CHEM101 Lecture Notes - Lecture 7: Chemical Equation, Stoichiometry, Chemical Reaction

Decomposition of Malachite

Here is what Malachite is Made Out Of

Malachite is a famous and very popular semi-precious stone. Its banded light and dark green bands are one-of-a-kind, and give it a unique ornamental quality unlike that of any other stone. The light and dark green bands are so distinctive that malachite may be one of the most easily recognized minerals by the general public. Malachite is popular in jewelry, especially Native American southwestern jewelry. The stones inlayed in silver make a nice alternative to traditional turquoise jewelry.

Malachite is a form of copper carbonate with the molecular formula Cu₂CO₃(OH)₂. When heated to around 200 �C, malachite decomposes into copper (II) oxide (CuO) and other products.

Reaction Stoichiometry

The Law of Conservation of Mass states that matter can neither be created nor destroyed. Therefore, the number of atoms of each element on the reactants side must be same as the number of atoms on the products side. This quantitative study of chemical reactions is called stoichiometry. The substances in a balanced reaction are related to each other by stoichiometric molar ratios. For example, magnesium reacts with hydrochloric acid in a 1:2 stoichiometric ratio in the balanced equation below.

Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

The molar ratios can be used as conversion factors to find the amount (in moles) of one substance given the amount of another. For example, given the number of moles of HCl, the amount of MgCl₂ can be determined as shown below.

nMgCl2=nHCl x (1mol MgCl2) / (2mol HCL)

The molar ratios are used to determine the amount in moles. However, the amount of reactant or product is often measured in grams. Mass can be converted to moles and vice versa as shown below.

n = m / (MM)

m = n x MM

where n is the number of moles, m is the mass of the product in g, and MM is its molar mass in g/mol.

To observe the Law of Conservation of Mass, you can perform different types of chemical reactions. For example, you can use a decomposition reaction to observe one compound being broken down into two or more elements or new compounds. For this reaction to take place, you must break existing bonds within the starting compound. Different sources of energy such as heat, light, or electricity may be used. Through a decomposition reaction, you can observe that the atoms in the original compound are not destroyed but are rearranged to form simpler substances, such as elements or compounds.

About This Lab

In this lab, you will test if the relationship between malachite and copper (II) oxide is stoichiometric. To do this, you will heat a known mass of malachite to produce copper (II) oxide. You will then compare the amount of copper (II) oxide obtained experimentally with the amount of copper (II) oxide expected from the balanced equation for the decomposition reaction.

1.Take a crucible from the Containers shelf and place it on the workbench.

2.Take a balance from the Instruments shelf and place it on the workbench.

3.Move the crucible onto the balance. Record the mass of the empty crucible to the nearest 0.001g in your Lab Notes.

4.Move the crucible onto the workbench.

5.Take copper (II) carbonate hydroxide from the Materials shelf and add 10 g to the crucible.

6.Move the crucible onto the balance and record the mass of the crucible containing copper (II) carbonate hydroxide in your Lab Notes.

7.Take a Bunsen burner from the Instruments shelf and place it on the workbench.

8.Move the crucible onto the Bunsen burner.

9.Double-click on the crucible to open its properties. Select Show Contents and click OK. This gives you a cut-away view of the crucible's contents.

10.Turn the Bunsen burner on to low flame by clicking the knob at the bottom once. Clicking multiple times will increase the intensity of the flame until it is turned off.

11.Watch the crucible until the malachite fully decomposes. You will see a distinct color change.

12.Move the crucible onto the workbench to cool.

13.Turn off your Bunsen burner.

14.Weigh the crucible with its contents and record the mass to the nearest 0.001 g in your Lab Notes.

Clear your station.

Lab Notes:

Numbers below refer to numbers in the above “About this Lab”

3. Empty crucible = 88 g

5. Added 10 g of Copper (II) carbonate hydroxide to crucible

6. Crucible with Copper (II) carbonate hydroxide = 98 g

11. to Black

14. after heating crucible with its contents = 95.195

Questions I have answered:

How many grams of malachite did you add to the crucible? 10g

How many grams of product were in the crucible after heating? 7.195

By how much did the mass of the crucible's contents change after heating? -2.805

The molar mass of malachite is 221.1 g/mol. How many moles of malachite were present before the reaction? 0.04523 mol

The product remaining in the crucible after heating is copper (II) oxide. The molar mass of copper (II) oxide is 79.55 g/mol. How many moles of copper (II) oxide were produced during the reaction?

0.0945 mol

Please answer the following:

Q1 a. Which of the following represents the balanced chemical equation for this reaction?

