashcards
What is meant by ‘rate of reaction’?
The change in concentration of reactants or products over time.
How can rate of reaction be measured?
● Initial rates method- i.e. the iodine clock reaction
● A continuous monitoring method- i.e. measuring the volume of gas released in a reaction over time.
What is an initial rates method?
The method involves measuring the initial rate of reaction for multiple different concentrations to observe how rate of reaction varies.
What is the equation for the iodine clock experiment?
H2O2(aq) + 2H+(aq) + 2I–(aq) → I2(aq) + 2H2O(l)
2S2O32–(aq) + I2(aq) → 2I–
(aq) + S4O62–(aq)
What happens during the iodine clock experiment?
The I2 produced reacts with all of the thiosulfate ions present.
Excess I2 remains in solution which then reacts with starch to form a blue-black solution.
Give an example of an initial rates method.
The iodine clock experiment
What are the issues with the iodine clock experiment?
● Some low I- concentrations may take too long to react.
● Delayed stopwatch reactions.
● Concentrations may not be exact due to measuring apparatus.
What is a continuous monitoring method?
This involves measuring the change in concentration of a reactant or product over time (or measuring volume of gas released) as the reaction progresses.
Give an example of a continuous monitoring method.
● Add a 6 cm strip of magnesium ribbon into a conical flask containing HCl, place a bung in the top of the flask and start the timer.
● Note down the volume of hydrogen gas collected every 15 seconds for a period of 2.5 minutes.
● Repeat for different HCl concentrations.
How would you analyse the data from a continuous monitoring experiment?
● Plot a graph of volume of hydrogen gas produced (y-axis) against time (x-axis) for each concentration of HCl. Draw a line of best fit.
● Draw a tangent at t = 0s for each line.
● To deduce the rate of each reaction, calculate the gradient of each tangent.
● Compare the calculated rate values.
What are some issues with this continuous monitoring experiment?
● Some gas may escape before the bung is added.
● The magnesium strips may be of different mass and surface area etc. which will affect the rate of reaction.
How do you prevent gas escaping?
How do you prevent gas escaping?
Place the solid reactant upright inside a sample tube in the conical flask, tipping the tube over by moving the conical flask
around to start the reaction.
