Chemical Equations Honors Chemistry Mr. Rivas

Chemical Equations A chemical reaction is a process in which substances are changed into different substances. The old substances are called reactants and the new substances are called products. A chemical equation describes what happens in a chemical reaction.

Example of a chemical change The reaction: When solid carbon reacts with oxygen gas, a new substance, carbon dioxide gas is formed and energy is released The equation: C(s) + O 2 (g)  CO 2 (g) Since this is an exothermic reaction we can also write: C(s) + O 2 (g)  CO 2 (g) + heat

Chemical Equation The chemical equation identifies: The original substances (reactants) and the resulting substances (products). The phase of each (s)Solid (l)Liquid (g)Gas (aq) Aqueous (dissolved in water) The number of moles of each substance

Chemical Equation The arrow  reads yields or produces. It separates the reactants to the left from the products to the right. Δ or ø means reactants must be heated. N.R. means no reaction indicates that the reaction is reversible.

Balancing Chemical Equations In a chemical reaction the mass of the products must be the same as the mass of the reactants. Steps: Write the unbalanced equation Balance the equation by making sure that the number of atom of each element is the same in the products and the reactants. Hint: Start by balancing an element that appears in only one reactant and one product

Example Tin oxide is heated with hydrogen to form tin metal and water vapor. Write the unbalanced equation SnO 2 (s) + H 2 (g) Δ Sn(s) + H 2 O(g) Identify which elements are not balanced. Here we have two O atoms in the reactants and only one in the products. We correct this by entering a coefficient of 2 for water …

Example This puts the hydrogen atoms out of balance. SnO 2 (s) + H 2 (g) Δ Sn(s) + 2 H 2 O(g) To fix this we enter a coefficient of 2 for hydrogen. SnO 2 (s) + 2 H 2 (g) Δ Sn(s) + 2 H 2 O(g) Remember: coefficients are multipliers. When we write 2H 2 O we mean 2x2 = 4 H atoms and 2x1 = 2 O atoms.

Remember… When balancing a chemical equation you must never change the formulas of the reactants or products. This means do not change the subscripts of the formulas. The balancing must be achieved by writing the appropriate coefficient(s).

Balancing Equations Exercises 1) H 2 + O 2  H 2 O 2) NaOH + H 2 SO 4  Na 2 SO 4 + H 2 O 3) Na + O 2  Na 2 O 2 4) N 2 + O 2  N 2 O 5 5) Cs + N 2  Cs 3 N 6) NH 3 + HCl  NH 4 Cl 7) Li + H 2 O  LiOH + H 2

Answers to balancing equations 1) 2 H 2 + O 2  2 H 2 O 2) 2 NaOH + H 2 SO 4  Na 2 SO H 2 O 3) 2 Na + O 2  Na 2 O 2 4) 2 N O 2  2 N 2 O 5 5) 6 Cs + N 2  2 Cs 3 N 6) NH 3 + HCl  NH 4 Cl 7) 2 Li + 2 H 2 O  2 LiOH + H 2

Worksheet Complete Worksheet 9.1