Chemical Equation notes

Chemical Equations

1.     Types of Reactions

a.     Reactions in which a solid is formed

                                               i.     A solid that is produced in a chemical reactant is called a precipitate.

                                             ii.     When a solid containing ions dissolves in water, the ions separate and move around independently.

                                            iii.     In order to know which product is the precipitate, solubility rules must be used.

1.     Write the reactants, as they actually exist before any reaction occurs. Remember that when a salt dissolves, its ions separate.

2.     Consider the various solids that could form. To do this, simply exchange the anions of the added salts.

3.     Use Reference #1 below to decide whether a solid forms and, if so, to predict the identity of the solid.

                                            iv.     There are three kinds of reactions in aqueous solutions (Reference #2)

1.     Molecular equation: shows the complete formulas of all reactants and products.

2.     Complete ionic equation: represents the actual forms of the reactants and products in solution.

a.     Only the electrolytes are split into ions but the precipitate is kept together since it is not dissolved.

b.     The equation reveals spectator ions, which do not directly participate in the reaction. The net ionic equation includes the compounds that are directly involved in the reaction.

b.     Reactions of Metals with Nonmetals (Oxidation-Reduction)

                                               i.     When a metal reacts with a nonmetal, an ionic compound is formed. The ions formed when the metals transfers one or more electrons to the nonmetals, the metal atom becoming a cation and the non-metal atom becoming an anion.

                                             ii.     Two nonmetals can also undergo an oxidation-reduction reaction. At this point we can recognize these cases only by looking for  as a reactant as a product. Redox reactions never happen in double displacement reactions.

c.     Different ways to classify reactions

                                               i.     Double-displacement reaction-a reaction where the two anions are exchanged. A precipitate is commonly formed.

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                                             ii.     Single-displacement reaction-The two types of anions are exchanged.

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                                            iii.     Combustion reaction-Involve oxygen to produce heat. Carbon dioxide is often in the products.

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                                            iv.     Synthesis reaction-When a compound is formed from simpler materials.

1.     Synthesis of water:

                                             v.     Decomposition reactions-A compound is broken down into simpler compounds. Usually done with an electrical current or heat.

1.     Decomposition of water:

2.     On the arrow you should put “heat” or “electric current”, depending on what is done to decompose the compound.

2.     Stoichiometry

a.     Molar ratios-conversion factor is the relationship between a reactant and a product.

b.     Make sure the element is in Standard, Temperature, and Pressure (STP). This is 273 Kelvin and 1 atm.

c.     Steps for calculating the masses of reactants and products:

                                               i.     Balance the equation.

                                             ii.     Convert the masses of the reactants or products to moles.

                                            iii.     Use the balanced equation to set up the appropriate molar ratios.

                                            iv.     Use the molar ratios to calculate the number of moles of the desired reactant or product.

                                             v.     Convert from moles back to mass.

d.     Steps to solve Stoichiometry Problems involving limiting reactants:

                                               i.     Write and balance the equation for the reaction

                                             ii.     Convert known masses of reactants to moles.

                                            iii.     Using the numbers of moles of reactants and the appropriate mole ratios, determine which reactant is limiting.

                                            iv.     Using the amount of the limiting reactant and the appropriate mole ratios, compute the number of moles of the desired product.

                                             v.     Convert from moles of product to grams of product, using molar mass.

e.     Percent Yield-percentage of the theoretical yield that was actually produced.

                                               i.     Example of Limiting Reactants and Percent Yield (Reference #3)

3.     Formulas of Compounds

a.     The Empirical Formula

                                               i.     The formula of a compound that expresses the smallest whole-number ratio of the atoms present.

1.     Ex. The Empirical formula for both  and  is .

b.     The Molecular formula

                                               i.     The actual formula of a compound—the one that gives the composition of the molecules that are present.

1.     Ex. Glucose’s molecular formula is, but the empirical formula is.

4.     Calculation of Compound Formulas (Reference #4)

a.     Steps for determining the empirical formula of a compound:

                                               i.     Obtain the mass of each element present (in grams).

                                             ii.     Determine the number of moles of each type of atom present.

                                            iii.     Divide the number of moles of each element by the smallest number of moles to convert the smallest number to 1.

1.     If all of the numbers so obtained are integers, they are the subscripts in the empirical formula.

2.     If one or more of these numbers are not integers, go on to step 4.

                                            iv.     Multiply the numbers you derived in step 3 by the smallest integer that will convert all of them to whole numbers.

1.     This set of whole numbers represents the subscripts in the empirical formula.

b.     Determining the molecular formula:

                                               i.     The molecular formula is always an integer multiple of the empirical formula.

1.     Ex. Glucose molecule:

                                             ii.     General Formula:

1.     If n=1, the molecular formula is the same as the empirical formula.