Chapter 18 - Rates of reaction Flashcards
How to measure rate using pH?
- The pH of a reaction can change overtime if H+ ions are used up or produced
- A pH probe can be used to measure the pH of a reaction at *regular* intervals
- You can then *calculate* the H+ ion concentration
Measuring rate if gas is produced
- Measure the amount of gas produced using a gas syringe (can collect gas if gas is toxic)
- Measure the loss of mass using a balance
How to measure rate if a reaction changes colour?
- Use a colorimeter (which measures the absorbance of light in a coloured sample)
- The more concentrated a reaction is, the darker the colour
assumptions for clock reactions
- the temperature of the reaction must remain constant
- the concentration of reactions doesn’t change significantly during the time period of the reaction
- the reaction has not proceeded too far when the end point is seen
- rate of reaction remains constant (so rate of clock reaction is good estimate for initial rate)
The Iodine clock reaction equation
Iodine clock experiment steps
- Add sodium thiosulfate and starch (indicator) to excess hydrogen peroxide to iodine water
- The sodium thiosulfate reacts immediately with the iodine produced
- When sodium thiosulfate ions run out, the iodine reacts with the starch to give a deep blue colour
What do you change in iodine clock experiment
-varying the CONCENTRATION of iodine (I2) and/or hydrogen peroxide (H2O2) and keeping everything else constant will result in the time taken for the blue/black colour to appear to change
- Using this information, we can work out the order of the reaction
Rate equation
Rate / moldm^-3 s^-1
Orders of reaction
- an order is the power to which the concentration is raised to in the rate equation, it tells us how the concentration of the substance affects rate
- Zero order : no effect on rate
How are orders determined
by experiment
Is the rate constant always constant?
The rate constant is only fixed at a particular constant, k increases when temperature increases
What do large values of k mean?
- The larger the value of k, the faster the rate of reaction
- Increasing the temperature (which increases the value of k) means that the particles have more kinetic energy and collide more often with the required activation energy, thus increasing the rate of reaction.
What is half life
the time it takes for half the reactant to be used up
What are half life graphs used for (concentration against time graph)
- Can be used to calculate the rate constant (k)
- 1st order: each half life is the same length
- Rate constant = (ln2/ half life)
What is the initial rates method (for finding the rate equation?
- Repeat the experiment several times but changing the concentrations of the reactants one at a time in each experiment
- Calculate the initial rate for each experiment
