Balancing Equations Chemical reactions occur when bonds (between the electrons of atoms) are formed or broken Chemical reactions involve changes in the chemical composition of matter the making of new materials with new properties energy changes: Bond breaking absorbs Energy (endothermic process) Bond making releases Energy (exothermic process) Symbols represent elements Formulas describe compounds Chemical equations describe a chemical reaction 1
Chemical Equations A chemical equation is written as an expression similar to a mathematic equation that can be compared to a recipe that a chemist follows in order to produce desired results. 2
Chemical Equations Their Job: Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al (s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) The numbers in the front are called stoichiometric coefficients The letters (s), (g), and (l) are the physical states of compounds. 3
Chemical Equations Because of the principle of the conservation of matter (matter can not be created or destroyed) an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Law of Conservation of Energy MUST ALSO BE FOLLOWED! Energy changes are written in (endo-/ exothermic reactions) Lavoisier, 1788 4
Chemical Equations All chemical equations have reactants and products. We express a chemical equation as follows: Reactants Products The arrow is equivalent to an = math. When we describe the equation we use the word yields or produces instead of equals Example C + O 2 CO 2 This reads carbon plus oxygen react to yield carbon dioxide 5
Symbols Used in Equations Solid (s) Liquid (l) Gas (g) Aqueous solution (aq) (dissolved in water) Catalyst H 2 SO 4 or Pt Escaping gas ( ) Change of temperature/ heat energy ( or + 3kJ or 3kJ) *there is no subtraction a negative sign means released/exothermic 6
Chemical Reactions and Chemical Equations Reactants Products Evidence that a reaction occurred: a) Color change b) Formation of a solid/precipitate c) Evolution of a gas/bubbling d) Evolution or absorption of heat 7
Balancing Equations When balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent) Think back to naming compounds/ determining formulas. NaCl exists, because Na is + and Cl is -, but NaCl 2 does NOT exist since you would not have a neutral compound! You can t just add a number to a formula to balance an equation. 8
Chemical Equations 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) This equation means 4 Al atoms + 3 O 2 molecules ---produces---> 2 molecules of Al 2 O 3 AND/OR 4 moles of Al + 3 moles of O 2 -- -produces---> 2 moles of Al 2 O 3 9
Steps to Balancing Equations Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. **And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! Take one element at a time, working left to right except for H and O. Metals, then nonmetals are a good way, too. Save H for next to last, and O until last. 10
Steps to Balancing Equations IF everything balances except for O, and there is no way to balance O with a whole number, double all the coefficients and try again. (Because O is diatomic as an element) (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units Check your answer to see if: The numbers of atoms on both sides of the equation are now balanced. The coefficients are in the lowest possible whole number ratios. (reduced) 11
Balancing Equations 2 H 2 (g) + O 2 (g) ---> 2 H 2 O(l) What Happened to the Other Oxygen Atom????? This equation is not balanced! Two hydrogen atoms from a hydrogen molecule (H 2 ) combine with one of the oxygen atoms from an oxygen molecule (O 2 ) to form H 2 O. Then, the remaining oxygen atom combines with two more hydrogen atoms (from another H 2 molecule) to make a second H 2 O molecule. 12
Balance this equation! Na + Cl 2 Na-1 Cl- 2 NaCl Na-1 Cl-1 **note that the number of sodiums balance but the chlorine does not. We will have to use coefficients in order to balance this equation. 13
Inserting subscripts Na + Cl 2 2 NaCl Na-1 Na- 1 2 Cl- 2 Cl- 1 2 ** Now the chlorine balances but the sodium does not! So we go back and balance the sodium. 14
Finally balanced! 2Na + Cl 2 2 NaCl Na-1 2 Na-1 2 Cl- 2 Cl-1 2 **Since the number of each element on the reactant side and the product side of the equation are equal, the equation is balanced. 15
Balancing Equations 2 Al(s) + 3 Br 2 (l) ---> Al 2 Br 6 (s) 16
Balancing Equations Sodium phosphate + iron (III) oxide sodium oxide + iron (III) phosphate 2 Na 3 PO 4 + Fe 2 O 3 ----> Na 2 O 3+ FePO 2 4 17