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CHAPTER 4 CHEMICAL REACTIONS: MAKING CHEMICALS SAFELY WITHOUT DAMAGING THE ENVIRONMENT. From Green Chemistry and the Ten Commandments of Sustainability , Stanley E. Manahan, ChemChar Research, Inc., 2006 email@example.com. 4.1. DESCRIBING WHAT HAPPENS WITH CHEMICAL EQUATIONS.
CHEMICAL REACTIONS: MAKING CHEMICALS SAFELY WITHOUT DAMAGING THE ENVIRONMENT
From Green Chemistry and the Ten Commandments of Sustainability, Stanley E. Manahan, ChemChar Research, Inc., 2006
In our own bodies:
A chemical reaction occurs: Glucose sugar reacts with oxygen to give carbon dioxide, water, and energy
Represented by a chemical equation:
C6H12O6 + 6O2 6CO2 + 6H2O (+ energy) (4.1.1)
Reactants: 6 C, 12 H, Products: 6 C, 12 H,
6 + 12 = 18 O 12 + 6 = 18 O
States of matter of reaction participants:
CaCO3(s) + 2HCl(aq)
CO2(g) + CaCl2(aq) + H2O(l) (4.1.2)
(s) for solid, (aq) for a substance in solution, (g) for gas, and (l) for liquid
denotes a reversible reaction
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) (4.1.3)
for heat added
A balanced chemical equation shows the same number of each kind of atom on both sides of the equation.
Balancing a chemical equation (different example from one shown in book):
MnO2 + C2H6 Mn + CO + H2O
There will have to be at least 2 C atoms and 6 H atoms on the right:
MnO2 + C2H6 Mn + 2CO + 3H2O
This gives 5 O atoms on the right, so the number of O atoms on the left must be a multiple of 5:
5MnO2 + C2H6 Mn + 2CO + 3H2O
This means that there must be 10 O atoms on the right, so multiply CO and H2O on the right by 2:
5MnO2 + C2H6 Mn + 4CO + 6H2O
There must be 4 C atoms and 12 H atoms on the left:
5MnO2 + 2C2H6 Mn + 4CO + 6H2O
The products must have 5 Mn atoms:
5MnO2 + 2C2H6 5Mn + 4CO + 6H2O
The equation should be balanced, check the results:
Reactants: 5 Mn, 10 O, 4 C, 12 H
Products: 5 Mn, 4 + 6 = 10 O, 4 C, 12 H
The following reaction occurs:
Fe(s) + H2SO4(aq) H2(g) + FeSO4(aq) (4.3.1)
The following reaction does not occur:
Cu(s) + H2SO4(aq) H2(g) + CuSO4(aq) (4.3.2)
Alternative way to make copper sulfate,CuSO4 :
2Cu(s) + O2(g) 2CuO(s) (4.3.4)
CuO(s) + H2SO4(aq) CuSO4(aq) + H2O(aq) (4.3.5)
Alternative reactions pathways for maximum safety, minimum byproduct, and utilization of readily available materials.
Two ways to prepare iron (II) sulfate, FeSO4.
Fe(s) + H2SO4(aq) H2(g) + FeSO4(aq) (4.3.1)
Could use scrap iron and waste sulfuric acid
Generates elemental H2, which is explosive
But H2 could be used in a fuel cell
FeO(s) + H2SO4(aq) FeSO4(aq) + H2O(aq) (4.3.6)
No dangerous H2
Could also use scrap iron and waste sulfuric acid
Yield is the percentage of the degree to which a chemical reaction or synthesis goes to completion.
Atom economy is defined as the fraction of reactants that go into final products.
Consider yield and atom economy for the preparation of HCl gas
By reaction of sodium chloride with sulfuric acid accompanied by heating to drive off HCl gas:
2NaCl(s) + H2SO4(l) 2HCl(g) + Na2SO4(s) (4.4.1)
When all of the NaCl and H2SO4 react, there is 100% yield.
Byproduct Na2SO4 gives less than 100% atom economy.
