Describing Chemical Reactions

Writing Chemical Equations In a chemical reaction, one or more substances (the reactants) change into one or more new substances (the products). Chemists use a chemical equation – a quick, shorthand notation – to convey as much information as possible about what happens in a chemical reaction.

Word Equations In shorthand for writing a description of a chemical reaction, the reactants are written on the left and the products on the right. Reactants  products Iron + oxygen  iron(III) oxide

Word Equations To write a word equation, write the names of the reactants to the left of the arrow separated by plus signs; write the names of the products to the right of the arrow, also separated by plus signs.

Chemical Equations A chemical equation is a representation of a chemical reaction. Equations that show just the formulas of the reactants and products are called skeleton equations.

Chemical Equations A skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products. Write the formulas of the reactants to the left of the yields sign (arrow) and the formulas of the products to the right.

Chemical Equations To indicate the physical states of substances you can put a symbol after each formula: (s) for solid (l) for liquid (g) for gas (aq) for an aqueous solution An aqueous solution is a substance dissolved in water.

Chemical Equations Fe(s) + O2(g)  Fe2O3 (s) In many chemical reactions, a catalyst is added to the reaction mixture. A catalyst is a substance that speeds up the reaction but is not used up in the reaction.

Chemical Equations A catalyst is neither a reactant nor a product, so its formula is written above the arrow in a chemical equation. H2O2(aq) H2O(l) + O2 (g) Manganese oxide catalyzes the decomposition of an aqueous solution of hydrogen peroxide to produce water and oxygen. MnO2

Explaining and Writing Skeleton Equations 1. write a sentence that describes this chemical reaction: Na(s) + H2O(l)  NaOH(aq) + H2(g)

Explaining and Writing Skeleton Equations 2. sulfur burns in oxygen to form sulfur dioxide. Write a skeleton equation for this chemical reaction. Include appropriate physical state symbols.

Balancing Chemical Equations An unbalanced equation does not indicate the quantity of the reactants needed to make the product. To balance an equation you need to change the coefficients – small whole numbers that are placed in front of the formulas in an equation in order to balance it.

Balancing Chemical Equations A chemical reaction is also described by a balanced equation in which each side of the equation has the same number of atoms of each element and mass is conserved. The law of conservation of mass states that an equation must be balanced to show mass is conserved.

Balancing Chemical Equations Representing a chemical reaction by a balanced chemical equation is a two-step process. To write a complete chemical equation, write the skeleton equation. Then use coefficients to balance the equation so that it obeys the law of conservation of mass.

Balancing Chemical Equations Hydrogen and oxygen react to form water. The reaction releases enough energy to launch a rocket. Write a balanced equation for the reaction. Step 1: Write correct formulas to give the skeleton equation H2(g) + O2(g)  H2O(l)

Balancing Chemical Equations Step 2: count the number of each kind of atom and balance by making each atom of a specific element on the left equal to each atom of a specific element on the right. 2H2(g) + O2(g)  2H2O(l)

Balancing Chemical Equations 3. Balance each equation: A. AgNO3 + H2S  Ag2S + HNO3 B. Zn(OH)2 + H3PO4  Zn3(PO4)2 + H2O

Balancing Chemical Equations 4. rewrite these word equations as balanced chemical equations. A. hydrogen + sulfur  hydrogen sulfide B. iron(III) chloride + calcium hydroxide  iron(III) hydroxide + calcium chloride

Balancing Chemical Equations 5. balance each equation. A. FeCl3 + NaOH  Fe(OH)3 + NaCl B. CS2 + Cl2  CCl4 + S2Cl2