Ch. 3 - Molecules, Compounds, and Chemical Equations

Finding the Molecular Formula from the Empirical Formula

- The molecular formula is simply a multiple of the empirical formula.

Chemical Formulas

- A *compound* is a distinct substance that is composed of atoms of two or more elements. - The compound can be described as a *chemical formula* by describing the number and type of each atom in the simplest unit of the compound. (molecules or ions) - Each element is represented by its letter symbol. - The number of atoms of each element is written to the right of the element as a subscript. (If there is only one atom, the 1 subscript is not written.) - Polyatomic ions are placed in parentheses.

Molecular Formula

- A molecular formula gives the actual number of atoms of each element in a molecule of a compound. - It does not describe the order of attachment, or the shape.

Acids

- Acids are molecular compounds that form H+ when dissolved in water. (aqueous) - Formula generally starts with H.

Empirical Formula

- An empirical formula gives the relative number of atoms of each element in a compound. - They use the simplest whole-number ratio of the atoms of elements in a compound. - It does not describe exactly how many atoms, the order of attachment, or the shape. - Example: The empirical formula for the molecular compound oxalic acid is CHO2. This means that there is 1 C atom and 1 H atom for every 2 O atoms in the molecule. The actual molecular formula is C2H2O4.

Chemical Bonds

- Chemical bonds are forces of attraction between atoms. - Compounds are made of atoms held together by chemical bonds. - The bonding attraction comes from attractions between protons and electrons.

Ionic Compounds

- Compounds of metals & nonmetals are made of ions. Their particles are cations and anions. - Metal atoms form cations*(+)*, nonmetal atoms form anions*(-)*.

Naming Binary Ionic Compounds for Metals with Variable Charge

- Contain metal cation + nonmetal anion. - Metal listed first in formula and name. 1. Name metal cation first, name nonmetal anion second. 2. Metal cation name is the metal name followed by a Roman numeral in parentheses to indicate its charge. (determine charge from anion charge) 3. Nonmetal anion named by changing the ending on the nonmetal name to -ide.

Naming Binary Ionic Compounds for Metals with Invariant Charge

- Contain metal cation + nonmetal anion. - Metal listed first in formula and name. 1. Name metal cation first, nonmetal anion second. 2. Cation name is the metal name. 3. Nonmetal anion named by changing the ending on the nonmetal name to -ide.

Covalent Bonds

- Covalent bonds result when two atoms *share* some of their electrons. - They are found when nonmetal atoms bond together.

Naming Monatomic Nonmetal Anions

- Determine the charge from position on the Periodic Table. (Group 4A = -4, 5A = -3, 6A = -2, 7A = -1) - To name anion, change ending on the element name to -ide.

Elements & Compounds

- Elements combine together to make an almost limitless number of *compounds*. - The properties of the compound are totally different from the constituent elements.

Polyatomic/Diatomic Elements

- Elements whose particles are multi-atom molecules. Certain elements occur as 2 atom (Diatomic) molecules. - The 7 diatomic elements (all gases) are: 1. Hydrogen (H2) 2. Nitrogen (N2) 3. Oxygen (O2) 4. Fluorine (F2) 5. Chlorine (Cl2) 6. Iodine (I2) 7. Bromine (Br2)

Formatting Chemical Equations

- Ex: CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) - CH4 and O2 are the *reactants*, and CO2 and H2O are the *products*. - The *(g)* after the formulas tells us the *state* of the chemical. - The number in front of each substance tells us the numbers of those molecules in the reaction (called *coefficients*).

Naming Oxyacids (Ion ends with -ate)

- If polyatomic ion name ends in -ate, then change ending to -ic suffix. - Write word acid at end of name.

Naming Oxyacids (Ion ends with -ite)

- If polyatomic ion name ends in -ite, then change ending to -ous suffix. - Write word acid at end of name.

Ionic Bonds

- Ionic bonds result when electrons have been *transferred* between atoms, resulting in oppositely charged ions that attract each other. - They are found when metal atoms bond to nonmetal atoms.

Chemical Reactions

- Reactions involve chemical changes in matter resulting in *new* substances. - Reactions involve rearrangement and exchange of atoms to produce new molecules. Elements are not transmuted. - "Reactants" → "Products"

Polyatomic Ions

- Several atoms attached together by covalent bonds into one ion. - Often identified by parentheses around ion in formula. - Name and charge of polyatomic ion do not change. - Name any ionic compound by naming cation first and then anion.

Chemical Equations

- Shorthand way of describing a reaction. - Provides information about the reaction: 1. Formulas of reactants and products. 2. States of reactants and products. 3. Relative numbers of reactant and product molecules that are required. 4. Can be used to determine weights of reactants used and products that can be made.

