BALANCING CHEMICAL EQUATIONS

REACTIONS CONSERVE MASS The Law of Conservation of Mass States that in ordinary chemical or physical changes, mass is neither created nor destroyed. React vinegar and baking soda Produces ________________ which is a gas. The products including this gas, if captured, is the same mass per mole as the reactants consumed.

REACTIONS REARRANGE ATOMS Products and reactants of a reaction are made up of the same number and types of atoms. The molecules may change but the atoms within them do not. If you have H, O, and C are the reactant side of a reaction, you must have ___, ___, and ____ on the product side. The ONLY thing we can change is the quantity of MOLECULES, or the number of moles of molecules

CONSIDER THE FOLLOWING: Sodium metal reacts with water to yield sodium hydroxide and hydrogen gas. Write out this word equation as a chemical equation. Reactants: Products: Number of: Na: ____, H ____, O ____

BALANCING EQUATIONS To show that a reaction satisfies the law of conservation of mass, its equation must be BALANCED. As we stated before, we can change ONLY the number of moles of molecules present Both of the reactants and products We change this by adding a coefficient out front of the reactant or product.

STOICHIOMETRIC COEFFICIENTS To balance an equation, you need to make the number if atoms for each element the same on the reactants’ side and on the products’ side. You CANNOT change the formulas of any of the substances. Only change the COEFFICIENT A small whole number that appears as a factor in front of a formula in a chemical equation.

MORE ON COEFFICIENTS H 2 O represents water (2 H, 1 O) 2 H 2 0 represents two moles of water (4 H, 2 O) H represents hydrogen peroxide (2 H, 2 O) A small change of the subscripts within the molecule will change the compound completely AND alter the entire reaction.

MORE ON COEFFICIENTS Determine the number of each atom in the following examples: _1_Fe 2 O 3 + _3_H 2  _3_ Ca(NO 3 ) 2 + _1_ C 6 H 12 O 6 + _6_ O 2  _6_ H _6_ CO 2

RULES TO CONSIDER WHEN BALANCING 1.Identify reactants and products If no equation is provided, identify the reactants and products and write an unbalanced equation for the reaction. (You may find it helpful to write a word equation first.) If not all chemicals are described in the problem, try to predict the missing chemicals based on the type of reaction.

RULES TO CONSIDER WHEN BALANCING 2.Count atoms Count the number of atoms of each element in the reactants and in the products, and record the results in a table. Identify elements that appear in only one reactant and in only one product, and balance the atoms of those elements first. Delay the balancing of atoms (often hydrogen and oxygen) that appear in more that one reactant or product. If a polyatomic ion appears on both sides of the equation, treat it as a single unit in you counts.

RULES TO CONSIDER WHEN BALANCING 3.Insert coefficients Balance atoms one element at a time by inserting coefficients. Count the atoms of each element frequently as you try different coefficients. Watch for elements whose atoms become unbalanced as a result of your work. Try the odd-even technique (double the odd number) if you see an even number of a particular atom on one side of an equation and an odd number of that atom on the other side.

RULES TO CONSIDER WHEN BALANCING 4.Verify your results Double-check to be sure that the numbers of atoms of each element are equal on both sides of the equation. Make sure your own work did not mess any “previously balanced” atoms.

PRACTICE: Balance the following example: Iron (III) oxide and hydrogen react to yield iron and water.

PRACTICE: Balance the following examples: ___P 4 + ___O 2  ___P 2 O 5 ___C 3 H 8 + ___O 2  ___CO 2 + ___H 2 O Silicon reacts with carbon dioxide to form silicon carbide, SiC, and silicon dioxide

NEVER CHANGE SUBSCRIPTS TO BALANCE A REACTION!!!

TRY THE FOLLOWING ___NH 3 + ___O 2  ___NO + ___ H 2 O Iron (II) hydroxide reacts with hydrogen peroxide to yield iron (III) hydroxide.

POLYATOMICS Polyatomics are no different. Let’s Practice ___HgCl 2 + ___AgNO 3  ___Hg(NO 3 ) 2 + ___AgCl Calcium phosphate and water are produced when calcium hydroxide reacts with phosphoric acid.

PRACTICE, PRACTICE, PRACTICE This will only get better with practice. You will soon spot clues and patterns that will allow you to balance reactions almost instantly. Be patient, it will come!! Quiz tomorrow: “Describing Chemical Reactions” “Balancing Chemical Reactions” Study both slideshows and practice balancing Page 274, #4, 5, and 6 are great balancing practice