Chapter 4: Chemical Reactions & Solution Stoichiometry

•OWL Deadline: 15-October-12 (7:50 AM)•Exam #2 on 15-Oct with Dr. Vining.•Types of Reactions•Solubility Rules•Aqueous Reactions and Stoichiometry•Oxidation-Reduction Reactions

Adding lead(II) nitrate to potassium iodide gives a yellow precipitate of lead(II) iodide. Precipitation reactions are important in biological organisms— for example, in the production of bone (calcium phosphate) and seashell (calcium carbonate).

Combination ReactionA reaction in which two or more substances combine to form a third substance.

For example: 2Na(s) + Cl2(g) 2NaCl(s)

Decomposition ReactionA reaction in which a single compound reacts to give two or more substances.

For example:2HgO(s) 2Hg(l) + O2(g)

Let’s look at some reactions:

Sodium metal reacts with oxygen gas to produce sodium oxide.-What kind of reaction ?-Write the reaction (equation)-Balance the equation

Aluminum Carbonate is heated and forms aluminum oxide and carbon dioxide gas.-What kind of reaction ?-Write the reaction (equation)-Balance the equation

Displacement ReactionA reaction in which an element reacts with a compound, displacing another element from it.

For example:Zn(s) + 2HCl(aq)

H2(g) + ZnCl2(aq)

Acid-Base Reaction (usually a type of Double Displacement Reaction that happens in solution)An acid that reacts with a base will usually for a salt and water.

For example:CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + HOH (l)

Precipitation Reaction (usually a type of Double Displacement Reaction in Solution)A reaction in which an or ion reacts with a compound or ion, displacing another element from it. A solid precipitate forms from the reactants in solution.

For example:MgCl2 (aq) + 2 AgNO3 (aq)

Mg(NO3)2 (aq) + 2 AgCl(s)

Gas Forming Reaction (usually a type of Double Displacement Reaction in Solution)A reaction in which an or ion reacts with a compound or ion, displacing another element from it. A solid precipitate forms from the reactants in solution.

For example:Na2CO3(aq) + 2HCl(aq) 2NaCl(aq) + H2O(l) + CO2(g)

Combustion ReactionA reaction in which a substance reacts with oxygen, usually with the rapid release of heat to produce a flame.

For example:4Fe(s) + 3O2(g) 2Fe2O3(s)

Let’s look at some reactions:

Calcium carbonate reacts with hydrochloric acid to produce water, carbon dioxide and calcium chloride .-What kind of reaction ?-Write the reaction (equation)-Balance the equation

A solution of barium chloride reacts with a solution of sodium sulfate. Barium sulfate precipitates. Sodium chloride remains in solution. -What kind of reaction ?-Write the reaction (equation)-Balance the equation

What happens in aqueous solution?Molecules and ions

become solvated.◦ The solvent is the larger

component, usually a liquid

◦ The solute is the minor component, usually starting as a solid or a liquid (but it can be a gas)

When water is the solvent, then the solute becomes hydrated

Nonelectrolytes can dissolve in water, but do not conduct charge ◦ They don’t form ions◦ CH3OH (l) CH3OH (aq) methanol dissolves in water

Where is this reaction important for your car?

Weak electrolytes dissolve, but only partially dissociate into their component ions. The solutions are weak conductors◦ CH3COOH (l) + H2O (l) CH3COO- (aq) + H3O+ (aq)

Strong electrolytes dissolve and completely dissociate into their component ions. The solutions are good conductors◦ NaCl (s) Na+ (aq) + Cl- (aq)

Non-, Strong- or Weak-Electrolytes?

Solubility of Ionic CompoundsSoluble: to dissolve. A solute that is soluble, dissolves in the solvent. ◦ Say that 10 times really

fast. There are specific

rules for predicting the solubility of ionic compounds.

When a solid forms from a solution, it is called a precipitate

Solubility Rules (know them)

1. Group IA and ammonium compounds are soluble.2. Acetates and nitrates are soluble.3. Most chlorides, bromides, and iodides are

soluble.Exceptions: AgCl, Hg2Cl2, PbCl2; AgBr, Hg2Br2, HgBr2, PbBr2; AgI, Hg2I2, HgI2, PbI2

4. Most sulfates are soluble.Exceptions: CaSO4, SrSO4, BaSO4, Ag2SO4, Hg2SO4, PbSO4

Reactions in Aqueous SolutionAcid-Base Reactions

◦ An acid + base produces a salt + water

◦ “Salt” refers to an ionic compound.

Acids produce H+ ions when dissolved in water. Bases increase the amount of OH- ions when dissolved in water.◦ Simplest definitions◦ What else do we know?

Taste pH Slippery?

Strong acids are strong electrolytes

Weak acids are weak electrolyotes

Acids can also be:◦ Monoprotic (one

proton produced)◦ Diprotic (two

protons produced)◦ Triprotic….

