Chemical Equations Study Guide •
When atoms are joined together, they form a bond.
Atoms bond together for a very simple reason: atoms like to have full valence shells. (8 electrons) Example: water - 2 Hydrogen atoms bonded to 1 Oxygen atom. H2O
When an atom combines with another atom (same element), this forms an element molecule .
Examples: H2 or Na3 (notice how it is 2 or more of the same element) • Compound Molecules: A combination of two or more different elements
Examples: NaCl or H2O (notice how there are two or more different elements) • A chemical equation is a symbolic representation of a chemical reaction. Equation Example: CH4 + 2 O2 → CO2 + 2 H2O • Chemical Formulas-show the elements & number of atoms of each element in a molecule Example: H2SO4 (This formula has 2 hydrogen atoms, 1 Sulfur Atom, and 4 Oxygen Atoms)
This same formula has a total of 7 atoms. If you are asked to find how many elements than all you do is count the different chemical symbols only. This formula has 3 different elements .
Subscripts show the number of atoms of that particular symbol only (the symbol to its left).
Example: 4H2O (the 2 is the subscript- it means there are two atoms of hydrogen)
The Coefficients represents the number of molecules of that compound or atom needed in the reaction. Example: 4H2O (the 4 is the coefficient- it means there are four compound molecules of water)
• A coefficient is distributed to ALL elements in a compound. For that same formula 4H2O, the coefficient of 4 distributes to the H and O. We multiply coefficients by the subscripts. The total atoms here would be 8 H(hydrogen) and 4 O (oxygen). •
CH4 + 2 O2 → CO2 + 2 H2O
Reading Chemical Equations
The two sides of the equation are separated by an arrow (yield).
The combinations of chemicals before (left) the arrow are called reactants . Combinations of chemicals to the right of the arrow are called Product .
The Law of Conservation of Mass states that in a chemical reaction, the quantity or amount of each element does not change.
This means that each side of the equation must represent the same quantity of each element; in other words have the same number of each kind of atom.
Na + O2 → Na2O We use a T- chart to figure if it is balanced
Na + O 2
In order for this equation to be balanced, there must be equal amount of Na on the left hand side and on the right hand side. Right now, there is 1 Na atom on the left but 2 Na atoms on the right. This equation is not balanced.