AQA Chemistry 5.3.1.3 · Combined and Triple
Mass changes in reactions
Mass is always conserved. When a balance reading rises or falls, a gas has joined or left. Explain both with particles, and (Higher) predict the change from the equation.
Triple Science · Higher tier
Start here
A fizzy drink on the scales.
You open a bottle of fizzy drink, stand it on kitchen scales and leave it for an hour.
If you had to guess, does the reading go up or down?
Open or sealed
Two balances, same reaction.
Marble chips (calcium carbonate) and dilute hydrochloric acid, on two balances. One flask is open; one is sealed. Conservation of mass holds in both: Conservation of mass covers the law itself.
Now predict each reading. Falls, rises, or stays the same?
Magnesium ribbon heated in an open crucible.
Predict.
Zinc and dilute sulfuric acid in an open test tube.
Predict.
Calcium carbonate heated in a sealed tube.
Predict.
Two solutions mixed in a sealed flask; a solid precipitate forms.
Predict.
Equation · mass of gas
In an open container the mass of gas that left (or joined) is the difference between the two readings. It is a subtraction, so there is no triangle: one line, both readings in the same unit.
mass of gas = mass before − mass after
If the reading rose (a metal gaining oxygen), swap the order: mass gained = mass after − mass before.
Higher · equation · moles from mass
Predicting the change from the equation.
On Higher you can predict how much mass a metal gains when it burns, from the balanced equation. The tool is the mole: n = m ÷ Mr. Moles have their own lesson, Moles; here is the triangle you need. Tap the quantity you want to find.
Equation
Moles from mass
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
n = m ÷ Mr
- m
- mass
- g
- n
- amount of substance
- mol
- Mr
- relative formula mass (Ar for an element)
- no unit
Three steps, each one taught on its own first: moles of the metal (n = m ÷ Ar), the ratio from the equation, then mass of oxygen (m = n × Mr). Mass-ratio working (48 g : 80 g) appears only as a check.
Think again · burning
“Burning destroys stuff, so the ash must weigh less than the magnesium ribbon.”
Your call
ChooseMagnesium ribbon is heated in an open crucible until it has all turned to white powder. How does the mass of the powder compare with the ribbon?
Sort
What does the balance do?
Place each experiment.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Explain
- An apparent change in mass, using the particle model.
- Calculate
- The mass of gas lost from two readings; (Higher) the mass gained from the equation.
- Predict
- Whether a reading rises, falls or stays the same.
Key fact
Some reactions may appear to involve a change in mass. The explanation is usually a gas: one that escapes from an open container (reading falls) or one from the air that becomes part of the product (reading rises). Sealed, nothing changes.
Key note · AQA 5.3.1.3 · Combined and Triple
Mass changes in reactions
- Some reactions may appear to involve a change in mass but can usually be explained because a reactant or product is a gas and its mass has not been taken into account.
- Reading falls: a gas escapes from an open container (marble chips and acid, zinc and acid, heating a carbonate). Gas particles move freely and leave, taking their mass with them.
- Reading rises: a metal reacts with oxygen from the air and the oxygen becomes part of the solid oxide (2Mg + O2 → 2MgO).
- Sealed container: nothing enters or leaves, so the reading stays the same. Mass of gas = mass before − mass after, both in the same unit.
- Higher: predict the mass change from the balanced equation in three steps: n = m ÷ Ar, the molar ratio, then m = n × Mr. 4.8 g Mg gains 3.2 g of oxygen to make 8.0 g MgO.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Reading rose when you expected a fall?
Balance readings in the practice are made up round numbers.