Cu2CO3(OH)2=2CuO+H2O

Cu2CO3(OH)2=CuO+CO2

Cu2CO3(OH)2=2CuO+H2O+CO2

Cu2CO3(OH)2=CuO+H2O+CO2

b. Based on the number of moles of malachite that you started with, how many grams of water were produced? The molar mass of water is 18.0153 g/mol. Choose the closest answer.

1.815 g 0.8148 g 1.630 g 1.982 g

c. Based on the number of moles of malachite that you started with, how many moles of CO₂were produced? Choose the closest answer.

1.000 mol 0.04523 mol 0.9000 mol 0.09045 mol

d. Based on the number of moles of malachite that you started with, how many grams of CO₂were produced? The molar mass of CO₂ is 44.01 g/mol. Choose the closest answer.

1.991 g 0.4401 g 0.04523 g 3.981 g

Q2a. Based on the number of grams of CO₂ and H₂O produced during the reaction, does this compensate for the observed loss of mass?

A. No, because this violates the law of conservation of mass

B. yes, this compensates for the mass lost in the reaction

C. No, because more copper oxide was produced

D. No because mass is conserved

b. You observed that the products in the crucible weigh less than the reactants that you added. How is this possible?

A. Mass was lost due to experimental error

B. The Malachite absorbed CO2 from the air

C. CO2 and H2O were released as gases

D. Mass was converted to energy

c. What kind of a reaction took place upon heating?

A. decomposition

B. single displacement

C. synthesis

D. Double displacement

d. Baking soda is used as a leavening agent in baking. When heated, baking soda undergoes a decomposition reaction to form carbon dioxide. Given the data below, which of the following reactions occurs during baking? The molar mass of baking soda is 84.007 g/mol and the molar mass of carbon dioxide is 44.01 g/mol.

Mass of empty crucible (g) = 55.000

mass of crucible with baking soda (g) = 57.000

mass of crucible and contents after the reaction (g) =56.260

A. 2NaHCO3= 2Na+2CO2 +H2O

B. 2NaHCO3 = Na2O + 2CO2 +H2O

C. NaHCO3 = NaOH +CO2

D. 2NaHCO3 = Na2CO3 + CO2 +H2O

1. (1 point) What are transferred in an oxidation-reduction reaction? *

• atoms

• electrons

• ions

• protons

2. (1 point) In the reaction of beryllium with fluorine, which atom is oxidized? *

• fluorine

• beryllium

• neither of them

• both of them

3. (1 point) Ag - - > Ag ^+ +1 e^- This equation represents the type of reaction called _____. *

• hydrolysis

• oxidation

• redox

• reduction

4. (1 point) What is the reducing agent in the following reaction? 2K + S - - > K2S *

• K

• S

• K2S

• None of the above

5. (1 point) The oxidation number of oxygen when it is in a compound other than a peroxide is ____. *

• -2

• -1

• 0

• +2

6. (1 point) What is the oxidation number for each atom in NH4F? *

• N=+3, H=-4, F=+1

• N=-3, H=+1, F=-1

• N=+3, H+1, F=-1

• N=-3, H=-1, F=+1

7. (1 point) Which atom has a change in oxidation number of +4 in the following redox reaction?K2Cr2O7 + H2O + S - - > KOH + Cr2O3 + SO2 *

• K

• Cr

• O

• S

Metal activity series for question 8

8. (1 point) Of the following metals, which ions are most easily reduced? Use the above table to answer the question. *

• magnesium

• lead

• copper

• sodium

9. (1 point) In a zinc-copper cell, Zn(s) | Zn^+2(1M) || Cu^+2(1M) | Cu(s), which electrode is positive? *

• Cu^+2

• Cu(s)

• Zn(s)

• Zn^+2

10. (1 point) How must a redox reaction that is to be used as a source of electrical energy be set up? *

• One half reaction must involve two metals.

• Two half reactions must use a metal wire electrodes.

• One half reactions must involve more than one electron.

• Two half reactions must be physically separated.

11. (1 point) In a voltaic cell at which electrode does reduction occur? *

• anode only

• cathode only

• both anode and cathode

• Either anode or cathode , depending on the metal

12. (1 point) What assures that there is no charge build up in a voltaic cell as oxidation and reduction occur? *

• one of the half cells

• the electrolyte solutions

• the moving electrons

• the salt bridge

13. (1 point) In the most common dry cell what material is the electrode that is the center rod made of? *

• copper

• graphite

• lead

• zinc

14. (1 point) To charge a lead storage battery you must ________. *

• apply a direct current to it.

• hold a magnet close to it.

• Use alternating current through it.

• increase the oxygen in it.

15. (1 point) In a hydrogen- oxygen fuel cell ______________. *

• hydrogen diffuses through the cathode.

• hydrogen and oxygen are mixed before entering the anode.

• oxygen diffuses through the anode.

• oxygen diffuses through the cathode.

