CI - Rates of reaction Flashcards
What does the reaction rate tell you?
How fast reactants are converted to products.
What is reaction rate?
The change in the amount of reactants or products per unit of time (normally per second).
What do the units of rate of reaction depend on?
What you are measuring.
E.g. if the reactants are in solution, the rate will be change in concentration per unit f time and the units will be, e.g. mol dm-3 s-1.
What is continuous monitoring?
Measuring how the amount of a product or reactant changes over the complete course of the reaction.
What are some examples of ways of measuring rate of reaction?
pH measurement Gas volume Loss of mass Colour change Titration
How can pH measurement be used to follow the rate of a reaction?
If one of the reactants or products is an acid or base you can follow the reaction by monitoring the pH of the reaction mixture.
You can do this by using a pH meter, or a pH probe connected to a data logger.
You can convert the pH data that you have collected into units of concentration using the equation [H+] = 10^-pH.
Any rate data that you calculate in this way will have the units of mol dm-3 time-1.
How can gas volume be used to follow the rate of a reaction?
If a gas is given off, you could collect it in a gas syringe and record how much you’ve got at regular time intervals.
For example, this would work for the reaction between an acid and a carbonate in which carbon dioxide gas is given off.
The rates that you calculate from this raw data will be in units of volume time-1 (e.g. cm3 s-1).
How can loss of mass be used to follow the rate of a reaction?
If a gas is given off, the system will lose mass. You can measure this at regular time intervals with a balance.
Any rates that you calculate from this raw data will be in units of mass time-1 (e.g. g s-1).
How can colour change be used to follow the rate of a reaction?
If one of you reactants or products is coloured, then you can track the colour change of a reaction using a colorimeter.
A colorimeter measures the absorbance of the solution (that’s the amount of light it absorbs).
The more concentrated the colour of the solution, the higher the absorbance is.
For example, in the reaction between iodine and propanone, the brown colour fades.
Once you have collected you absorbance data, you can use it to calculate the concentration of the coloured substance using a calibration curve.
How can titration be used to follow the rate of a reaction?
You can monitor the concentration of a reactant or product in solution by taking small samples of the reaction mixture at regular time intervals and titrating them.
Once you’ve taken a sample of the reaction mixture, you need to slow down the reaction happening in it (otherwise the concentration will be changing while you’re trying to measure it). To do this you can dilute the sample with deionised water, cool it, or add another chemical that will stop the reaction happening.
From what graph can you work out the rate of reaction from?
A concentration-time graph.
What can you do with the data collected by repeatedly measuring the concentration of one of the reactants or products over the course of a reaction?
Plot a concentration-time graph.
What is the rate at any point of the reaction given by on a concentration-time graph?
The gradient (slope) at that point.
What does the gradient at a given point on a concentration-time graph give information about?
The rate of reaction at that point.
How to you find the gradient of a curve on a concentration-time graph?
Draw a tangent to the curve.
