CHE 111 - Module 4

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0. CHE 111 - Module 4. CHAPTER 4 &amp; 5 LECTURE NOTES. 0. Stoichiometry &amp; Balancing Equations. Remember we stated in the previous chapter that stoichiometry is the study of the quantitative relationships between the amounts of reactants and products in chemical reactions.

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### CHE 111 - Module 4

CHAPTER 4 & 5

LECTURE NOTES

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Stoichiometry & Balancing Equations
• Remember we stated in the previous chapter that stoichiometry is the study of the quantitative relationships between the amounts of reactants and products in chemical reactions.
• We use BALANCED equations to understand stoichiometric relationships of the elements and compounds within a chemical reaction.

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The Balanced Equation

2Al(s) + 3Br2(l) Al2Br6(s)

2mol of Al : 3mol of Br2 : 1mol of Al2Br6

2 atoms of Al = 2 atoms of Al

6 atoms of Br = 6 atoms of Br

The number of the same atom of each element must be equal on each side of the equation.

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A Closer Look at the Equation

2Al(s) + 3Br2(l) Al2Br6(s)

• The chemicals on the left are the reactants and the right are the products.
• The coefficient in front of the chemical denotes the stoichiometric relationship.
• The numerical subscriptrepresents the number of atoms present in the molecule.
• The letter subscripted denotes the phase of matter.
For example the following is balanced.

CH4 + 2O2 CO2 + 2H2O

Try to balance the following:

Fe2S3 + O2 Fe2O3 + S

Al+ H2SO4 Al2(SO4)3 + H2

Ca + Al2Br6 CaBr2 + Al

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Balancing Equations
Balanced Equations

• 2Fe2S3 + 3O2 2Fe2O3 + 6S
• 2Al+ 3H2SO4 Al2(SO4)3 + 3H2
• 3Ca + Al2Br6 3CaBr2 + 2Al

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Types of Reactions
• Combination Reactions
• Decomposition Reactions
• Displacement (Single-Replacement) Reactions
• Metathesis (Double-Replacement) Reactions
• Combustion Reactions

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Combination Reactions
• A combination reaction is a reaction where two substances chemically combine to form another substance.

A + B AB

2Na(s) + Cl2(g) 2NaCl(s)

P4(s) + 6Cl2(g) 4PCl3(s)

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Decomposition Reaction
• A decomposition reaction is when a single compound decomposes into two or more other substances.

AB A + B

2KClO3(s) 2KCl(s) + 3O2(g)

2NO2(g) 2NO(g) + O2(g)

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Displacement Reaction
• A displacement (single replacement) reaction is a reaction where one element displaces another element.

A + BC B + AC

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

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Metathesis Reaction
• A metathesis (double replacement) reactionis a reaction where two compounds switch cations to form two new compounds.

A+B- + C+D- A+D- + C+B-

CaCl2(aq) + Na2CO3(aq) CaCO3(s) + 2NaCl(aq)

AgNO3(aq) + KCl(aq) ???

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SOLUBILITY
• Solubility – the amount of a substance that can be dissolved in a given quantity of solvent (like water) at a specific temperature
• Unsaturated – amount of substance less than saturated
• Saturated – the exact amount at solubility
• Supersaturated – excess amount of substance

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How Solubility Influences Rxn
• When a substance is soluble in water, it will appear with a subscript of (aq) meaning that the substance is broken up into it’s ions incorporated into the water lattice.
• When a substance is insoluble in water, it will be written with a subscript of (s), (l), or (g) and will precipitate out of solution.
Solubility of Ionic Compounds in Water

Soluble CompoundsExceptions

Insolubility of Ionic Compounds in Water

Insoluble CompoundsExceptions

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A Look at Metathesis Again
• Looking back at slide 10 to the first reaction: when the cations rearranged, the CaCO3 being insoluble by our definition is recorded as CaCO3 (s). The CaCO3 would precipitate out of solution as a solid.
• Looking at AgNO3(aq) + KCl(aq) ?, we can rearrange the cations and conclude that the AgCl is a solid and will precipitate out of solution