Let’s look at some reactions….Autoprotolysis of waterNitric acid plus sodium hydroxide

(SA + SB)Oxalic acid plus sodium hydroxide

(WA + SB)Acetic acid plus water (WA + WB)Acetic acid plus lithium hydroxide

(WA + SB)Ammonia plus water (WB + WA)

Precipitation ReactionsYou MUST know solubility rulesOrdinarily double displacement reactions

3 CaCl2 (aq) + 2 Na3PO4 (aq) Ca3(PO4)2 (s) + 6 NaCl (aq)If the polyatomic ions are constant, you can balance them as “groups”

Ammonium chloride + lead (II) nitrate solutions are mixed.◦ Does a reaction occur? ◦ If so, is there a precipitate?◦ Write and balance the chemical equation

Lithium phosphate and potassium iodide are mixed.

Net Ionic Equations (NIE)In many reactions, there are important

compounds/ions and then there are spectators◦ Spectator ions are present in the solution, but don’t

actually react or change what they are associated with◦ They are sometimes an ion in a strong electrolyte

Net Ionic Equations eliminate spectators

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

Ag+(aq) + NO3

-(aq) + Na+

(aq) + Cl-(aq) AgCl(s) + Na+

(aq) + NO3(aq)

Ag+(aq) + Cl-(aq) AgCl(s)

Potassium iodide reacts with lead (II) nitrate.Does a reaction take place?Write the balanced equation.Identify strong electrolytes, soluble

compounds.Identify spectator ions.Write Net Ionic Equation.

Net Ionic Equations & Acid/Base ReactionsAcids and bases are strong or weak

electrolytes.

Lithium hydroxide reacts with phosphoric acid to produce lithium phosphate and water.

Net Ionic Equations & Gas Forming Reactions

Even though non-electrolytes are a big part of the reaction (the gas), there are still spectators.

Calcium carbonate reacts with hydrochloric acid to produce carbon dioxide, water and calcium chloride.

Oxidation Reduction (REDOX) ReactionsNow we start looking at when

electrons move from one atom to another ◦Charges Change

Oxidation: when electrons are lost

Reduction: when electrons are gained◦Because the charge is reduced

Half-ReactionsHalf-reactions are like mini net

ionic equations.◦Oxidation half-reaction◦Reduction half-reaction

They are used to identify the two processes and balance the overall equation.◦Plus, lots of times a chemist is only

worried about one half of the reaction.

Oxidation Numbers (charges)The charge on some ion or species

◦Cations (+) or anions (-)◦Now we are looking at:

Elements becoming ions Ions changing their charge

2 HCl(aq) + Zn(s) ZnCl2(aq) + H2(g)Write the half-reactionsIdentify oxidizing and reducing agents

Chromate, dichromate, sulfite, sulfate, nitrite, nitrate. What are the oxidation numbers in each?

What is the oxidation number of Cr in dichromate, Cr2O7

2-?

Cr O2(oxidation number of Cr) + 7(-2) = -22(oxidation number of Cr) + (-14) = -22(oxidation number of Cr) = -12Oxidation number of Cr = +6

Solution Stoichiometry……

In the titration above, the indicator changes color to indicate when the reaction is just complete.

Molarity (M): moles solute/liter of solution◦The solution volume is the FINAL volume

(includes everything.◦Molarity changes with temperature, which can

be a problem because solutions expand and contract.

◦One of the more common units we will use

Molality (m): moles of solute / kg of solvent◦The mass of a solvent used is constant,

regardless of temperature◦A significant unit in geochemistry, especially

when dealing with various melted rocks (hot boiling magma)….

How much sodium chloride do I need to make a 1.0 liter solution that is 0.35 M NaCl?

A solution is made from 45.0 g of Calcium Nitrate dissolved in 2.0 liters of water. What is the concentration (M) of each ion in the solution?

Describe how you would make a 2.0 molar solution of phosphoric acid (500 mL).

Stoichiometry Calculations

250 mL of a 1.0 M calcium chloride solution is mixed with a solution of sodium phosphate (in excess). After the reaction is complete, the resulting volume is 1.0 liters.◦Write the balanced equation◦Identify any precipitates◦Calculate the amount of any

precipitate that is formed◦What is the resulting concentration

of calcium ion in solution?

Acid Base Titrations

The titrant is added to the buret.The equivalence point is when

the moles of titrant are exactly the amount needed to react completely with the analyte

An indicator is a substance that is used to show the equivalence point (a dye of some sort)◦The indicator changes color at the

end point-this is sometimes a little different from the equivalence point.

In the titration above, the indicator changes color to indicate when the reaction is just complete.

2.00 grams of oxalic acid dihydrate are titrated to the endpoint with 25.0 mL of sodium hydroxide. What is the molarity of the NaOH solution?

The standardized NaOH solution above is used to titrate 10.0 mL of sulfuric acid.◦What is the molarity of the sulfuric acid

solution◦How many grams of sulfuric acid were in

the solution?