Chemical Reactions - Part 2 A Change of Properties How can you be sure a chemical reaction is happening and not a physical change? Ex: boiling water (bubbles, gas) We have signs but… The most important sign: a new substance forms

And the properties of these new substances are very different from those of the starting substances.

A chemical bond is a force that holds atoms together in a molecule. Bonds: Holding Molecules Together A chemical bond is a force that holds atoms together in a molecule. For a chemical reaction to occur, old bonds must be broken, while new bonds form.

In order for bonds to break, the molecules must run into each other with enough energy to break their bond. Once the bonds are broken, the atoms rearrange and form new bonds.

SECTION 2 Every element on the periodic table has a chemical symbol, or abbreviation, by which it is easily identified. Example: He These symbols are put together to represent molecules. Molecules are represented by chemical formulas. Examples: CO2 H2SO4

Chemical Formulas A chemical formula is a short way to represent a substance. It is formed from chemical symbols and numbers. The symbols in the formulas represent the elements.

The numbers in the formula represent how many atoms of each element are in the substance. The numbers in a formula are called subscripts. This number is written after and below the element. Example: H2 If there is no subscript, it means there is only one atom of that element. Never insert a subscript of one. It is just understood to be there. Example: CO2

If the substance is a covalent compound, then its name will often help you write the formula. The names of covalent compounds often contain number prefixes, which helps you know what subscripts to insert. (chart p 393) Example: dinitrogen monoxide N2O

Phosphorus pentachloride Dinitrogen trioxide Try Try these: Sulfur trioxide Phosphorus pentachloride Dinitrogen trioxide

What About Formulas for Ionic Compounds? Metal and nonmetal make ionic compounds. To write the formula for an ionic compound, make sure the compound’s charge is 0. The formula must have subscripts that cause the charges the ions to cancel out.

Use oxidation numbers! The oxidation states of the elements involved in the molecule determine if subscripts must be added. Example: 2+ 1- The oxidation state of an atom tells you its bonding capacity.

A Review Oxidation Numbers for Those Who Need a Refresher How to Determine Oxidation State: You must know the number of valence electrons to determine oxidation state. If the atom has 1, 2, or 3 valence electrons, it will give them away in an effort to have a filled outermost energy level. If the atom gives away electrons, its oxidation state will be positive because it now has more protons than it does electrons. Depending on how many valence electrons it gave away, the atom’s oxidation state will be 1+, 2+, or 3+. If an atom has 5, 6 or 7 valence electrons, it will take electrons to get up to 8, have a filled outer energy level, and become stable. If an atom takes valence electrons, it has a negative oxidation state because it now has more electrons than it has protons. Depending on how many electrons it took, the atom will have an oxidation state of 1-, 2-, or 3-.

When a chemical formula is written, the positive oxidation state element is always written first in the formula. The oxidation states within a compound must add up to equal zero because the compound is neutral. In order to make the oxidation states equal zero, subscripts are inserted in the chemical formula. Example: Mg2+Cl1- = MgCl2

Chemical Equations A chemical equation uses symbols and formulas to describe a chemical reaction. The components of a chemical equation are: a. symbols b. formulas c. subscripts d. coefficients e. arrow