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Types of Metathesis Reactions
• Three classifications of metathesis reactions
• Precipitation reaction - formation of a solid

Pb(NO3)2(aq) + Na2CO3(aq) PbCO3(s) + 2NaNO3(aq)

• Neutralization reaction - formation of water

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

• Gas formation reaction - CO2, H2S, SOx, & NOx are typically formed

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Precipitation Reactions
• A solid precipitate is produced in the rearrangement of cations as follows:

Pb(NO3)2(aq) + Na2CO3(aq) PbCO3(s) + 2NaNO3(aq)

• The Ionic Equation is expressed as:

Pb+2 + 2NO3- + 2Na+ + CO3-2 PbCO3(s) + 2Na+ + 2NO3-

• After neglecting the spectator ions, the net ionic equation will look like:

Pb+2(aq) + CO3-2(aq) PbCO3(s)

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Reviewing Ionic Compounds

Ca+2 + 2Cl- CaCl2

Each ion comes together based on charge to form an overall neutral ionic compound.

3Ca+2 + 2PO4-3 Ca3(PO4)2

The cation and the polyatomic ion come together based on charge to form an overall neutral ionic compound.

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Net Ionic Equations (NIE)
• If you were given the reactants Ca(NO3)2 and Na3PO4 you should be able to predict the precipitate and write a balance equation, the ionic equation, and the net ionic equation (NIE) for this reaction.
• The NIE for these reactants is as follows:

3Ca+2(aq) + 2PO4-3(aq) Ca3(PO4)2(s)

carbonate ion CO3-2

sulfate ion SO4-2

sulfite ion SO3-2

hydroxide OH-

phosphate PO4-3

permanganate MnO4-

chromate CrO4-2

dichromate Cr2O7-2

ammonium NH4+

oxalate C2O4-2

bicarbonate HCO3-

cyanide ion CN-

acetate C2H3O3-

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Common Polyatomic Ions

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Neutralization Reaction
• A neutralization reaction is a reaction that occurs between an acid and a base with the production of a salt and water.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

acid base salt water

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Gas Formation Reaction
• A gas formationreaction is a metathesis reaction that generates a gas as a product.
• Metal carbonates or bicarbonates + acid
• Metal sulfides + acid
• Metal sulfites + acid
• Ammonium salts and strong base

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Metal Carbonates
• Metal carbonates or bicarbonates when combined with an acid form salt, water and carbon dioxide gas.

Na2CO3(aq)+ 2HCl(aq) 2NaCl(aq)+ H2O(l)+ CO2(g)

• Where CO2 gas is given off

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Metal Sulfides
• Metal sulfides when combined with an acid form salt and hydrogen sulfide gas.

Na2S(aq)+ 2HCl(aq) 2NaCl(aq)+ H2S(g)

• Where H2S gas is given off

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Metal Sulfites
• Metal sulfites when combined with an acid form salt, water, and sulfur dioxide gas.

Na2SO3(aq)+ 2HCl(aq) 2NaCl(aq)+ H2O(l)+ SO2(g)

• Where SO2 gas is given off

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Ammonium Salts
• Ammonium salts when combined with a base produce salt, water and ammonia.

NH4Cl (aq)+ NaOH(aq) NaCl(aq)+ H2O(l)+ NH3(g)

• Where ammonia gas is given off

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Combustion Reactions
• A combustion reaction is a reaction with molecular oxygen to form products in which all elements are combined with oxygen.

CH4 + 2O2  CO2 + 2H2O

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Limiting Reactants
• One of the reactants is in limited supply and thus restricts the amount of product formed.
• Think of it as: If you wanted to bake a batch of peanut butter cookies and the recipe calls for 1 cup of peanut butter and all you have is ½ a cup, even though you have all the other ingredients, you can at most make ½ a batch of cookies.

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Limiting Reactants (cont.)
• Consider the combustion reaction:

CH4 + 2O2 CO2 + 2H2O

• How much CO2 can be produced if you have 0.13g of methane and 0.45g of O2?

