1 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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1 Evidence for a Chemical Reaction Chemical reactions give a visual signal. Examples: Rusted steel & bleached hair, Clues that a Chemical Reaction (rxn)
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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.