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Syllabus
Syllabus
The rate of a chemical reaction can be found by measuring the quantity of a reactant used or the quantity of product formed over time:
mean rate of reaction = quantity of reactant used / time taken
mean rate of reaction = quantity of product formed / time taken
The quantity of reactant or product can be measured by the mass in grams or by a volume in cm3 .
The units of rate of reaction may be given as g/s or cm3/s.
Higher Tier students are also required to use quantity of reactants in terms of moles and units for rate of reaction in mol/s.
Students should be able to:
calculate the mean rate of a reaction from given information about the quantity of a reactant used or the quantity of a product formed and the time taken
draw, and interpret, graphs showing the quantity of product formed or quantity of reactant used up against time
draw tangents to the curves on these graphs and use the slope of the tangent as a measure of the rate of reaction
(HT only) calculate the gradient of a tangent to the curve on these graphs as a measure of rate of reaction at a specific time.
What does this mean?
What does this mean?
What does rate mean?
What does rate mean?
The rate of a reaction is another word for the speed of the reaction.
So a high rate of reaction means a fast reaction that's over quickly & a low rate means a slow reaction that takes a long time.
To measure a rate of a reaction we either measure the speed a product is made, or the speed a reactant is used up.
Measuring the speed by collecting the product.
Measuring the speed by collecting the product.
If the reaction makes a gas (acid +carbonate or acid + metal) then the gas can be collected in a gas-syringe or an upturned measuring cylinder.
We would need to write down the volume of gas collected every 20 seconds.
If we plotted a graph of the amount of gas against time it would increase quickly, then slow down until it stopped reacting altogether.
Measuring the speed a product escapes.
Measuring the speed a product escapes.
Rather than collecting the product we could simply weigh the reactants and their container.
As the gas escapes the mass decreases and we write it down every 20 seconds.
The cotton wool in the top allows the gas to escape but collects any water that might be thrown upwards when bubbles burst.
Mass would decrease quickly at the start, then slow until it stopped being lost altogether.
The graphs
The graphs
In both cases, the reaction is fastest at the start and we can see this by the steepness of the curve.
In both cases, the reaction is fastest at the start and we can see this by the steepness of the curve.
Later, the reaction slows as the reacting substances are used up.
So, the curve continues to go up but less steeply.
When one reactant runs out, the reaction is over.
No more gas is made and the line stops going up (horizontal)
Changing variables.
Changing variables.
We could carry out the same experiment but change one variable each time to see what effect it has.
Variables that we may choose to change are:
The temperature of the acid
The concentration of the acid
The surface area of the marble chips/metal - the particle size
With a different reaction (such as the decomposition of Hydrogen Peroxide) we might add various catalysts.
A catalyst speeds up a reaction without being used up.
Mean Rate
Mean Rate
In the example above, 20cm3 of gas was made in 20 seconds
That's a rate of 1cm3/s - but the reaction was faster than that at the very start and slower than that at 20 sec.
So, this is the mean rate of reaction.
Exams may ask you for this but....
Tangents and Initial Rate!
Tangents and Initial Rate!
We're usually interested in the initial rate of a reaction - the rate at the start when it is fastest.
To do this we must draw a tangent to the curve at 0 seconds
or
The gradient of the tangent is then calculated and is equal to the rate of reaction.
You really should have done this in Maths by now
Pick two convenient points on your tangent.
Gradient = Difference in y coordinates / Difference in x coordinates
In the example above the units would be cm3/s - because these are the units on the graph.
If the graph showed mass loss then the units would be g/s.
A HT paper might expect you to convert grams to moles and the units to mol/s
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