Malachite Decomposition
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Description

Mass relationships are at the heart of chemistry. A known mass of malachite, CuCO3Cu(OH)2 is heated to produce copper(II) oxide. The mass of CuO found is compared to the amount predicted.

Safety

Hot glass and hot objects can cause burns. Hot glass and cold glass look exactly alike. Hold a finger near a previously-heated object for several seconds before touching it to detect heat. Do not grasp hot objects. Do not place hot objects on balance pans. Wear goggles and apron. Know the location of the fire equipment; review use of that equipment before beginning work. Wash hands after the experiment.

Procedure

  1. Obtain a glass Pasteur pipet with a short tip. set up a ring stand with a ring and Nichrome gauze for heating. The gauze should be about 3 cm above the end of the barrel of a bunsen burner. Using a hot bunsen burner flame, heat the empty pipet to drive off water. Heat the tip for brief periods only. The tip will bend and seal if it is heated too long. Sweep the flame from the large end to the tapered end. Repeat several times. Cool to room temperature.

  2. Determine the mass of the pipet to the nearest 0.001 g.

  3. Preweigh a sample of 0.45-0.55 g of malachite on a balance paper. Using a small spatula, transfer the malachite into the pipet through its large open end. Malachite is filled in the same manner as CuSO4•5H2O.
  4. Determine the mass of the pipet plus malachite to the nearest 0.001 g.

  5. Using a non-luminous flame, heat the malachite. Begin at the open end, and heat slowly and carefully moving the burner toward the tapered end. When heated, the green malachite is transformed to brown and finally black copper(II) oxide. When the entire sample has turned jet black, sweep out the remaining water by moving the flame from the large end of the tube to the tapered end. Repeat until no water is observed.

  6. Turn off the burner and allow the pipet to cool to room temperature.
  7. Determine the mass of the pipet plus copper(II) oxide to the nearest 0.001 g.
  8. Repeat the heating and cooling steps until a constant weight is obtained.

Data Analysis

  1. Predict the mass of CuO expected to result from complete reaction of the malachite.
  2. Compare the mass obtained from that predicted.
  3. Discuss the reason for any discrepancy.

Questions

  1. Write a balanced equation for the malachite decomposition reaction.
  2. Suggest a way to demonstrate that CO2 is a product of the reaction.
  3. Predict the mass of copper(II) oxide expected from heating 0.500 g malachite.
  4. Suppose the mass actually obtained after one heating was 0.375 g. Suggest a possible reason for the problem.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 042 Malachite Decomposition

Watch the movies.

Use this sample data for the calculations and to answer the questions.

mass heated pipet - 2.913 g
mass pipet + malachite - 3.376 g
mass pipet + CuO 1st - 3.244 g
mass pipet + CuO 2nd - 3.245 g

Answer the questions.

Curriculum-

This activity fits in as soon as mass relations are studied. Excellent quantitative results are expected. Related experiments include dehydration of CuSO4•5H2O (Experiment 041) and the conversion of NaHCO3 into Na2CO3 (Experiment 043).

Safety-

Time-

Teacher Preparation: 10 minutes

Class Time: 45 minutes (Cooling periods may be used to discuss data analysis or other topics.)

Materials-

Disposal-

These solids may be wrapped in paper and discarded with ordinary solid refuse.

Lab Hints-

Data Table-

mass heated pipet- 2.913 g
mass pipet + malachite- 3.376 g
mass pipet + CuO 1st- 3.244 g
mass pipet + CuO 2nd- 3.245 g

Data Analysis-

mass pipet + malachite 3.376 g
mass heated pipet 2.913 g
mass malachite 0.463 g
mass pipet + CuO 2nd 3.245 g
mass heated pipet 2.913 g
mass CuO 0.332 g
CuCO3Cu(OH)2 --> 2 CuO + CO2 + H2O
0.463 g CuCO3Cu(OH)2 x (2 moles CuO/1 mole CuCO3Cu(OH)2)
= 0.463 g malact x (159.08 g CuO/221.12 g CuCO3Cu(OH)2)
= 0.333 g CuO
The mass of CuO found, 0.332, is within balance error of the amount predicted.

Answers-

Q1. Write a balanced equation for the malachite decomposition reaction.
A1. CuCO3Cu(OH)2 --> 2 CuO + CO2 + H2O
Q2. Suggest a way to demonstrate that CO2 is a product of the reaction.
A2. Use a tube at the end of the heated pipet to catch gas produced and bubble that gas through limewater. A white precipitate suggests that CO2 may be a product.
Q3. Predict the mass of copper(II) oxide expected from heating 0.500 g malachite.
A3. 0.500 g malact x (159.08 g CuO/221.12 g CuCO3Cu(OH)2)
= 0.360 g CuO
Q4. Suppose the mass actually obtained after one heating was 0.375 g. Suggest a possible reason for the problem.
A4. Masses above 0.360 suggest that the sample was not heated adequately. Heating to constant mass should give a result closer to the predicted result.

CoopLearn-

The data from many groups should be gathered and analyzed to get a good impression of the precision of the experiment. Assign some groups the related experiments 41 and 43.

Key Words 1-

stoichiometry, mass relations, decomposition reaction,

Elements-

Cu C


This static was created at 2:04:47 PM on Monday, April 20, 2015