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Chapter 8

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  • Chemical Reactions and Chemical Quantities

Climate Change and Combustion of Fossil Fuels: The Greenhouse Effect

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  • Greenhouse gases in the atmosphere:
    • Allow sunlight to enter the atmosphere
    • Warm Earth’s surface
    • Prevent some of the heat generated by the sunlight from escaping
      • The balance between incoming and outgoing energy from the sun determines Earth’s average temperature.

Global Warming

  • Scientists have measured an average >0.8 °C rise in atmospheric temperature since 1860.
  • During the same period, atmospheric CO2 levels have risen >25%.
  • Are the two trends causal?

How Much Matters

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  • It’s important to know how much CO2 is in the air. We’re going to learn how to calculate that

Physical Changes in Matter

What is a physical change?

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  • Changes that alter only the state or appearance, but not composition, are physical changes.
  • The atoms or molecules that compose a substance do not change their identity during a physical change.
  • When water boils, it changes its state from a liquid to a gas.
  • The gas remains composed of water molecules, so this is a physical change.

Chemical Changes in Matter

What is a chemical change?

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  • Changes that alter the composition of matter are chemical changes.
  • During a chemical change, atoms rearrange, transforming the original substances into different substances.
    • A rusting nail is a chemical change.
    • Rust occurs when the iron atoms exchange electrons with oxygen atoms— combining to form a NEW or different chemical substance.
    • Iron rust is the compound iron(III) oxide (Fe2O3).

Clicker 1

  • A chemical change
    • A) occurs when methane gas is burned.
    • B) occurs when paper is shredded.
    • C) occurs when water is vaporized.
    • D) occurs when salt is dissolved in water.
    • E) occurs when powdered lemonade is stirred into water.

Chemical Reactions

Writing and Balancing Chemical Reactions

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  • Reactions involve chemical changes in matter resulting in new substances.
  • Reactions involve rearrangement and exchange of atoms to produce new molecules.
  • A chemical reaction is a written statement.
    • It states the quantities, the chemical identity of the substances in the reaction, and their physical state.
    • Example:
    • CH4(g) + O2(g) → CO2(g) + H2O(l)
    • Reactants → Products

Chemical Equations - Short hand for Describing a Chemical Reaction

  • Chemical equations:
  • Provide information about the reaction
    • Molecular or ionic formulas of reactants and products
    • States of reactants and products
  • Gas (g), liquid (l), solid (s), and aqueous (aq)
    • Relative numbers of reactant and product molecules that are required
    • Can be used to determine weights of reactants used and products that can be made

The Quantities in Chemical Reactions

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  • The amount of every substance used and made in a chemical reaction is related to the amounts of all the other substances in the reaction.
    • Law of conservation of mass
    • Balancing equations by balancing atoms
  • The study of the numerical relationship between chemical quantities in a chemical reaction is called stoichiometry.

Practice Problem: Balancing and Writing Chemical Equations

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  • Write a balanced equation for the reaction between solid cobalt (III) oxide and solid carbon to produce solid cobalt and carbon dioxide gas

Practice Problem: Balancing and Writing Chemical Equations

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  • Write a balanced equation for the combustion of gaseous butane (C4H10) with gaseous oxygen to give carbon dioxide and gaseous water.

Clicker 2

  • What is the stoichiometric coefficient for oxygen?
    • B2H6(g) + O2(g) -> B2O3(s) + H2O(g)
  • What is the stoichiometric coefficient for oxygen?
    • A) 1
    • B) 2
    • C) 3
    • D) 4
    • E) 6

Clicker 3

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  • Balance the following equation with the smallest whole number coefficients. Choose the answer that is the sum of the coefficients in the balanced equation. Do not forget coefficients of “one.”
    • Cr2(SO4)3 + RbOH -> Cr(OH)3 + Rb2SO4
    • A) 10
    • B) 12
    • C) 13
    • D) 14
    • E) 15

Reaction Stoichiometry: What Is it about?

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  • The coefficients in a chemical reaction specify the relative amounts in molecules and in moles of each of the substances involved in the reaction.
    • 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)
      • This could mean molecules or moles
  • What the above BALANCED chemical equation says:
    • 2 molecules of C8H18 react with 25 molecules of O2 to form 16 molecules of CO2 and 18 molecules of H2O.
    • 2 moles of C8H18 react with 25 moles of O2 to form 16 moles of CO2 and 18 moles of H2O.
      • 2 mol C8H18 : 25 mol O2 : 16 mol CO2 : 18 mol H2O

Cooking Stoichiometry: Making Pizza

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  • The number of pizzas you can make depends on the amount of ingredients you use.
  • 1 crust + 5 oz. tomato sauce + 2 cups cheese → 1 pizza
  • This relationship can be expressed mathematically.
    • 1 crust : 5 oz. sauce : 2 cups cheese : 1 pizza
  • We can compare the amount of pizza that can be made from 10 cups of cheese:
    • Since 2 cups cheese : 1 pizza, then:
      • 10 cups of cheese = 5 pizzas

Mole-to-Mole Conversions

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  • In the same way that the ratio was used from the pizza recipe example, the stoichiometric ratio acts as a conversion factor between the amount in moles of a reactant to moles of a product.
    • 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)
    • Reactant to product:
      • Stoichiometric ratio: 2 moles C8H18 : 16 moles CO2
  • Stoichiometric ratio can be between:
    • Reactant to reactant
      • Stoichiometric ratio: 2 moles C8H18 : 25 moles O2
    • Product to product:
      • Stoichiometric ratio: 16 moles CO2 : 18 moles H2O

How Many Moles of CO2 Form If 22.0 Moles of C8H18 Are combusted (Burned)?

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Mole-to-Mass and Mass-to-Mass Conversions

  • Stoichiometric ratios can be used as a conversion factor between the amount in grams (mass) of a reactant used to determine mass (grams) of a product made.
  • Strategy: A is reactant and B is the product.
  • Mass of A → Moles of A → Stoichiometric ratio B : A → Moles B → Mass B
  • mass A × (1 mole A/mol.mass A) × (mole B/mole A) × (mol.mass B/1 mol B)
Term Definition
physical change changes that alter only the state or appearance, but not composition
chemical changes changes that alter the composition of matter
stoichiometry the study of the numerical relationship between chemical quantities in a chemical reaction
stoichiometric ratio acts as a conversion factor between the amount in moles of a reactant to moles of a product