Given the atomic masses H 1.0, Cl 35.5, Na 23.0, and O 16.0 gives the following:
Mass of desired product = 2 (1.0 + 35.5) = 73.0 (4.4.3)
Total mass product = 2 (1.0 + 35.5) + (2 23.0 + 32.0 + 4 16.0) = 215 (4.4.4)
Alternatively, the following occurs with 100% atom economy:
H2(g) + Cl2(g) 2HCl(g) (4.4.2)
Carbon monoxide burns in air:
2CO + O2 2CO2 (4.5.1)
CO is generated by automobile engines and is an undesirable air pollutant.
CO is eliminated by reaction with oxygen over an automotive exhaust catalytic converter.
The metals on the surface of the catalytic converter act as a catalyst to enable the above reaction to occur efficiently.
A catalyst speeds up a chemical reaction without itself being consumed.
Enzymes are specialized proteins that act as biological catalysts.
For example, aerobic respiration, in which glucose reacts with oxygen in living organisms using enzyme catalysts and producing energy.
C6H12O6 + O2 6CO2 + 6H2O + energy (4.5.2)
Enzymes participate in many life processes including
•Protein synthesis • Repair damaged DNA •Detoxification
Chemical kinetics deals with rates of chemical reactions.
Combination reaction or addition reaction in which two substances come together to form a new substance
C + O2 CO2
CaO + SO2 CaSO3
Addition reactions are 100% atom economical.
Decomposition reaction in which a compound decomposes to two or more products.
Example is electrolysis of water to produce elemental hydrogen and oxygen by passing an electrical current through water made electrically conducting with a dissolved salt, such as Na2SO4
2H2O(l)2H2(g) + O2(g) (4.6.3)
Can be 100% atom economical, but may be less than 100% because of side reactions.
Decomposition reaction to make sodium carbonate, Na2CO3, from sodium bicarbonate, NaHCO3
2NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(g) (4.6.4)
to produce sodium carbonate, Na2CO3, commonly used as an industrial chemical to treat water, in cleaning solutions, and as an ingredient of glass.
Substitution or replacement reaction is one such as the reaction of iron and sulfuric acid,
Fe(s) + H2SO4(aq) H2(g) + FeSO4(aq) (4.3.1)
This reaction is also evolution of a gas.
A double replacement or metathesis reaction, in which two compounds trade ions or other groups.
H2SO4(aq) + Ca(OH)2(aq)CaSO4(s) + 2H2O(l) (4.6.5)
This is also a neutralization reaction in which an acid and a base react to produce water and a salt.
Precipitation reactions produce precipitates of insoluble substance that come out of water solution:
CaCl2(aq) + Na2CO3(aq)CaCO3(s) + 2NaCl(aq) (4.6.5)
Calcium removal from water is water softening.
• Calcium can cause scale in pipes
• Calcium precipitates soap in a useless solid form
Oxidation-reduction reactions, frequently called redox reactions
Use of oxidation to describe the reaction of a substance with oxygen:
2Ca + O22CaO
Calcium metal is oxidized.
Elemental oxygen is reduced to produce the oxide ion, O2- in CaO.
When a chemical species loses electrons in a chemical reaction it is oxidized and when a species gains electrons it is reduced.
Whenever a chemical species combines with elemental hydrogen, it is reduced.
FeO + H2Fe +H2O(4.7.2)
In this case the Fe in FeO is reduced to iron metal and the hydrogen in elemental H2 is oxidized to H2O.
When elemental oxygen reacts to produce chemically combined oxygen, it is acting as an oxidizing agent and is reduced.
Oxidation-reduction in photosynthesis,
6CO2 + 6H2O + h C6H12O6 + 6O2 (4.7.3)
h represents light energy
Oxidation-reduction in respiration
C6H12O6 + 6O2 6CO2 + 6H2O + energy(4.1.1)
Oxidation of fossil fuel
CH4 + 2O2 CO2 + 2H2O + energy(4.7.4)
Electrolysis of Water in Which H2O is Oxidized at One Electrode and Reduced at the Other
Oxidation of fossil fuels and other materials in producing energy
Hydrogen and carbon in hydrocarbons are in reduced form, such as in ethane, C2H6.
