RP 7 - Measuring Rate of Reaction Flashcards
What’s 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 (measuring the volume of has released in a reaction overtime)
What’s the initial rates method to measure rate of reaction?
The method involves measuring the initial rate of reaction for multiple different concentrations to observe how rate of reaction varies
Give an example of an initial rates method
• The ‘Iodine Clock’ experiment:
- H2O2 (aq) + 2H+ (aq) + 2I- (aq) -> I2 (aq) + 2H2O (l)
- 2S2O3^2- (aq) + I2 (aq) -> 2I- (aq) + S4O6^2- (aq)
• 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
- Time how long it takes for this blue-black colour to appear. You can vary [I-] to then determine the order with respect to the iodide ions
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’s 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 Mg ribbon into a conical flask containing HCl, place a bụng 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 the continuous monitoring method?
- 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 reduce the rate of each reaction, calculate the gradient of each tangent
- Compare the calculated rate values
What should the set up for the continuous monitoring method look like?
*There’s a ‘Conical Flask’ with a liquid inside of it and a ‘Rubber Bung’ at the top connected to a ‘Laboratory Tubing’ which is connected to a ‘Gas Syringe’
What are some issues with the continuous monitoring method?
• Some gas may escape before the bụng is added
• The Mg strips may be of different mass and surface area which will affect the rate of reaction
How do you prevent gas from escaping in the continuous monitoring method?
Place the solid reactant upright inside the sample tube in the conical flask, tipping the tube over by moving the conical flask around to start the reaction
