Balancing Equations. n Chemical reactions occur when bonds (between the electrons of atoms) are formed or broken. n Chemical reactions involve

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1 Balancing Equations n Chemical reactions occur when bonds (between the electrons of atoms) are formed or broken n Chemical reactions involve n changes in the chemical composition of matter n the making of new materials with new properties n energy changes: n Bond breaking absorbs Energy (endothermic process) n Bond making releases Energy (exothermic process) n Symbols represent elements n Formulas describe compounds n Chemical equations describe a chemical reaction

2 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.

3 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.

4 Chemical Equations n n n n 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

5 Chemical Equations n All chemical equations have reactants and products. n We express a chemical equation as follows: Reactants Products n The arrow is equivalent to an = math. When we describe the equation we use the word yields or produces instead of equals n Example C + O 2 à CO 2 n This reads carbon plus oxygen react to yield carbon dioxide

6 Balancing a Chemical Equation n A chemical equation is balanced when the ions or atoms found on the reactant side of the equation equals that found on the product side. n The arrow can be considered the balance point.

7 Symbols Used in Equations nsolid (s) nliquid (l) ngas (g) naqueous solution (aq) (dissolved in water) ncatalyst H 2 SO 4 or Pt nescaping gas ( ) nchange of temperature/ heat energy (D or + 3kJ or 3kJ) *there is no subtraction a negative sign means released/exothermic

8 Chemical Reactions and Chemical Equations Reactants Products Driving forces: a) Color change b) Formation of a solid/precipitate c) Evolution of a gas d) Evolution or absorption of heat

9 Balancing Equations n 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. nchanging the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent) n 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.

10 Subscripts vs. Coefficients n The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number, of molecules of the compound.

11 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 produces---> 2 moles of Al 2 O 3

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13 Steps to Balancing Equations There are four basic steps to balancing a chemical equation. 1. 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! 2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. 3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. 4. Check your answer to see if: n The numbers of atoms on both sides of the equation are now balanced. n The coefficients are in the lowest possible whole number ratios. (reduced)

14 Some Suggestions to Help You Some helpful hints for balancing equations: n 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. n 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) n (Shortcut) Polyatomic ions that appear on both sides of the equation should be balanced as independent units

15 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.

16 Balancing Equations Sodium phosphate + iron (III) oxide à sodium oxide + iron (III) phosphate 2 Na 3 PO 4 + Fe 2 O > 3 Na 2 O + 2 FePO 4