16. (1 point) When electrical energy is used to cause a chemical reaction we call this process _____. *

• electronation

• electrolysis

• hydration

• reduction

17. (1 point) Which of the following is true about an electrolytic cell? *

• It changes chemical energy into electrical energy.

• It is the type of cell used in electrocution

• It uses an electric current to make a nonspontaneous reaction go.

• All of the above

18. (1 point) The products formed in the net reaction of the electrolysis of water are ____. *

• aqueous hydrogen ion and hydroxyl ion

• hydrogen gas and oxygen gas

• hydrogen gas and liquid oxygen

• water vapor

19. (1 point) What occurs in electroplating? *

• decomposition of an ionic compound

• decomposition of a salt layer

• deposition of a metal layer on a material

• deposition of a salt layer on a metal

Open response Questions : Answer questions 20 through 22 in complete sentences

20. ( 5 points) Write the equation for the reaction of calcium and oxygen ( 1 point). Write the oxidation numbers for both the reactants and the products ( 2 points) What is the reducing agent and why? ( 2 points)

21. ( 5 points) Tell how a voltaic cell works. Be sure to mention the various parts of the voltaic cell and what they have to do with how it works.

22. (4 points) What are the signs given to the electrodes in both the voltaic and electrolytic cells? What reaction (oxidation or reduction) is occurring at each?

Having trouble with thse questions. Please answer all questions correctly. Will provide a thumbs up.

1.Which of these equations is the balanced reaction between nitrous acid solution and solidmagnesium hydroxide ?

A.2NH₃(aq) + MgOH (s) Mg(NH₄)₂(aq) + 2 H₂O(l)

B.2HNO₂(aq) + Mg(OH)₂(s) Mg(NO₂)₂(aq) + 2 H₂O(l)

C.2HNO₃(aq) + Mg(OH)₂(s) Mg(NO₃)₂(aq) + 2 H₂O(l)

D.2HNO₂(aq) + MgOH (s) Mg(NO₃)₂(s) + 2H₂O(l)

2.Balance the following chemical equation:

C₃H₇OH(l) + O₂(g)CO₂(g) + H₂O(g)

a. C₃H₇OH(l) + 4 O₂(g)3 CO₂(g) + 4 H₂O(g)

b.2 C₃H₇OH(l) + 9 O₂(g)6 CO₂(g) + 8 H₂O(g)

c.C₃H₇OH(l) + 5 O₂(g)3 CO₂(g) + 4 H₂O(g)

d.C₃H₇OH(l) + O₂(g)CO₂(g) + H₂O(g)

3.Which species would be considered a spectator ion based on the following equation?

Ba²⁺ + 2NO₃^− + 2Na⁺ + SO₄^2- → BaSO₄(s) + 2NO₃^− + 2Na⁺

SO₄^2-

2NO₃^−

Ba²⁺

BaSO₄

4.Barium sulfate is often ingested to aid in the diagnosis of GI tract disorders. The equation for its production is Ba²⁺ + SO₄^2- → BaSO₄(s). What type of equation is this?

A full equation

A net ionic equation

A total ionic equation

An oxidation-reduction equation

5.Which species is being reduced in this reaction?

CuO(s) + H₂(g) Cu(s) + H₂O(g)

H₂O

CuO

Cu

H₂

6.Which of the following is an oxidizing agent?

H₂

NH₃

O₂

H₂O

7.The following chemical equation,

HC₂H₃O₂(aq) + KOH(aq) KC₂H₃O₂(aq) + H₂O(l),

is an example of a(n) _______ reaction.

a.oxidation-reduction and acid-base

b.acid-base

c.precipitation

d.oxidation-reduction and precipitation

e.oxidation-reduction

8.What substance is oxidized in the following reaction?

4HCl + MnO₂ → Cl₂ + 2H₂O + MnCl₂

a.Mn in MnO₂

b.O in MnO₂

c.Cl in HCl

d.H in HCl

9.What is the reducing agent in the following reaction?

4HCl + MnO₂ → Cl₂ + 2H₂O + MnCl₂

Cl₂

MnCl₂

HCl

MnO₂

10.All redox reactions involve oxygen.

True

False

11.The term formula weight can be used with respect to which of the following?

A.ionic compounds

B.covalent compounds

C.both a and b

D.neither a nor b

12.What is the molecular weight of ibuprofen, C₁₃H₁₈O₂?

A.206.3 amu

B.377.7 amu

C.29.0 amu

D.289.4 amu

13.The mass of an atom of ¹²C is exactly 12 atomic mass units. Which of the following describes a mole of ¹²C atoms?