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Percent Yield
• The maximum amount of product that can be obtained from a chemical reaction is the theoretical yield.
• The actual amount produced in a chemical process is the actual yield.
• The percent yield is equal to the actual yield divided by the theoretical yield times 100%.

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Redox Reactions
• Oxidation of an element takes place when electrons are lost from the valence shell of the element.
• Reduction of an element takes place when electrons are added to the valence shell of the element.
• Redox reactions show the transfer of electrons that takes place during oxidation and reduction.

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Redox Reactions (cont.)
• All oxidation and reduction reactions involve transfer of electrons between substances.
• View CD-ROM screen 5.12
• Ag+ accepts electrons for Cu and is reduced to Ag and Cu loses electrons to Ag+ and is oxidized to Cu+2 in the following redox rxn:

2Ag+(aq) + Cu(s) 2Ag(s) + Cu+2(aq)

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Redox Reactions (cont.)
• The oxidation half reaction is :

Cu(s) – 2e- Cu+2(aq)

• The reduction half reaction is:

2Ag+(aq) + 2e- 2Ag(s)

• Cu is called the reducing agent because it caused Ag+ to be reduced; and Ag+ is called the oxidizing agent because it caused Cu to be oxidized.

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Determining Oxidation Numbers
• Each atom in a pure element has an oxidation number of zero.
• For monoatomic ions, the ox. number is equal to it’s ionic charge.
• F is always –1, other halogens are –1 as well except with oxygen or fluorine.
• The ox. number for H is +1 except with hydrides (CaH2).and O is –2 except with peroxides (H2O2).
• The  ox.# must = 0 for a compound or = to the overall charge of polyatomic ion being considered.

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Balancing Redox Reactions
• We can use the balance of electrons transferred in a redox reaction to help us balance the overall equation.
• Consider the unbalanced equation:

Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)

• The balanced equation takes into consideration the oxidation of the Zn and the reduction of the H+.
• Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

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Molarity

Molarity = Moles of Solute

Liters of Total Solution

• Symbol for molarity is M
• Units are moles/Liter
Solution Preparation
• To prepare a 1.0M solution of NaCl, you would determine how many grams of NaCl is contained in 1.0 moles of NaCl and then dissolve that amount in a 1.0L volumetric flask. You would then qs with distilled H2O.
• 1.0M NaCl = 1 mole (or 58.44g) NaCl

1.0L of solution

• How much NaCl would you use to make a 0.1M solution of NaCl?

As 1/10 of a mole = 5.844g NaCl, you would dissolve

0.1mole (5.844g) of NaCl in 1.0L of solution.

Acids

An acid is defined as follows:

• Arrhenius – releases H+ when dissolved in H2O
• Bronsted-Lowrey – a substance that can donate a proton to another substance
• Lewis – a substance that can accept a pair of electrons from another atom to form a new bond
Bases

A base is defined as follows:

• Arrhenius – releases OH- when dissolved in H2O
• Bronsted-Lowrey – a substance that can accept a proton from another substance
• Lewis – a substance that can donate a pair of electrons to another atom to form a new bond
pH and Concentrations of Acids and Bases

pH = -log [H+]

1 – acidic – 7 – basic – 14

When dealing with [H+] less than 0.1M (pH=1), we use activity coefficients instead of pH.

pH of Household Items
• pH of vinegar = 2.80
• pH of soda = 2.90
• pH of orange juice = 3.80
• pH of pure water = 7.00
• pH of blood = 7.40
• pH of ammonia = 11.00
• pH of oven cleaner = 11.7
Titration
• A method for quantitative analysis of a substance by essentially complete reaction in solution with a measured quantity of a reagent of known concentration.
• Often used in redox reactions
• Many redox reactions go rapidly to completion in aqueous media to determine the equivalency point.
• Typically used for neutralization reactions.
• Acid is titrated with a base using an indicator to determine the equivalency point of the neutralization reactions.