AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Calculating rates of reactions: The quantity of reactant or product can be measured by the mass in grams or by a volume in cm³.
To follow a reaction, you need a measurable quantity that changes as reactants are used up or products form.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
To follow a reaction, you need a measurable quantity that changes as reactants are used up or products form. Mass in grams suits reactions that release a gas into the air, because the total mass falls as the gas escapes; a balance reading in g is recorded at timed intervals. Volume in cm³ suits reactions that produce a gas you can collect, for example in a gas syringe or over water; the volume is read at timed intervals. Either measurement can be plotted against time and used to find a rate. Choose the method that matches the reaction: if a gas is produced, volume or mass loss both work; if no gas forms, another property such as colour or pH may be needed. Record readings at regular times and keep other variables controlled.
What earns the marks
- Identify mass in grams as a measurement of the quantity of reactant or product, often using a balance.
- Identify volume in cm³ as a measurement of the quantity of reactant or product, often using a gas syringe or measuring cylinder.
- Link mass loss to a gas escaping from an open reaction vessel.
- Link volume increase to a gas being produced and collected.
- Select a suitable measurement method for a described reaction and justify the choice.
- Record measurements at regular time intervals so that a rate can be calculated.
Common mistakes and how to put them right
Using mass loss when the reaction is in a sealed vessel, so no gas escapes.
Put it right by choosing volume of gas produced instead.
Confusing cm³ with cm² or writing the unit without the raised exponent.
Put it right by writing cm³ for volume.
Recording only a single reading rather than a series over time.
Put it right by taking readings at regular intervals so a rate can be found.
Exam tips
- Name the apparatus you would use, such as a balance for mass or a gas syringe for volume, to show your method is practical.
- State the unit with every measurement, using g for mass and cm³ for volume.
- When comparing methods, mention what is controlled, such as temperature or concentration, so the comparison is fair.
Try the lesson
The start of the Calculating rates of reactions: The quantity of reactant or product can be measured by the mass in grams or by a volume in cm³. lesson, made from checked teaching notes for this specification point.
Have a go first
A student measures the mass of a reaction mixture before and after a gas escapes. Which unit is used to record the mass of the mixture?
Listen
To follow a reaction, you need a measurable quantity that changes as reactants are used up or products form. Mass in grams suits reactions that release a gas into the air, because the total mass falls as the gas escapes; a balance reading in g is recorded at timed intervals. Volume in cm³ suits reactions that produce a gas you can collect, for example in a gas syringe or over water; the volume is read at timed intervals. Either measurement can be plotted against time and used to find a rate. Choose the method that matches the reaction: if a gas is produced, volume or mass loss both work; if no gas forms, another property such as colour or pH may be needed. Record readings at regular times and keep other variables controlled. A strong answer earns marks for these points. Identify mass in grams as a measurement of the quantity of reactant or product, often using a balance. Identify volume in cm³ as a measurement of the quantity of reactant or product, often using a gas syringe or measuring cylinder. Link mass loss to a gas escaping from an open reaction vessel. Link volume increase to a gas being produced and collected. A common slip: Using mass loss when the reaction is in a sealed vessel, so no gas escapes. Instead, choosing volume of gas produced instead. Exam tip: Name the apparatus you would use, such as a balance for mass or a gas syringe for volume, to show your method is practical.
Key words
Say what each word means before you turn the card.
Mass loss and gas escape
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Gas volume collection
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Measuring reaction progress
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Choosing apparatus and units
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See every definition
- Mass loss and gas escape
- Mass in grams suits reactions that release a gas into the air, because the total mass falls as the gas escapes.
- Gas volume collection
- Volume in cm³ suits reactions that produce a gas you can collect, for example in a gas syringe or over water.
- Measuring reaction progress
- A measurable quantity that changes as reactants are used up or products form is needed to follow a reaction.
- Choosing apparatus and units
- A balance is used for mass in g and a gas syringe for volume in cm³, and the method must match the reaction.
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