A.a sample which weighs exactly 12 atomic mass units

B.a sample which contains 6.02 × 10²³ atoms of ¹²C

C.both a and b

D.neither a nor b

14.The mass of an atom of ¹²C is exactly 12 atomic mass units. Which of the following describes a mole of ¹²C atoms?

A.a sample which contains 6.02 × 10²³ atoms of ¹²C

B.a sample which weighs exactly 12.011 grams

C.both a and b

D.neither a nor b

15.What is the molar mass of water, H₂O?

A.18.02 g

B.18.02 amu

C.17.01 g

D.17.01 amu

16.What is the molar mass of glucose, C₆H₁₂O₆?

A.180.2 amu

B.180.2 g

C.60.05 amu

D.60.05 g

17.In the reaction 2H₂O → 2H₂ + O₂, 2.0 mol water will produce how many grams of O₂?

32

64

16

36

18.In the reaction 2CH₄ + O₂ → 2CO + 4H₂, how many molecules of CO will be produced by the complete reaction of 1.60 grams of CH₄?

3.01 × 10²²

2.00

12.0 × 10²³

6.02 × 10²²

19.Suppose 2.43 g of magnesium is reacted with 3.20 g of oxygen: 2Mg + O₂ → 2MgO. How many grams of MgO will be produced?

1.27

4.03

5.63

8.06

20.Which of the following conversion factors would allow one to calculate the amount of PbI₂produced from a given amount of NaI in the following unbalanced reaction?

Pb(NO₃)₂ + Nal → Pbl₂ + NaNO₃

a.

b.

c.

d.

21.The unbalanced equation for the reduction of copper(I) sulfide with oxygen is given below. What mass of copper is produced from the reduction of 7.50 g Cu₂S?

Cu₂S(s) + O₂(g) Cu(s) + SO₂(g)

18.8 g

5.99 g

1.50 g

9.39 g

2.99 g

22.How many grams of aluminum oxide are produced by the complete reaction of 19.8 g aluminum with excess oxygen?

10.5 g aluminum oxide

37.4 g aluminum oxide

49.9 g aluminum oxide

74.8 g aluminum oxide

23.If 5 grams of CO and 5 grams of O₂ are combined according to the reaction, which is the limiting reagent?

2CO + O₂ → 2CO₂

O₂

CO and O₂

CO₂

CO

24.In an industrial process, nitrogen is reacted with hydrogen to produce ammonia. The percent yield of this particular process is 26%. How much hydrogen is needed to produce 4.8 kg ammonia?

1.5 kg

7.0 kg

0.85 kg

3.3 kg

25.A percentage yield greater than 100% has been reported in a scientific paper you are assigned to read. What would be a reasonable explanation for this?

a. The product was measured more than once.

b. It must be a typographical error since yields can't exceed 100%.

c. A lack of training of the professor who published the study.

d. Small measurement errors could have caused this to occur.

26.In an industrial process, nitrogen is reacted with hydrogen to produce ammonia. The percent yield of this particular process is 24.8%. How much hydrogen is needed to produce 3.7 kg ammonia?

1.8 kg

1.2 kg

4.4 kg

12 kg

2.7 kg

27.The following reaction has a percent yield of 90.5%. How many grams of carbon dioxide are produced from 78.5 g calcium carbonate via this reaction?

2 HNO₃(aq) + CaCO₃(aq) Ca(NO₃)₂(aq) + H₂O(l) + CO₂(g)

34.5 g

21.6 g

38.1 g

31.2 g

28.When gas molecules dissolve in liquids to form a saturated system, the process is usually

exothermic.

endothermic.

independent of the nature of the liquid.

temperature independent.

29.The reaction below would be considered.

CaCO₃ + heat → CaO + CO₂

an exothermic combination.

an exothermic decomposition.

an endothermic combination.

an endothermic decomposition.

30.The combustion of one mole of propane, C₃H₈, releases 531 kcal. How much heat is released by the combustion of 1.20 moles of propane?

A.584 kcal

B.690 kcal

C.443 kcal

D.637 kcal

31.The combustion of one mole of propane, C₃H₈, releases 531 kcal. How much heat is released by the combustion of 4.40 g of propane?

A.531 kcal

B.5.31 × 10⁴ kcal

C.53.1 kcal

D.5.31 × 10³ kcal

32.Metabolism of one mole of glucose, C₆H₁₂O₆, releases 670 kcal. How much heat is released by the combustion of 0.300 moles of glucose?

A.201 kcal

B.2.41 × 10³ kcal

C.186 kcal

D.2.23 × 10³ kcal

33.Metabolism of one mole of glucose, C₆H₁₂O₆, releases 6.70 × 10² kcal. How much heat is released by the combustion of 50.0 g of glucose?

A.201 kcal

B.2.41 × 10³ kcal

C.186 kcal

D.2.23 × 10³ kcal