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Evidence for a Chemical Reaction Chemical reactions give a visual signal.

Examples: Rusted steel & bleached hair,

Clues that a Chemical Reaction (rxn) has Occurred

1. Color changes

2. Solid forms

3. Bubbles form

4. Heat is produced or absorbed by monitoring the change in temperature

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Chemical EquationsTwo important types of information:1. The identities of the reactants and products.2. The relative numbers of each. Physical States Symbol State (s) solid (l) liquid (g) gas (aq) dissolved in water (in aqueous solution)

Examples include 2K(s) + 2H2O(l) H2(g) + 2KOH(aq)

2H2(g) + O2(g) 2H2O(g)

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Types of Rxns

Single displacement Double displacement Combustion Synthesis Decomposition Acid-base neutralization

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Single displacement

A + BC AC + B Cu(s) + 2AgNO3(aq) Cu(NO3)2(aq) + 2Ag(s)

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Double displacement

AB + CD AC + BD 2KI(aq) + Pb(NO3)2(aq) 2KNO3(aq) + PbI2(s)

When solid formed precipitate

Can we predict the formation of a precipitate? Yes: solubility rules

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Solubility

The ability for a substance to dissolve in a medium

Soluble substances can form a 0.10-molar solution at 25 °C.

If unable to dissolve, said to be insoluble

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Solubility Rules

Check it out: http://web.clark.edu/aaliabadi/CHEM131_fall200

6_solubility_rules.htm Let’s take a look

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Solubility

What happens when you dissolve NaCl in water? It breaks up into individual cations and anions They are surrounded by water molecules There are strong water-ion interactions

Give the equation for the ionization of the following compounds: Na2CO3

Al2(SO4)3

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Combustion

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Requires oxygen gas and a hydrocarbon Yields carbon dioxide and water

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Synthesis

A + B AB 2Mg(s) + O2(g) 2MgO(s)

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Decomposition

AB A + B 2KClO3(s) 2KCl(s) + 3O2(g)

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Acid-base neutralization (a special type of DD)

HX(aq) + MOH(aq) MX(aq) + H2O(l)

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)

CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H2O(l)

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More on chemical equations

Can be expressed three ways Molecular Ionic Net-ionic

See next three slides

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Molecular equation

2KI(aq) + Pb(NO3)2(aq) 2KNO3(aq) + PbI2(s)

Gives the complete chemical equation Includes all the bells-and-whistles

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Ionic equation

2K+(aq) + 2I-

(aq) + Pb2+(aq) + 2NO3

-(aq) 2K +

(aq) + 2NO3-(aq) + PbI2(s)

Displays all the components in their broken-down form Shows all ions in soln

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Net-ionic equation

Pb2+(aq) + 2I-

(aq) PbI2(s)

Shows only that which creates a rxn Gets rid of spectator ions

Ions that still remain in solution and don’t play a role

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PracticeWrite the balanced molecular, ionic, and net-ionic equations (if

necessary), including symbols of states, for each of the chemical reactions below. Also, name the type of rxn.

1. Solid magnesium metal reacts with liquid water to form solid magnesium hydroxide and hydrogen gas.

2. Solid ammonium dichromate decomposes to solid chromium (III) oxide, gaseous nitrogen, and gaseous water.

3. Gaseous ammonia reacts with gaseous oxygen to form gaseous nitrogen monoxide and gaseous water.

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More practice

4. When solutions of strontium bromide and magnesium sulfate are mixed, a ppt is formed.

5. Copper metal is thrown in to a beaker containing highly concentrated nitric acid. This exothermic reaction yields copper (II) nitrate, nitrogen dioxide gas, and water.

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Oxidation-Reduction Reactions: a subset Oxidation-Reduction Reaction: or redox reactions

Rxns in which one or more electrons are transferred. Oxidation: Loss of electrons Reduction: Gain of Electrons Example: 2Na(s) + Cl2(g) 2NaCl(s)

Sodium is oxidized to Na+

Chlorine is reduced to Cl-

Chlorine is oxidizing agent Itself being reduced

Sodium is reducing agent Itself being oxidized

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Oxidation-Reduction Reactions Identify what is oxidized/reduced, and what is the

oxidizing/reducing agent.

2Cu(s) + O2(g) 2CuO(s)

Ba(s) + H2O(l) Ba(OH)2(aq) + H2(g)

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Oxidation-Reduction Reactions Between Nonmetals

Oxidation Loss of Electrons/gain in oxygens/loss of hydrogens

Reduction Gain of Electrons/gain in hydrogens/loss of oxygens

Oxidizing Agent Causes the elements to be oxidized: electron acceptor

Reducing Agent Causes the elements to be reduced: electron donor

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ExampleCH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Reactants: C = -4 H = +1 O = 0

Products: C = +4 H = +1 O = -2

So which are oxidized and reduced?

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Practice

The combustion of C2H5SH to yield carbon dioxide, water, and sulfur dioxide

State what is being oxidized/reduced and what is the oxidizing/reducing agent

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Solubility

Oil and water do not mix. Oil is non-polar and water is polar. Oil floats on the surface of water because its density is less than that of water.

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Solution Composition:An Introduction Unsaturated solution

A solution that has not reached the limit of solute that will dissolve

Saturated solution A solution that contains as much solute as will

dissolve at a given temperature

Supersaturated solution A solution that contains more than its allowable

limit of dissolved solute Demo

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Solution Composition: Molarity

Molarity Number of moles of

solute per volume of solution in liters.

Example: A solution that is 1.0 M contains 1.0 mol per 1 L of solution.

moles of solute molMolarity = M = =

liters of solution L

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Example

Calculate the concentration of a solution prepared by dissolving 1.00 g of ethanol, C2H6O, in 2.50 liters of water

Practice: How many grams of NaCl are needed to make 1.25 liters of a 0.050 molar solution?

-3mol 11.00 g ( ) ( ) = 8.68 10 M

46.069 g 2.50 L

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More practice

Give the concentrations of the ions in each of the following solutions:

0.10 M Na2CO3

0.010 M Al2(SO4)3

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More

Stock solution Solution that is routinely used and purchased or

prepared in concentrated form

Dilution The process of adding more water to a solution

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Making dilutions

The equation for solving dilution problems is given below.

C1 x V1 = C2 x V2

Moles of solute after dilution = Moles of solute before dilution

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Practice

What volume of 12 M HCl must be taken to prepare 0.75 L of 0.25 M HCl?

12M x V1 = 0.25M x 0.75L

V1 = 0.016L of acid

So what’s the next step?

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Stoichiometry ofSolution Reactions Calculate the mass of lead (II) sulfate formed when 1.25

L of 0.0500 M lead (II) nitrate and an excess of sodium sulfate are mixed.

Na2SO4(aq) + Pb(NO3)2(aq) 2NaNO3(aq) + PbSO4(s)

0.0500 mol 1 mol 303.3 g1.25 L ( ) ( ) ( ) = 19.0 g

L 1 mol mol

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Example

Calculate the mass of barium oxalate formed when 0.0255 L of 0.0305 M barium chloride and 0.0354 L of 0.0257 M rubidium oxalate are mixed.