Chemical Formula Stoichiometry Review AP Chemistry– M. Gonzalez Rm. 613

Overview Types of formulas 1. Simplest or empirical formula Simplest/Empirical formula gives the simplest whole number ratio between the number of atoms of different elements in the compound. Molecular formula indicates the number of atoms of each type in a molecule. The molecular formula may be the same as the simplest formula— Ex: Water. They can also represent elements as well as compounds. Types of formulas 1. Simplest or empirical formula 2. Molecular formula

Hydrates Sometimes to represent a compound may go beyond the simplest and simple formula. The compound is written in such a way to suggest the structure of the compound. Hydrates are usually solids, ionic in nature, which contain a water molecule within the crystal lattice. Example of a hydrate: BaCl2 ∙ 2H2O

Calculations Percent composition Determination of percent composition from experimentation Simplest formula from % composition Molecular formula from simplest formula

Chemical Equations Balancing equations Indicating the states of reactants and products symbols— Mole relations in Balanced chemical equations Mass relations in equations Types of reactions Stoichiometry

Symbols

Simple patterns of chemical reactivity Combination/synthesis reaction Two or more substances combine to form one product A + B AB Decomposition reaction One substance undergoes a reaction to produce two or more substances AB A + B

Simple patterns of chemical reactivity Single replacement/displacement reaction one element replaces a similar element in a compound Most SR reactions takes place in aqueous solution. The amount of energy involved in this type of reaction is usually smaller than the amount involved in synthesis or decomposition reactions. A + BX AX + B

Simple patterns of chemical reactivity Double Replacement/ Displacement Reaction The ions of two compounds exchange places in an aqueous solution to form two new compounds. One of the compounds is usually a precipitate (ppt), an insoluble gas that bubbles out of solution, or a molecular compound, usually water. The other compound is usually soluble and remains dissolved in solution. AX + BY AY + BX

Simple patterns of chemical reactivity Combustion reaction – A substance combines with oxygen releasing a large amount of energy in the form of light or heat. Example: The burning of natural gas, propane, and wood. C3H4 + 5O2 3CO2 + 4H2O

Simple patterns of chemical reactivity Neutralization/Acid-Base Reaction A reaction between an acid and a base to produce a salt and water. HCl + NaOH NaCl + HOH More on this when get to acids and bases.

Activity Series of metals The ability of an element to react is referred to as the activity of the element. The more readily an element reacts with other substances the greater its activity. An activity series is a list of elements organized according to the ease with which they undergo certain chemical reactions.

Activity Series Metals Li Rb K Ba Sr Ca Na Mg Al Mn Zn Cr Fe Cd Co Ni Sn Pb H2 Sb Bi Cu Hg Ag Pt Au Can react with cold water and acids replacing hydrogen Can react with steam and acids replacing hydrogen Can react with acids, replacing hydrogen React with oxygen, forming oxides Fairly unreactive Forms oxides only indirectly

Activity Series Non-metals (Halogens) F2 Cl2 Br2 I2

Generalization based on activity series An element will replace from a compound in aqueous solution any of those elements below it in the activity series. The lager the interval between the elements, the greater the tendency for the replacement reaction to occur. Any metal above Mg will replace Hydrogen from water Any metal above Co will replace Hydrogen from steam Any metal above Hydrogen reacts with acids, replacing Hydrogen

Generalization based on activity series Any metal above silver reacts with oxygen, forming oxides; those near the top react rapidly. Any metal below mercury forms oxides only indirectly (not by reaction with O2). The more active the metal the more strongly it holds onto oxygen in an oxide and the more strongly the oxide resists decomposition into its elements upon heating. Oxides of metals below Cu decompose with heat alone. Oxides of metals below Cr yield metals when heated with hydrogen.

Generalization based on activity series Oxides of metals above Fe resist conversion to the free metal when heated with oxygen. Elements near the top of the series are never found free in nature. Elements near the bottom are often found free in nature.

Stoichiometry The mass relationships between reactants and products in a chemical equation. Types: Mole to mole Mole to mass/Mass to mole Mass to Mass

Limiting Reactants In a lab setting, a reaction is rarely carries out with exactly required amounts of each of the reactants. In most cases, one or more reactants is present in excess. When all of one reactant is used up, no more product can be formed even if there is more of other reactants available. The substance that is completely used of first in a reaction is called the limiting reactant. The limiting reactant is the reactant that limits the amount of the other reactants that can combine and the amount of products formed in a chemical reaction. The substance that is not used up completely in a reaction is called the excess reactant.

Practice Problem The most important commercial process for converting N2 from the air into nitrogen containing compounds is based on the reaction of N2 and H2 to form ammonia NH3. N2 (g) + 3H2 (g) 2NH3 (g) How many moles of ammonia can be formed from 3.0 moles of nitrogen and 6.0 moles of hydrogen?

Summarize reaction data in a table N2 (g) 3H2 (g) 2NH3 (g) Before reaction 3.0 mol 6.0 mol 0 mol Change (reaction) -2.0 mol -6.0 mol +4.0 mol After reaction 1.0 mol 4.0 mol

Theoretical and percent yield Theoretical yield is the maximum amount of product that can be produced from a given amount of reactant. Actual yield is the amount of product actually obtained. Percent yield is the ratio of the actual yield to the theoretical yield multiplied by 100.