Evidence for a Chemical Reaction

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

2
Chemical Equations

• Two 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)

3
Types of Rxns

• Single displacement
• Double displacement
• Combustion
• Synthesis
• Decomposition
• Acid-base neutralization

4
Single displacement

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

5
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

6
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

7
Solubility Rules

• Check it out
• http//web.clark.edu/aaliabadi/CHEM131_fall2006_so
  lubility_rules.htm
• Lets take a look

8
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

9
Combustion

• CH4(g) 2O2(g) ? CO2(g) 2H2O(l)
• Requires oxygen gas and a hydrocarbon
• Yields carbon dioxide and water

10
Synthesis

• A B ? AB
• 2Mg(s) O2(g) ? 2MgO(s)

11
Decomposition

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

12
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)

13
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

15
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 dont play
  a role

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Practice

• Write 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

20
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)

21
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?

23
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

24
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 CompositionAn 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.

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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?

28
More practice

• Give the concentrations of the ions in each of
  the following solutions
• 0.10 M Na2CO3
• 0.010 M Al2(SO4)3

29
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

30
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

31
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 whats the next step?

32
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)

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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.