Download 19. Mass changes in chemical reactions

130
Microscale Chemistry
THE ROYAL
SOCIETY OF
CHEMISTRY
19. Mass changes in chemical reactions
Topic
The nature of chemical reactions, scientific investigation.
Level
Pre-16.
Timing
10 min.
Description
In this experiment students measure the mass of various reactant solutions before and
after reaction to see whether there has been any change in mass.
Apparatus (per group)
▼
One student worksheet
▼
Part of a well-plate (the three-well plate is cut from the standard 24-well plate
using a hacksaw. A class set can be cut from a single well-plate.
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Access to a balance that reads to 0.01g.
Chemicals (per group)
Solutions contained in plastic pipettes, see p. 2
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Sodium carbonate
0.5 mol dm–3
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Calcium nitrate
0.5 mol dm–3
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Hydrochloric acid
1 mol dm–3
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Magnesium ribbon
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Marble chips (small).
Observations
Students should find that there is a negligible difference in mass before and after
mixing the sodium carbonate/calcium nitrate solution but there is some difference in
mass in the magnesium or marble chip reaction with hydrochloric acid.
The success of this experiment depends on careful working by students and on the
reliability of the balance and its proper use. Other combinations of substances could
be examined and the experiment could be used as an investigation.
As an outcome of this experiment students should appreciate that matter is neither
created nor destroyed in chemical reactions and that this is a very fundamental
aspect of chemistry. It should also help them in balancing chemical equations!
Note
You will need a balance that reads to 0.01 g.
Safety
Students must wear eye protection.
It is the responsibility of the teacher to carry out a risk assessment.
44
Microscale Chemistry
THE ROYAL
SOCIETY OF
CHEMISTRY
19. Mass changes in chemical reactions
In this experiment you will be doing two chemical reactions to see whether any mass
changes occur.
Instructions
Part A
The reaction between sodium carbonate and calcium nitrate
1.
Put two plastic pipettes containing the solutions of sodium carbonate and
calcium nitrate in the outer two wells of the mini well-plate (see below).
2.
Place on a balance and record the mass.
3.
Put 20 drops of sodium carbonate solution into the middle well followed by 20
drops of calcium nitrate solution.
4.
Record any changes you see and write an equation for the reaction.
5.
Reweigh the complete apparatus and record the mass.
1.
Is there a difference in the masses before and after the reaction? Explain your
answer.
Question
Part B
The reaction between marble and hydrochloric acid
1.
Place one piece of marble chip and the pipette containing the hydrochloric
acid in two of the wells in the mini well-plate.
2.
Add 10 drops of hydrochloric acid to the well containing the marble chip.
3.
Record any changes you see and write an equation for the reaction.
4.
When the reaction has finished reweigh the complete apparatus and record the
mass.
1.
How do your answers compare with those in Part A? Explain your answers.
Question