Rate of Reaction
Learning Objectives
• Describe and evaluate practical methods for investigating the rate of reaction,
including change in mass of a reactant or a product and the formation of a
gas
Types of Reactions
• Respiration
• Combustion [when you burn anything in the presence of oxygen]
• Displacement [halide displacement, when a more reactive halogen displaces a less
reactive one]
• Photosynthesis
• Neutralisation [acid and alkali react]
• Precipitation reaction [two aqueous solutions mix to form a precipitate]
• Think of an example of a fast chemical reaction and a slow chemical reaction.
What is rate of reaction?
• Reactions that occur at di erent speeds
• Change that occurs per unit time
• Time measured in
• Seconds
• Minutes
• Hours
• Months
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• Days
In what context would speed of a reaction matter?
Observing Rate of Reaction
Reaction between Zinc and Hydrochloric Acid
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Products? Zinc chloride and hydrogen
Observations? E ervescence/bubbling
Chemical Equation, with state symbols? Zn (s) + 2 HCl (aq) —> ZnCl2 (aq)+ H2 (g)
How would you measure the rate of this reaction?
Observing Rate of Reaction
• You could measure the rate of reaction, by measuring:
• The amount of zinc used per minute
• The amount of hydrochloric acid used per minute
• The amount of zinc chloride produced
• The amount of hydrogen produced
Observing Rate of Reaction
• For the reaction, what would be the easiest way to observe the rate of
reaction using common laboratory apparatus.
Zinc
Results
• How would the volume of hydrogen change over time?
• It will increase because the reactants will react and continue produce hydrogen
• Will the volume increase by same amount in each interval of time?
• No
• Reason for answer?
• Because the reactants are being used up
• What would the graph look like?
A: Initially, the reaction will be the fastest
Or most amount of hydrogen produced or
the graph will be the steepest because
initially maximum amount of reactants are
available
B: the gradient of the graph decreases or
graph becomes less steep because
reactants are being used up
C: either one or both the reactants have
been used up
Reactant that nishes up rst? Limiting
reagent
The one the remains behind? In excess
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Rate of Reaction = volume of hydrogen / time
= 40 / 5 = 8 cm3 / min
Rate of reaction at the second minute = 25/2 = 12.5 cm3/min
Understanding the graph
Observing Rate of Reaction
• Calcium Carbonate + Hydrochloric Acid —>
• What are the products formed?
• Calcium chloride + carbon dioxide + water
• Observations?
• E ervescence/ bubbling
• Chemical Equation?
• CaCO3 (S) + 2 HCl (aq) —> CaCl2 (aq) + CO2 (g) + H2O (l)
• The mass of reactants = mass of products [some mass of products is being lost]
• As the reaction proceeds what will happen to the mass of the container with the reactants in it?
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• It will decrease
Marble chips - calcium carbonate
Mass of container will start to decreased as carbon
dioxide is being produced and lost, as the reaction
proceeds
Cotton wool - it allows the carbon dioxide to
escape but makes sure the reactants don’t
With time the mass of the container will start to
decrease
Suitable for heavier gases but not for lighter ones, e.g. hydrogen gas
Results
• How would the mass change over time?
• Decrease
• Will the mass decrease by same amount in each interval of time?
• No
• Reason for answer?
• Reactants are being used up
• What would the graph look like?
Results