Reactions and Reactivity: Review

• Chemical reactions occur when matter combines or decomposes forming new types of matter.
• Chemical equations must be balanced to reflect the law of conservation of mass.
• Synthesis and decomposition reactions are essentially opposite—the first has a single compound as a product, the latter has a single compound as a reactant.
• In displacement reactions, one substance replaces another substance in a compound.
• Synthesis reactions produce a single compound as a product.
• Decomposition reactions begin with a compound and produce simpler substances which may be elements, compounds or both.
• Combustion reactions always involve combination with oxygen. Combustion of hydrocarbons always produces carbon dioxide and water if enough oxygen is present.
• Single replacement reactions begin with an element and a compound and produce a new element and new compound. Single replacement reactions only occur when the free element in the reactants is more active than the combined element in the reactant compound.
• Double replacement reactions occur in solutions and result from the exchange of cations with anions.
• A working knowledge of solubility charts for compounds, activity of metals and weak electrolytes is necessary to predict whether or not a reaction actually occurs.
• Three types of equations can be written for many reactions:
  • Molecular equations include all of the compounds and elements taking part in the reaction, including their physical state—solid, liquid, gas or aqueous.
  • Ionic equations are written when soluble ionic compounds are involved in the reaction.
  • Net ionic equations are used to show only the ions which react to form products. Spectator ions are not shown in net ionic equations.
• The mole is a very large number, 6.02 × 10²³ molecules or atoms.
• The molar mass of an element is the mass of 6.02 × 10²³ atoms of that element and is equal to the atom’s average atomic mass in grams, found on the Periodic Table.
• The molar mass of a compound is calculated by adding the molar masses of all of the atoms in the compound, accounting for the correct number of each kind of atom.
• The number of moles in a sample is found by dividing the mass of the sample by the molar mass of the substance.
• Coefficients in balanced equations represent the number of moles as well as the number of molecules and atoms.
• The mole ratio from the equation can be used to show relationships between substances in a chemical equation.
• The molar mass can be used to calculate the amount of each substance required or produced during a reaction, if an amount of starting material is known.
• The stoichiometric amount of a product of a chemical reaction calculated from the amount of starting material, mole ratios from the balanced equation, and the molar masses is called the theoretical yield.
• The actual amount of product obtained in an experimental procedure is called the experimental yield.
• Percent yield is obtained by calculating the ratio of the experimental yield to the theoretical yield and multiplying by 100 to convert the ratio to a percentage.
• Sometimes amounts of both reactants are given in a situation for an experiment. In order to determine the theoretical amount of product the limiting reactant must first be determined.
• Kinetics is the study of rates of chemical reactions.
• There are two kinds of rate laws, the differential and integrated rate laws which show how the rate depends on the concentration of reactants.
• The half-life of a reaction is the time required for the concentration of a reactant to become half of its initial value. Half-lives are calculated differently for different orders of reactions.
• A reaction mechanism is a proposed series of elementary steps by which a reaction is thought to proceed. The slowest step in a reaction mechanism is the rate-determining step.
• The collision model assumes that molecules must collide to react. A number of factors influence the ability of molecules to have effective collisions.
• Activation energy is the minimum energy molecules must possess for the reaction to proceed.

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