http://educ.queensu.ca/%7Escience/main/concept/chem/c02/C02LAVR2.htm
Prior to this activity Review:
- Molecular Formulas and Molecular Mass
- Amount of Substance in Moles (moles = mass/Molecular Mass)
- Chemical Equations ( balancing)
MASSES OF REACTANTS AND PRODUCTS
HANDOUT~Laboratory : Decomposing Malachite (Toxic)(25 mins.)
The IUPAC name for malachite is copper(II) hydroxide carbonate (common name that
appears in chemical supply catalogues - basic copper carbonate). Copper(II) hydroxide
carbonate is a green double salt with the chemical formula Cu(OH)2 CuCO3(s).It
decomposes completely when heated to 200°C according to the following chemical
equation:
Cu(OH)2 CuCO3(s) _ 2CuO(s) + CO2 + H2O
(Before starting the experiment, have students predict what the products may be. After
having figured out the products have students write out the balanced equation for the
reaction. This is then followed by having students predict how many moles of CuO will
be produce if one mole of malachite is used. Ask students to develop a hypothesis)
Materials
safety glasses, porcelain dish, hot plate, glass stirring rod, malachite (not more than 3g,
balance(+0.01g), spatula.
Problem
How is the mass of copper(II) oxide formed from the decomposition of malachite related
to the mass of malachite reacted?
(students may work individually or in pairs depending on availability of materials)
Experimental procedure
1) Determine the mass of malachite used (assign varying amounts of malachite used, ~
0.25,0.50,1.25,..,3.00g).
2) Heat sample (stir gently at the same time) until colour changes from green to black.
3) Determine the mass of the black product.
Results - record everybody s data in a table form on the blackboard or the overhead.
Name
moles
weight of
weight of
number of moles
number of
Malachite used
CuO(s) produced
of Malachite
CuO(s)
(g)
(g)
used
produced
Data Analysis (discussion in class, 20 mins.)
1) Combine results of each student or each pair of students in a graph (malachite (g) vs.
CuO (g)) on a huge piece of square line paper (visible to the class) or on a square line
overhead and solve the problem above. Also provide reference data to predict future
decomposition.
2) Each student calculate the number of moles of reactant used and also calculate the
number of moles of product formed.
3) Compare the mole ratios of malachite to CuO obtained by the students.
Questions ~ What is the theoretical mole ratio? Do the results of the students correspond
to theoretical value? Explain. How can we predict the number of moles of product
produced, given that we know the number of moles of reactants used, in any reaction?
Test Students Understanding (10 mins.)
~ From the graph of reference data, predict the mass of CuO produced by decomposing
the following masses of malachite used: 0.30g, 1.55g, and 2.70g.
~ Similarly calculate the theoretical values using the mole ratio.(MW of malachite =
221.13g/mol MW of CuO + 79.55g/mol)
Solution: malachite _ 2CuO(s) + carbon dioxide + water
mole ratio 1 : 2 : 1 : 1
mass 0.30g ?
# of moles 0.30 g = 1.36 x 10-3mol m
221.13g/mol
m = # of moles of malachite used (mol) x 2 mol
1 mol
= 1.36 x 10-3 x 2 mol = 2.72 x 10-3 mol
Thus the mass of CuO = 2.72 x 10-3 mol x 79.55 g = 0.22 g
1 mol