Chapter 3 Stoichiometry: Calculations with Chemical Formulas and Equations

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Chapter 3Stoichiometry Calculations with
Chemical Formulas and Equations
CHEMISTRY The Central Science 9th Edition
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Chemical Changes

• Chemical properties describes the reaction a
  substance undergo to form new substances.
• The study of chemical changes is at the heart of
  chemistry.
• Some chemical changes or simple and some or
  complex.
• For example, changes that occur in your brain and
  eyes allow you to see and think.
• In this chapter, chemical changes will be used
  to explore the quantity of substances consumed
  and produced in a chemical reaction.
• Chemical Equations are used to describe chemical
  reactions.

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Defining Chemistry

• Lavoisier mass is conserved in a chemical
  reaction.
• His careful measurements turned chemistry into a
  science (Father of Chemistry)
• Investigate the reaction of hydrogen with oxygen
  to produce water (2H2 O2 ? 2H2O).

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Chemical Equations

• The chemical equation for the formation of water
  can be visualized as two hydrogen molecules
  reacting with one oxygen molecule to form two
  water molecules
• 2H2 O2 ? 2H2O

Products
Reactants
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Reading Chemical Equations

• The plus sign () means react and the arrow
  points towards the substance produce in the
  reaction.
• The chemical formulas on the right side of the
  equation are called reactants and after the arrow
  are called product.
• The numbers in front of the formulas are called
  stoichiometric coefficients.
• 2Na 2H2O ? 2NaOH H2
• Stoichiometric coefficients numbers in front of
  the chemical formulas give ratio of reactants
  and products.

Coefficient
Reactants
Products
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Understanding Chemical Equations
Coefficients and subscripts included in the
chemical formula have different effects on the
composition.
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Balancing Chemical Equations

• Law of conservation of mass matter cannot be
  lost in any chemical reactions.

O
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Class Practice Problem

• Balance the following equations
• (a) Na(s) H2O(l) NaOH(aq) H2(g)
• (b) Al(s) HCl(aq) AlCl3(aq) H2(g)
• (c) C2H4(g) O2(g) CO2(g) H2O(l)

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Combination and Decomposition Reactions

• In Combination reactions two or more substances
  react to form products
• 2Mg(s) O2(g) ? 2MgO(s)
• The Mg has combined with O2 to form MgO (ionic
  compounds).
• Decomposition reactions is when one substance
  undergoes a reaction to produce two or more
  substances
• 2NaN3(s) ? 2Na(s) 3N2(g)
• (the reaction that occurs in an air bag)
• The NaN3 has decomposed into Na and N2 gas.

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Summarization of Reactions
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Patterns in Chemical Reactivity

• The periodic table can be used to predict how
  elements will react in a combination reaction
• 2K(s) 2H2O(l) ? 2KOH(aq) H2(g)
• All alkali metals will react with water to form
  the hydroxide compound and hydrogen.
• Thus, if let M represent the alkali metal, we
  able to write
• 2M(s) 2H2O(l) ? 2MOH(aq) H2(g)
• Alkali metal water ? Metal hydroxide
  hydrogen

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Formula Weights

• Formula and Molecular Weights
• Formula weights (FW) is the sum of the atomic
  weights of each atom in the chemical formula.
• FW (H2SO4) 2AW(H) AW(S) 4AW(O)
• 2(1.0 amu) (32.0 amu) 4(16.0)
• 98.0 amu
• If the chemical formula is also its molecular
  formula then the weight is called the molecular
  weight (MW).
• MW(C6H12O6) 6(12.0 amu) 12(1.0 amu) 6(16.0
  amu)

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Formula Weights

• Percentage Composition from Formulas
• Percent composition is the atomic weight for each
  element divided by the formula weight of the
  compound multiplied by 100

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Class Practice Problem

• Calculate the FW of C12H22O11.
• Calculate the percent composition of H2O.

• 12C 12 x 12.01 144.12
• 22H 22 x 1.01 22.22
• 11O 11 x 16.00 176.0
• C12H22O11 342.34 amu

• H 11.21
• O 88.79

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Molar Mass

• Molar mass mass in grams of 1 mole of substance
  (units g/mol, g.mol-1).
• Experimentally, 1 mole of 12C 12 g, which can
  be written as 12g/mol.

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The Mole

• The unit we use to express the quantity of atoms,
  ions, and molecules that an object contains is
  called mole.
• Mole convenient measure chemical quantities.
• The actual number of atoms, ions, or molecules in
  1 mole of something 6.0221367 ? 1023
  (Advogadros number) of that thing.
• Thus,
• 1 mole of 12C atoms 6.02 x 1023 12C atoms
• 1 mole of H2O molecules 6.02 x 1023 molecules
• 1 mole of NO3- ions 6.02 x 1023 ions

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Visualizing The Mole Concept
Different Units
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Class Practice Problem

• How many C atoms are in 0.350 mol of C6H12O6?

• C atoms 0.350 mol C6H12O6 (6.02 x 1023
  molecules/1 mole C6H12O6)(6 C atoms/1 molecule)
  1.26 x 1024 C atoms

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Class Practice Problem

• Converting moles to mass
• Calculate the number of moles of glucose C6H12O6
  in 5.380 g of C6H12O6.

• Moles of C6H12O6 5.380 g C6H12O6 (1 mole
  C6H12O6/ 180 g C6H12O6) 0.02989 mol C6H12O6

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Class Practice Problem

• Converting mass to particles
• Calculate the number of atoms of Cu in 3 g of Cu?

• Atoms of Cu 3 g Cu (1 mole Cu/180 g Cu)(6.02 x
  1023 atoms Cu/1 mole Cu) 3 x 1023Cu atoms

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Empirical Formulas from Analyses

• Start with mass of elements (i.e. empirical
  data) and calculate a formula, or
• Start with the formula and calculate the mass
  elements.
• For example
• Ascorbic acid contains 40.92 percent C, 4.58
  percent H, and 54.50 percent O by mass. What is
  the empirical Formula?

The experimentally determined molecular weight is
176 amu.
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Empirical Formulas from Analyses

• Molecular Formula from Empirical Formula
• Once we know the empirical formula, we need the
  MW to find the molecular formula.
• Example 3.14, page 98
• Subscripts in the molecular formula are always
  whole-number multiples of subscripts in the
  empirical formula

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Quantitative Information from Balanced Equations

• Balanced chemical equation gives number of
  molecules that react to form products.
• Interpretation ratio of number of moles of
  reactant required to give the ratio of number of
  moles of product.
• These ratios are called stoichiometric ratios.
• NB Stoichiometric ratios are ideal proportions
• Real ratios of reactants and products in the
  laboratory need to be measured (in grams and
  converted to moles).

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Limiting Reactants

• If the reactants are not present in
  stoichiometric amounts, at end of reaction some
  reactants are still present (in excess).
• Limiting Reactant one reactant that is consumed

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Limiting Reactants
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Limiting Reactants

• Theoretical Yields
• The amount of product predicted from
  stoichiometry taking into account limiting
  reagents is called the theoretical yield.
• The percent yield relates the actual yield
  (amount of material recovered in the laboratory)
  to the theoretical yield

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The End of Chapter 3The test will cover Chapters
1-3, Scheduled for September 28, 2005Chemistry
Feud will be held on Monday, September 26,
2005Homework 3.9, 3.11, 3.15, 3.17, 3.19,
3.21, 3.25, 3.27, 3.31, 3.33, 3.43, 3.47