Many raw materials are partially oxidized hydrocarbons, such as ethanol, C2H6O, which can be made by:
2C2H6 + O2 2C2H6O(4.7.5)
Alternate biosynthesis of ethanol by fermentation of carbohydrates:
C6H12O6 2C2H6O + 2CO2 (4.7.6)
Formula mass: The sum of the atomic masses of all the atoms in a formula unit of a compound.
Molar mass: Where X is the formula mass, the molar mass is X grams of an element or compound, that is, the mass in grams of 1 mole of the element or compound.
2C2H6 + 7O2 4CO2 + 6H2O (4.8.1)
In terms of moles, 2 moles of C2H6 react with 7 moles of O2 to yield 4 moles of CO2 and 6 moles of H2O.
Given the atomic masses H 1.0, C 12.0, and O 16.0 the molar mass of C2H6 is 30.0 g/mol, that of O2 32.0 g/mol, that of CO2 is 44.0 g/mol, and that of H2O = 18.0 g/mol.
For the reaction: 2C2H6 + 7O2 4CO2 + 6H2O
In terms of the minimum whole number of moles reacting and produced
• 2 moles of C2H6 with a mass of 2 30.0 g = 60.0 g of C2H6
• 7 moles of O2 with a mass of 7 32.0 g = 224 g of O2
• 4 moles of CO2 with a mass of 4 44.0 g = 176 g of CO2
• 6 moles of H2O with a mass of 6 18.0 g = 108 g of H2O
The total mass of reactants is
60.0 g of C2H6 + 224 g of O2 = 284.0 g of reactants
and the total mass of products is
176 g of CO2 + 108 g of H2O = 284 g of products
The calculation of quantities of materials involved in chemical reactions is addressed by stoichiometry.
Based upon the law of conservation of mass which states that the total mass of reactants in a chemical reaction equals the total mass of products.
Holds true because matter is neither created nor destroyed in chemical reactions.
The mole ratio method of stoichiometric calculations is based upon the fact that the relative numbers of moles of reactants and products remain the same regardless of the total quantity of reaction.
2C2H6 + 7O2 4CO2 + 6H2O (4.9.1)
At the mole level, this chemical equation states that 2 moles C2H6 react with 7 moles of O2 to produce 4 moles of CO2 and 6 moles of H2O.
For 10 times as much material, 20 moles C2H6 react with 70 moles of O2 to produce 40 moles of CO2 and 60 moles of H2O.
Suppose that it is given that 18.0 g of C2H6 react. What is the mass of O2 that will react with this amount of C2H6? What mass of CO2 is produced? What mass of H2O is produced?
To solve this problem, the following mole ratios are used:
To solve for the mass of O2 reacting use the following steps:
A. Mass of C2H2 reacting
B. Convert to moles of C2H2
C. Convert to moles of O2
D. Convert to mass of O2
Given the molar mass of C2H6 as 30.0 g/mol, the molar mass of O2 (18.0 g/mol), and the mole ratio relating moles of O2 to moles of C2H6,
The masses of CO2 and H2O produced are calculated as follows:
Total mass of reactants, 18.0 g C2H6 + 67.2 g O2 = 85.2 g
Total mass of products, 52.8 g CO2 + 32.4 g H2O = 85.2 g
One of the reactants is almost always a limiting reactant.
Example: Reaction of 100 g of elemental zinc (atomic mass 65.4) and 100 g of elemental sulfur (atomic mass 32.0) are mixed and heated undergoing the following reaction:
Zn + S ZnS (4.9.1)
What mass of ZnS, formula mass 97.4 g/mol, is produced?
If 100 g of zinc react completely, the mass of S reacting and the mass of ZnS produced would be given by the following calculations:
Only 48.9 g of the 100 g of S react, so zinc is the limiting reactant.
The mass of Zn required to react with 100 g of sulfur would be 204 g of Zn, but only 100 g of Zn is available.
The mass of product calculated from the mass of limiting reactant in a chemical reaction is called the stoichiometric yield of a chemical reaction.
By measuring the actual mass of a product produced in a chemical reaction and comparing it to the mass predicted from the stoichiometric yield it is possible to calculate the percent yield.
