Combining Elements, Chemical Reactions Chemistry Combining Elements, Chemical Reactions

Why Do Elements Combine? Elements combine to become more stable. Elements become stable by filling their outermost level of electrons.

What is a Compound? A compound is what results when two or more elements combine. A binary compound is a compound formed from two elements. Example: CaF2 , Calcium fluoride.

How Do Elements Combine? Elements combine through chemical bonding. There are 2 types of chemical bonding: Ionic Bonding Covalent Bonding

Ionic Bonding Ionic bonding happens when elements combine by gaining or losing electrons. Example: Na + Cl Atoms become charged as electrons are transferred from Na to Cl. Na+Cl- Opposite charged atoms attract. Attraction force forms an ionic bond between elements and a new compound is formed. NaCl This type compound is called an ionic compound. Note: the compound as a whole is neutral.

Elements Bond in Predictable Ways Metals will lose electrons and become “+” charged. Non-metals will gain electrons and become “-“ charged.

Covalent bonding happens when elements combine by sharing electrons. For example: Cl + Cl Each atom has the same number of protons and electrons, therefore each have a neutral charge. :: Cl :. .: Cl :: Each atom is looking for one electron to fill outer energy level (8) so each atom shares one electron with the other atom. :: Cl ::: Cl :: Since electrons are shared, no atom gains or loses electrons. The result is a new molecule with a neutral charge. Cl2 Note: the force of attraction between the “ –” electrons and both “+” nucleus’ is what holds the molecule together.

What is a Molecule? A molecule is the neutral particle formed as a result of atoms sharing electrons. Example: H. + .H  H:H Two hydrogen atoms each share one electron to form a more stable hydrogen molecule, or “H2”.

Polar and Non-polar Molecules Molecules can be classified as polar or non-polar. Polar molecules are molecules that have two opposite charged ends, or poles.

Why Do Poles Form? The nucleus of the atom containing more protons (here Oxygen with +8 nucleus) has a greater force of attraction for the “-” electrons than the less attractive Hydrogen’s with +1 in each nucleus. As a result, the oxygen end of a water molecule has a slight negative charge while the hydrogen end has a slight positive charge. The result is a polar molecule.

Non-polar Molecules Non-polar molecules are molecules that do not have charged ends. Example: Cl2 ::Cl:::Cl:: The equal number of protons in both nuclei results in an equal force of attraction for the shared electrons, therefore, no poles are created.

Non-polar Molecules

Writing Chemical Formulas “Nomenclature” is the system of shorthand used for writing chemical formulas and equations.

Nomenclature Terms - Subscript A subscript is a number placed below and to the right of an elements symbol. This number indicates how many atoms of that element are present. (the number 1 does not need to be written since the symbol already indicates one atom is present). Examples: O2 = 2 oxygen atoms Cl2 = 2 chlorine atoms H2O= 2 hydrogen atoms and 1 oxygen atom C6H12O6 = 6 carbon atoms and 12 hydrogen atoms and 6 oxygen atoms

Nomenclature Terms - Superscript A superscript is a number placed above and to the right of an elements symbol. This number indicates the charge of an atom, or the atom’s oxidation number. Examples: O-2 means that the oxygen gained 2 “-”electrons and now has a -2 charge. K+1 means that the potassium atom has lost one electron and has a +1 charge.

Nomenclature Terms - Oxidation Number An oxidation number is the number of electrons that an atom gains, loses or shares when bonding with another atom. Example: when Lithium (Li) gives away an electron it becomes Li+1. The +1 is the oxidation number; it indicates that lithium has lost one “-” electron and the result is a +1 charge on the atom.

Nomenclature Terms - Coefficient A coefficient is the number in front of each “item” in a chemical equation. The coefficient shows how many of each “item” there is in the equation. Example: 6 H2 + 6 O  6 H2O

How Do We Write Chemical Formulas? Using a series of steps allows us to correctly write chemical formulas.

