Calculating rates of reactions: The units of rate of reaction may be given as g/s or cm³/s.

The idea

Rate of reaction measures how quickly a reactant is used up or a product is formed. When the quantity is a mass, the rate is the mass change divided by the time taken, giving units of grams per second, g/s. When the quantity is a gas volume, the rate is the volume change divided by the time taken, giving units of cubic centimetres per second, cm³/s. For example, if 24 cm³ of gas is collected in 8 s, the mean rate is 24 cm³ ÷ 8 s = 3 cm³/s. If a reaction mixture loses 6.0 g in 30 s, the mean rate is 6.0 g ÷ 30 s = 0.20 g/s. The unit follows directly from the measured quantity, so choosing the right unit depends on what is being measured.

What earns the marks

• State that rate of reaction is the change in amount of reactant or product divided by the time taken.
• Identify that a mass change measured in grams over time in seconds gives the unit g/s.
• Identify that a gas volume change measured in cm³ over time in seconds gives the unit cm³/s.
• Calculate a mean rate by dividing the measured change by the time taken and attach the correct unit.
• Recognise that the unit must match the quantity measured, so mass data give g/s and gas volume data give cm³/s.

Common mistakes and how to put them right

• Writing the unit as g or cm³ without the per second part.

  Put it right by remembering that rate always involves time, so the unit must include /s.

• Dividing time by the change instead of change by time.

  Put it right by using rate = change ÷ time.

• Using g/s for a gas volume measurement or cm³/s for a mass measurement.

  Put it right by matching the unit to the quantity actually measured.

Exam tips

• Always write the division explicitly, for example 24 cm³ ÷ 8 s = 3 cm³/s, so the unit is clearly earned.
• Check whether the question gives mass or gas volume before choosing g/s or cm³/s.
• If a graph is provided, read the change from the axis label and divide by the time interval to keep the unit consistent.