Balancing Chemical Equations Chapter 10

Objectives: Identify the reactants and products in a chemical equation From the word equation for a chemical reaction, write and balance the formula equation for the reaction Identify a chemical reaction as one of the five major reaction types

Chemical Equations: Reactant 1(s) + Reactant 2(aq)  Product 1(aq) + Product 2(g) A chemical equation is the shorthand describing the interaction of compounds at the molecular level Reactants, or the starting materials appear to the left of the arrow The arrow is read as “yield” or “react to produce” Products, or the end materials, appear to the right of the arrow Symbols are also used to represent the physical state of matter of the reactants and products. (s) solid (l) liquid (g) gas (aq) solutions of a substance dissolved in water

Word Equations Iron is a solid and chlorine is a gas. They react to product a brown cloud composed of particles of iron (III) chloride. Word equation: iron (s) +chlorine (g)  iron(III) chloride (s) Read this as: “iron and chlorine react to produce iron(III) chloride”

Skeleton Equations A skeleton equation uses chemical formulas instead of words to describe a chemical equation, but lacks the numbers to describe the proportionality of the chemical reaction. Fe(s) + Cl2(g)  FeCl3(s) If carbon and sulfur solids react to form a carbon disulfide solution, what is the skeleton equation? Make certain to indicate the physical states of the reactants. C(s) + S(s)  CS2(aq)

Sample Time! Write the skeleton equation. In water, iron(III) chloride reacts with sodium hydroxide, producing solid iron(III) hydroxide and sodium chloride. FeCl3(aq) + NaOH(aq) Fe(OH)3(s) + NaCl(aq)

Sample Time! Write the skeleton equation. Liquid carbon disulfide reacts with oxygen gas, producing carbon dioxide gas and sulfur dioxide gas. CS2(l) + O2(g)  CO2(g) + SO2(g)

Recall: The Law of the Conservation of Matter states that matter can neither be created nor destroyed.

Balancing Chemical Equations Because a skeleton equation does not indicate the proportionality of the reactants, it does not show how the Law of the Conservation of Matter is obeyed. E.g. Fe(s) + Cl2(g)  FeCl3 This shows the identity and physical states of the reactants and products, but not the ratios in which they combine. We add coefficients to this formula to identify the correct ratios and thereby produce a balanced chemical equation. How would you balance this equation? Add the proper coefficients to each reactant and the product, remembering that matter is neither created or destroyed. 2Fe(s) + 3Cl2(g)  2FeCl3

Final Tips: Chemical Equations The process in which a sample dissolves in water will be indicated by equations such as the following:  H2O + C12H22O11(s)  C12H22O11(aq) A chemical equation is a statement of what can happen, not necessarily what will happen. The following equation, for example, does not guarantee that hydrogen will react with oxygen to form water. 2 H2(g) + O2(g)  2 H2O(g)

Sample Time! Write the balanced equation. H2(g) + Cl2(g)  HCl (g) H2(g) + Cl2(g)  2HCl (g) NaOH(aq) + CaBr2(aq)  Ca(OH)2 (s) + NaBr(aq) 2NaOH(aq) + CaBr2(aq)  Ca(OH)2 (s) + 2NaBr(aq) H2(g) + O2(g)  H2O(g) 2 H2(g) + O2(g)  2 H2O(g) Al(s) + Fe2O3(s)  Al2O3(s) + Fe(l) 2 Al(s) + Fe2O3(s)  Al2O3(s) + 2 Fe(l)

Sample Time! Write the balanced equation. Example: Consider what happens when propane (C3H8) burns in air to form CO2 and H2O. Write the skeleton equation: C3H8(g) + O2(g)  CO2(g) + H2O(g) Balance the skeleton (it’s okay to ignore states at this point): 1 C3H8 + _____ O2  3 CO2 + _____ H2O 1 C3H8 + _____ O2  3 CO2 + 4 H2O 1 C3H8 + 5 O2  3 CO2 + 4 H2O There are now three carbon atoms, eight hydrogen atoms, and 10 oxygen atoms on each side of the equation. The balanced equation for this reaction is therefore written as follows. C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

Sample Time! Write the balanced equation. Write a balanced equation for the reaction that occurs when ammonia burns in air to form nitrogen oxide and water. ___ NH3 (g) + ___ O2 (g)  ___ NO (g) + ___ H2O (l) 1 NH3 (g) + ___ O2 (g)  1 NO (g) + ___ H2O (l) 2 NH3 (g) + 5/2 O2 (g)  2 NO (g) + 3 H2O (l) 4 NH3 (g) + 5 O2 (g)  4 NO (g) + 6 H2O (l)

Homework! Do the balancing equations homework tonight. Tomorrow, we will discuss reaction types and the homework.