Equations for Chemical Reactions Foothill Chemistry

What is a Chemical Reaction? A process by which one or more substances are transformed into one or more different substances. The equation for a chemical reaction represents the identities and relative molecular or molar amounts of the reactants and products using symbols and formulas. Example: (NH4)2Cr2O7(s)  N2(g) + Cr2O3(s) + 4H2O(g)

Indications of a Chemical Reaction Change in energy as heat or light or cold Production of a gas (bubbles) Formation of a precipitate Color change / smell change

Characteristics of Chemical Equations Equation must represent known facts Reactants and products must be identified Through experimentation or reference sources Equation must contain the correct formulas for the reactants and products The chemical formulas have to be correct The law of conservation of mass must be satisfied Mass is not created or destroyed

Steps in Writing a Chemical Equation Write a word equation – an equation in which the reactants and products are represented by words Methane + Oxygen react to yield (make / produce / form) Carbon Dioxide + Water Replace the names of the products and reactants with their proper chemical formulas Use an arrow  to represent the “react to yield” statement. CH4(g) + O2(g)  CO2(g) + H2O(g) Add the necessary coefficients to balance the equation This step meets the “Conservation of Mass” requirement CH4(g) +2O2(g)  CO2(g) + 2H2O(g)

Symbols of Chemical Reactions Explanation  “yields”; points to results of the reaction (l) Liquid state of matter  Reversible reaction (the reaction goes in both directions) (aq) Aqueous solution (dissolved in water) (s) Solid state of matter or precipitate (g) Gaseous state of matter  Precipitate (not solid)  Indicates a gaseous product

Symbols or Words above the  Δ or the word, ”heat”, above the arrow means that the reactants are heated in the reaction # atm, the number symbol will be an numeric symbol and will indicate the atmospheric pressure at which the reaction is carried out The word, “pressure” indicates that the pressure at which the reaction is carried out is higher than normal atmospheric pressure #oC – The temperature at which the reaction is carried out (e.g. 0oC) (Chemical formula) – The formula of a catalyst. It may be an element or a chemical. The catalyst increases the reaction rate without being used in the reaction.

Reversible Reaction A chemical reaction in which the products re-form the original reactants. If that reaction occurs equally in both directions, the arrow lengths will be equal. If they occur greater in one direction or another, the arrows will show a longer arrow for the reaction that produces a greater amount of product.

Chemical Reaction Significance The coefficients of a chemical reaction indicate relative, not absolute, amounts of reactants and products The relative masses of the reactants and products can be determined from the reaction’s coefficients. The reverse reaction has the same relative amounts of substances as the forward reaction.

Balancing Chemical Equations Identify names of the reactants and products and write a word equation. Write a formula equation by substituting correct formulas for the reactants and products, along with any other necessary symbols. Balance the formula equation according to the law of conservation of mass. Balance the different types of atoms one at a time Balance the atoms that appear only once on each side of the equation Balance polyatomic ions that appear on both sides of the equation as single units Balance H and O atoms after all of the other atoms have been balanced Count the atoms to be sure the equation is in balance.

Word Problem Practice zinc and lead(II) nitrate yield zinc nitrate and lead aluminum bromide and chlorine yield aluminum chloride and bromine sodium phosphate and calcium chloride yield calcium phosphate and sodium chloride aluminum and hydrochloric acid yield aluminum chloride and hydrogen gas potassium chlorate, when heated, yields potassium chloride and oxygen gas calcium hydroxide and phosphoric acid yield calcium phosphate and water copper and sulfuric acid yield copper(II) sulfate and water and sulfur dioxide hydrogen and nitrogen monoxide yield water and nitrogen

Types of Chemical Reactions Synthesis Reaction Decomposition Reaction Single Displacement (Replacement) Reaction Double Displacement (Replacement) Reaction Combustion Reaction

Make It and Break It Synthesis Reaction (Composition Reaction) Two or more substances combine to form a new compound A + X  AX “Make It” Decomposition One substance reacts to form two or more simpler compounds AX  A + X “Break It”

Common Types of Reactions Synthesis Making a sulfide or oxide by reacting with sulfur or oxygen 16 Rb(s) + S8(s)  8Rb2S(s) C(s) + O2(g)  CO2(g) Reacting with Halogens (Group 17 or 7A) 2Na(s) + Cl2(g)  2NaCl(s) Reacting with Oxides and Water CaO(s) + H2O(l)Ca(OH)2(s)

Common Types of Reactions Decomposition Breaking a Binary Compound Electrolysis (Breaking using Electricity) 2H2O(l)  2H2(g) + O2(g) Metal Carbonates – Produces metal oxide +CO2 Metal Hydroxides – Produces metal oxide + water Metal Chlorate – Produces metal chloride + O2 Acid Decomposition – Produces non-metal oxide and water (can be a dioxide or trioxide) Electricity

Switcheroo Reactions Single-displacement (sometimes single- replacement) One element replaces a similar element in a compound A + BX  AX + B (Cation replacement) Y + BX  BY + X (Anion Replacement) Double-Displacement (sometimes double- replacement) The ions of two compounds exchange places in an aqueous solution to form two new compounds. AX + BY  AY + BX

Common Types of Reactions Single Displacement Displacement of a metal in a compound by another metal (cations) 2Al(s) + 3Pb(NO3)2(aq)  2 Al(NO3)3 (aq) + 3 Pb(s) Displacement of hydrogen in water by a metal (anions) More reactive metals (farther left on table) form hydroxides and hydrogen gas 2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g) Less reactive metals react with steam to form a metal oxide and hydrogen gas 3Fe(s) + 4H2O(g)  Fe3O4(s) + 4H2(g)

Common Types of Reactions Single Displacement Displacement of hydrogen in acid by metal (cations) Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) Displacement of halogens (anions) 2NaBr(aq) + Cl2(g)  2NaCl(aq) + Br2(l) ** Halogens can only displace halides if they are higher on the periodic table than the one that is being replaced. Br2 + KCl(aq)  No Reaction

Common Types of Reactions Double Displacement Formation of a Precipitate 2KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2KNO3(aq) Formation of a Gas FeS(s) +2HCl(aq)  H2S(g) + FeCl2(aq) Formation of Water HCl(aq) + NaOH(aq)  H2O(l) + NaCl(aq)

BOOM! Combustion Reaction A substance combines with oxygen, releasing a large amount of energy in the form of heat and light Burning natural gas, propane, and wood are also examples of combustion reactions C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g)