Steps For Writing The Chemical Formula For Binary Compounds Write the symbol of the element with “+” oxidation number (all metals and hydrogen). Example: Ca Next write the symbol of the element with the “-” oxidation number (all nonmetals). Example: Ca F Write in the oxidation numbers for each element. Example: Ca+2 F-1 Balance the formula. There must be an equal number of “+” and “-“ charges in the completed formula to have a neutral compound. Example: Ca+2 2F-1 Put in subscripts so the sum of the charges is equal to zero. Example: Ca1F2 , drop the unnecessary “1” and get CaF2.

Criss-Cross Method A Short Cut is to use the criss-cross method. Change the oxidation number to a subscript for the other element Example: Ca+2 F-1 becomes Ca1F2 Drop the unnecessary “1” and get CaF2.

Naming Binary Compounds Write the name of the first element. Example: Calcium. Write the root of the name of the second element. Ex: Calcium Flour. Add the suffix “ide” to the root. Example: Calcium Fluoride.

How Do Elements Combine? Elements combine through chemical bonding. Chemical bonding involves chemical reactions.

What Is Involved In Chemical Reactions? The theory of Conservation of Matter states that matter can not be created nor destroyed.

What Is Involved In Chemical Reactions? Reactants are the elements or molecules involved at the start of a chemical reaction. Products are the results of the chemical reaction. What we are left with after the reaction.

How Do We Know What Happens During Chemical Reactions? Chemical equations are a way of showing what is taking place during a chemical reaction by using numbers and symbols. Ex: Ag + H2S  Ag2S + H2

What is a Balanced Equation? A balanced equation is when there are the same number of atoms of each element on both sides of the chemical equation. What we start with = what we end with. Ex: 2Ag + H2S  Ag2S + H2

Balancing Chemical Equations Describe the chemical reaction in words. Ex: silver nitrate plus sodium chloride yields silver chloride plus sodium nitrate. Write the chemical equation using formulas and symbols. Ex: AgNO3 + NaCl  AgCl + NaNO3 Check for balance. (make a table) Determine the coefficients (if necessary).

Balance The Chemical Equation For How The Human Body Makes Energy Glucose plus oxygen produces carbon dioxide and water. C6H12O6 + O2  CO2 + H2O Balanced = C6H12O6 + 6O2  6CO2 + 6H2O

Why Are Chemical Reactions Important? Life could not exist without chemical reactions. In chemical reactions atoms rearrange themselves forming all of life’s substances and compounds.

Types of Chemical Reactions There are four (4) types of chemical reactions, all are based on the way atoms rearrange themselves during the reaction. These 4 are synthesis reactions, decomposition reactions, single replacement, double displacement.

Synthesis Reaction Synthesis reactions are when 2 or more substances combine to form another substance. Most synthesis reactions give off energy in the form of heat and light. General formula for synthesis reactions: A + B  AB Example: Aluminum + oxygen produces aluminum oxide Al + O2  Al2O3 Balanced: 4 Al + 3O2  2 Al2O3

Decomposition Reaction Decomposition reactions are when one substance breaks down into simpler substances. Most decomposition reactions require the addition of energy (need energy) General formula for decomposition reactions: AB  A + B Example: Carbonation (bubbles) in soda Decomposition of carbonic acid yields water and carbon dioxide. H2CO3  H2O + CO2

Single Replacement Reaction Single replacement reactions are when one element replaces another element in a compound. General formulas for single replacement reactions: A + BC  AC + B (the positive ion (B) is replaced (by A)) D + BC  BD + C (the negative ion (C) is replaced (by D)) Example: Tarnish Aluminum plus silver sulfide yields aluminum sulfide plus silver 2Al + 3AgS  Al2S3 + 3Ag

Double Displacement Reactions Double displacement reactions are when the positive ion of one compound replaces the positive ion of another compound forming two new compounds. General formula for double displacement reactions: AB + CD  AD + CB Most acid-base reactions are double displacement reactions. Often a precipitate forms in these reactions. Example: Antacid plus stomach acid yields precipitate plus water Magnesium hydroxide (antacid) plus hydrochloric acid (stomach acid) yields magnesium chloride (precipitate) plus water. Mg(OH)2 + 2HCl  MgCl2 + 2H2O