Stoichiometry

• Stoichiometry is the area of study that examines the quantities of substances consumed and produced in chemical reactions
• Stoichiometry


  • It is built on an understanding of atomic masses, chemical formulas, and the law of conservation of mass (atoms are neither created nor destroyed during any chemical reaction, the changes that occur during any reaction merely rearrange the atoms)
  • The focus is on the use of chemical formulas to represent reactions and on the quantitative information we can obtain about the amounts of substances involved in reactions
• Atomic mass


  The mass of an atom depends on the number of electrons, protons, and neutrons it contains
• Atomic mass unit (amu)


  1 amu is defined as a mass exactly equal to one-twelfth the mass of one carbon-12 atom
• The atomic mass of an element is based on the average mass of the stable (nonradioactive) isotopes of the element
• Molecular mass


  The sum of the masses of the atoms in the molecule of the substance
• Formula mass


  The term used for ionic compounds, it is the same calculation as molecular mass
• Mole (mol)


  The amount of a substance that contains as many particles (atoms, molecules or ions) as there are atoms in exactly 12 g of carbon-12 isotope
• Avogadro's number


  The numerical value of 6.02 x 10^23
• Solving problems dealing with the number of particles present in a given number of moles of a substance

  1. Use Avogadro's number as part of the conversion factor

  2. Relate the number of objects present to the number of moles present
• Molar mass


  The mass in grams of one mole of a substance, with the unit grams per mole (g/mol)
• Molar mass is numerically equal to the substance's atomic mass, formulas mass or molecular mass
• Solving chemical-formula-based problems


  1. Use the figure to determine any of the three quantities (mass, number of particles, moles) when one is provided

  2. The only "transitions" allowed are those between quantities (boxes) connected by arrows

  3. The conversion factor is based on the concept associated with each arrow
• Mole


  Unit used to measure the amount of a substance, equal to the number of atoms in 12 grams of carbon-12
• Solving chemical-formula-based problems


  1. Dimensional analysis

  2. Molar number (Avogadro's number)

  3. Molar mass
• Subscript in chemical formula


  Conversion factor to convert from moles of compound to moles of element within a compound
• Law of definite proportion


  The elements in a compound are present in a definite proportion by mass
• Percent composition


  The composition of the compound expressed as the percent by mass of the elements
• Empirical formula


  Specifies the smallest whole-number ratio of atoms of the elements in a compound
• Molecular formula


  Specifies both the relative and the actual number of atoms in a molecule
• Chemical equation


  A written statement that uses chemical symbols and chemical formulas instead of words to describe the changes that occur in a chemical reaction
• Reactants


  The substances present at the start of a chemical reaction
• Products


  New materials with new chemical properties produced in a chemical reaction
• Conventions for writing chemical equations


  • Correct formulas of reactants are written on the left side
  • Correct formulas of products are written on the right side
  • Reactants and products are separated by an arrow pointing toward the products
  • Plus signs are used to separate different reactants or different products
• Valid chemical equation


  • It must be consistent with experimental facts
  • There must be the same number of atoms of each kind on both sides of the chemical equation
• Major categories of chemical reactions

  • Combination (Synthesis) reaction
  • Decomposition reaction
  • Single-replacement (Substitution) reaction
  • Double-replacement reaction
  • Combustion reaction
• Combination (Synthesis) reaction


  A chemical reaction in which a single product is produced from two or more reactants
• Decomposition reaction


  A chemical reaction in which a single reactant is converted into two or more simpler substances/products (elements or compounds)
• Decomposition reactions


  • 2NH4NO3 → 4H2O + 2N2 + O2
  • 2AgBr → 2Ag + Br2
  • 2H2O → 2H2 + O2
• Single-replacement (Substitution) reaction


  A chemical reaction in which an atom or molecule replaces an atom or group of atoms from a compound
• Single-replacement reactions


  • Fe + CuSO4 → Cu + FeSO4
  • F2 + 2NaCl → Cl2 + 2NaF
  • 4PH3 + Ni(CO)4 → 4CO + Ni(PH3)4
• Double-replacement reaction


  A chemical reaction in which two substances exchange parts with one another and form two different substances
• Double-replacement reactions


  • AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
  • 2KI (aq) + Pb(NO3)2 (aq) → PbI2 (s) + 2KNO3 (aq)
• Combustion reaction


  A chemical reaction between a substance and oxygen (usually from air) that proceeds with the evolution of heat and light (usually from a flame)
• Combustion reactions


  • C4H8 + 6O2 → 4CO2 + 4H2O
  • 4NH3 + 5O2 → 4NO + 6H2O
• Equation coefficient


  A number placed to the left of a chemical formula in a chemical equation that changes the amount, but not the identity, of the substance
• Balancing a chemical equation


  Adjust the number of reactant or product molecules by adding coefficients to satisfy the law of conservation of mass
• Atoms are neither created nor destroyed in a chemical reaction, only rearranged
• The mass of the reactants and the mass of the products are the same, because both contain exactly the same number of atoms of each kind present
• Mechanics of balancing a chemical equation


  Determine the coefficients needed to balance the equation