Symbols Used in Equations

- Symbols used to indicate state are placed after the chemical: - (*g*) = gas; (*l*) = liquid; (*s*) = solid (*aq*) = aqueous = dissolved in water - Energy symbols are used above the "reaction arrow" for decomposition reactions: - Greek symbol "delta" = heat - hv = light - shock = mechanical - elec = electrical

Balancing Chemical Equations

- To show a given reaction obeys the Law of Conservation of Mass, the equation must be balanced. - We adjust the numbers of molecules so there are equal numbers of atoms of each element on both sides of the arrow. - To obtain the number of atoms of an element, multiply the *subscript* by the *coefficient*. - Ex: combustion of methane: Incorrect: CH4(g) + O2(g) → CO2(g) + H2O(g) Correct/Balanced: CH4(g) + *2*O2(g) → CO2(g) + *2*H2O(g)

Writing Formulas for Acids

- When name ends in acid, formula starts with H. - Write formulas as if ionic, even though it is molecular. - Hydro- prefix means it is binary acid, no prefix means it is an oxyacid. - For oxyacid, if ending is -ic, polyatomic ion ends in -ate; if ending is -ous, polyatomic ion ends in -ous.

Naming Binary Acids

- Write a hydro- prefix. - Follow with the nonmetal name. - Change ending on nonmetal name to -ic. - Write the word acid at the end of the name.

Formula-to-Name Rules for Ionic Compunds

- Write systematic name by simply naming the ions: - If cation is: metal with invariant charge = metal name metal with variable charge = metal name(charge) polyatomic ion = name of polyatomic ion - If anion is: nonmetal = stem of nonmetal name postfixed with -ide polyatomic ion = name of polyatomic ion.

Prefixes & Suffixes for Polyatomic Ions

-ate ion *Ex: chlorate = ClO3(−)* -ate ion + 1 O: same charge, *per- prefix* *Ex: perchlorate = ClO4(−)* -ate ion - 1 O: same charge, *-ite suffix* *Ex: chlorite = ClO2(−)* -ate ion - 2 O: same charge, *hypo- prefix, -ite suffix* *Ex: hypochlorite = ClO(−)*

Subscript Prefixes

1 = mono- (not used on first nonmetal) 2 = di- 3 = tri- 4 = tetra- 5 = penta- 6 = hexa- 7 = hepta- 8 = octa- 9 = nona- 10 = deca- - Drop last "a" if element name begins with a vowel.

What is Always True in a Balanced Equation?

The number of atoms of each kind is the same for both the products and reactants. (Law of Conservation of Mass)

Finding an Empirical Formula

1. Convert the percentages to grams. - Assume you start with 100 g of the compound. (100% = 100g) 2. Convert grams to moles using the molar mass of each element. 3. Write a pseudoformula using moles as subscripts. 4. Divide all by the *smallest* number of moles - If result is within 0.1 of whole number, round to that whole number. 5. Multiply all mole ratios by number to make all whole numbers.

Patterns for Polyatomic Ions

1. Elements in the same column form similar polyatomic ions. - Same number of O's and same charge. - Ex: ClO3− = chlorate, so BrO3− = bromate 2. If the polyatomic ion starts with H, add hydrogen- prefix before name and add 1 to the charge. - Ex: CO3(2−) = carbonate, so HCO3− = hydrogen carbonate.

Writing Names of Binary Molecular Compounds of Two Nonmetals

1. Write name of first element in formula. - Element furthest left and down on the Periodic Table. - Use the full name of the element. 2. Write the name of the second element in the formula with an -ide suffix. (as if it were an anion, however, remember these compounds do not contain ions) 3. Use a prefix in front of each name to indicate the number of atoms. - Never use the prefix mono- on the first element.

Writing Formulas for Ionic Compounds

1. Write the symbol for the metal cation and its charge. 2. Write the symbol for the nonmetal anion and its charge. 3. Charge (without sign) becomes subscript for other ion. 4. Reduce subscripts to smallest whole number ratio. 5. Check that the sum of the charges of the cations cancels the sum of the anions.

How To Remember the Diatomic Elements

An easy-to-remember mnemonic for the diatomic elements is: *Have* - H - Hydrogen (H2) *No* - N - Nitrogen (N2) *Fear* - F - Fluorine (F2) *Of* - O - Oxygen (O2) *Ice* - I - Iodine (I2) *Cold* - Chlorine (Cl2) *Beer* - Bromine - (Br2)

Molecular Compounds

Compounds whose particles are molecules made of only nonmetals.

Atomic Elements

Elements whose particles are single atoms.

Types of Chemical Bonds

There are two general types of bonding between atoms found in compounds: *ionic* and *covalent*.