Suppose that a water solution containing 25.0 g of CaCl2 was mixed with a solution of sodium sulfate,
CaCl2(aq) + Na2SO4(aq) CaSO4(s) + 2NaCl(aq) (4.10.2)
Removed by filtration and dried, the precipitate was found to have a mass of 28.3 g, the measured yield. What was the percent yield?
The stoichiometric yield of CaSO4 calculated by the mole ratio method is 30.6 g CaSO4
The percent yield is calculated by the following
If the molar concentration of a solution is known, the number of moles may be measured by the volume of the solution (see measuring glassware below:
Titration uses a buret to measure the volume of a solution with a known concentration of a reagent required to react exactly with another substance in solution
Reagent added from the buret until a measured end point is reached indicating that the reaction is complete
• Volume used with stoichiometry to measure amount of substance
The pertinent equations relating to solution concentration and stoichiometry are the following:
Consider a sample consisting of basic lime, Ca(OH)2, molar mass 74.1 g/mol, and dirt with a total sample mass of 1.26 g. Using titration with a standard acid solution it is possible to determine the mass of basic Ca(OH)2 in the solution and from that calculate the percentage of Ca(OH)2 in the sample. Assume that the solid sample is placed in water and titrated with 0.112 mol/L standard HCl (concentration designated MHCl), a volume of 42.2 mL (0.0422 L) of the acid being required to reach the end point. The dirt does not react with HCl, but the Ca(OH)2 reacts as follows with the mole ratio given below
Ca(OH)2 + 2HCl CaCl2 + 2H2O
1 mol Ca(OH)2
2 mol HCl
At the end point the number of moles of HCl can be calculated by
MolHCl = LitersHCl x MHCl
The calculation of the percentage of Ca(OH)2 in the sample is given by the following:
The Solvay process consists of saturating a sodium chloride solution (brine) with ammonia gas (NH3), then with carbon dioxide, then cooling it to precipitate solid NaHCO3:
NaCl + NH3 + CO2 + H2O NaHCO3(s) + NH4Cl
The sodium bicarbonate product is heated to produce sodium carbonate, Na2CO3, a chemical with many industrial uses:
2NaHCO3 + heat Na2CO3 + H2O(g) + CO2(g)
The CO2 from this reaction is recirculated back to the first reaction above.
Ammonia is made by the following reaction, which requires heat, high pressures and a catalyst:
3H2 + N2 2NH3
Ammonia is reclaimed from the reaction solution by adding lime, Ca(OH)2, made from heating limestone and reacting the CaO product with water:
CaCO3 + heat CaO + CO2 (calcination of limestone)
CaO + H2O Ca(OH)2
When Ca(OH)2 is added to the spent solution from which NaHCO3 has precipitated, the ammonia is evolved and reclaimed:
Ca(OH)2 (s) + 2NH4Cl(aq) 2NH3(g)+ CaCl2(aq) + 2H2O(l)
Although this reaction reclaims ammonia, it generates large quantities of calcium, chloride, CaCl2, which has few commercial uses and tends to accumulate as waste
The overall reaction for the Solvay process is
CaCO3 + 2NaCl Na2CO3 + CaCl2
from which the stoichiometric atom economy 48.8% (mass of Na2CO3 product divided by total mass of reactants)
In practice, the yield is less due to incomplete precipitation of NaHCO3 and other factors
It is green in that
It uses inexpensive, abundantly available raw materials in the form of NaCl brine and limestone (CaCO3). A significant amount of NH3 is required to initiate the process with relatively small quantities to keep it going.
It maximizes recycle of two major reactants, ammonia and carbon dioxide. The calcination of limestone provides ample carbon dioxide to make up for inevitable losses from the process, but some additional ammonia has to be added to compensate for any leakage.
The Solvay process is not green because it requires extraction of non-renewable NaCl and CaCO3 (although they are abundant), generates excess, potentially waste CaCl2, uses relatively large amounts of energy, has a relatively low atom economy
In the U.S. and some other countries Na2CO3.NaHCO3.2H2O (trona) is mined and the Solvay process is not used.