8.1 Describing Chemical Reactions May 12, 2015

8.1 Describing Chemical Reactions A chemical reaction is the process by which one or more substances are changed into one or more different substances. In any chemical reaction, the original substances are known as the reactants and the resulting substances are known as the products. According to the law of conservation of mass, the total mass of reactants must equal the total mass of products for any given chemical reaction. A chemical equation represents, with symbols and formulas, the identities and relative molecular or molar amounts of the reactants and products in a chemical reaction.

8.1 Describing Chemical Reactions There are four ways to tell if a chemical reaction has taken place: 1. Evolution of energy as heat and light. 2. Production of a gas. 3. Formation of a precipitate. 4. Color change.

8.1 Describing Chemical Reactions The following rules must be satisfied when writing out chemical equations: 1. The equation must represent known facts. All reactants and products must be identified. 2. The equation must contain the correct formulas for the reactants and products.

8.1 Describing Chemical Reactions 3. The law of conservation of mass must be satisfied. The same number of atoms of each element must appear on each side of the chemical equation. A coefficient can be used which is a small whole number that appears in the front of a formula in a chemical equation.

8.1 Describing Chemical Reactions Writing out a chemical equation: Step 1: Write out a word equation. A word equation is an equation in which the reactants and products in a chemical reaction are represented by words. Don’t forget the yield  sign and the state of each reactant or product. (Gas, liquid, solid, aqueous) Step 2: Replace the words in the chemical equation with basic formulas. A formula equation represents the reactants and products of a chemical reaction by their symbols or formulas. Step 3: Balance both sides of the equation. Use coefficients to balance both sides of the equation. Make sure you have the same number of atoms of each element on each side of the equation.

8.1 Describing Chemical Reactions Balancing Tips: Usually the elements of hydrogen and oxygen are balanced LAST. Balance “rare” elements, that is, elements that you have the fewest of in the equation FIRST. Balance atoms of elements that are combined and that appear only once on each side of the equation. Balance polyatomic ions as single units. Don’t confuse coefficients with subscripts which are part of each element or compound. Remember, when you place a coefficient in front of an element or compound that has a subscript, you have to multiply the coefficient by the subscript to get the total number of atoms present. (g) – gas state, (l) – liquid state, (s) – solid state, (aq) – aqueous state. Know what state each product and reactant is in! NO CHARGES!

8.1 Describing Chemical Reactions In many reactions, as soon as the products begin to form, they immediately begin to react with each other and re-form the reactants. In other words, the reverse reaction also occurs. A reversible reaction is a chemical reaction in which the products re- form the original reactants. The reversibility of a reaction is indicated by writing two arrows pointing in opposite directions.

8.1 Describing Chemical Reactions Reactions can also be translated into words from formulas. Consider the following reaction: 2HgO(s)  2Hg(l) + O 2 (g) This reaction can be written as “When heated, solid mercury (II) oxide yield liquid mercury and gaseous oxygen. Remember, when you are writing out word equations, always remember the state each reactant or product is in.

8.1 Describing Chemical Reactions Sample Problems: Write word and formula equations for hydrogen peroxide in an aqueous solution that decomposes to produce oxygen and water. Write word and formula equation for when solid zinc metal reacts with aqueous copper sulfate to produce solid copper metal and aqueous zinc sulfate.

8.1 Describing Chemical Reactions Translate the following equation into sentences: 2ZnO(s) + C(s)  2Zn(s) + CO 2 (g) Na 2 O(s) + 2CO 2 (g) + H 2 O(g)  2NaHCO 3 (s) Write a balanced chemical equation for each: Nitrogen dioxide gas reacts with water to form aqueous nitric acid and nitrogen monoxide gas. Solid potassium chlorate decomposes to form solid potassium chloride and oxygen gas.