Chemical Reactions Chemistry – Chapter 10

Reactions and Equations Chemical reaction – process by which the atoms of 1 or more substances are rearranged to form different substances Representing Chemical Reactions ◦ Reactants – starting substances ◦ Products – substances formed during reaction ◦ Equations used by chemists show the direction in which the reaction progresses  Represented by an arrow

◦ Equations use symbols to show physical states  Solids – (s)  Liquids – (l)  Gases – (g)  Water solution – (aq) ◦ Skeleton equation – uses chemical formulas to identify reactants and products  Ex:Fe(s) + Cl 2 (g)FeCl 3 (s) ◦ Chemical equation – equation that shows the number of atoms of each reactant and each product is equal on both sides of the arrow  Shows that matter is conserved during a reaction

Balancing Chemical Equations ◦ Coefficient – number written in front of a reactant or product  Whole numbers  Not written if value is one ◦ Steps for balancing equations:  1. Write skeleton equation for reaction  2. Count the atoms of the elements in reactants  3. Count the atoms of the elements in products  4. Change the coefficients to make the number of atoms of each element equal on both sides of the equation  5. Write the coefficients in their lowest ratio

Classifying Chemical Reactions Synthesis Reaction – chemical reaction in which 2 or more substances react to produce a single product Combustion Reaction – oxygen combines w/ a substance and releases energy in the form of heat and light ◦ Some (NOT ALL) combustion reactions are also synthesis reactions

◦ Hydrocarbons contain C and H and burn in O to yield CO 2 and H 2 O  Ex: methane, gasoline, diesel Decomposition Reaction – single compound breaks down into 2 or more elements or new compounds ◦ Often require energy source (e.g., heat)

◦ Ex: Ammonium nitrate breaks down into dinitrogen monoxide and water: NH 4 NO 3 (g)N 2 O(g) + 2H 2 O(g) ◦ Ex: Air bags inflate rapidly as sodium azide pellets decompose producing nitrogen gas: 2NaN 3 (s)2Na(s) + 3N 2 (g) Replacement Reaction – replacement of an element in a compound ◦ Single-replacement reaction – reaction in which the atoms of one element replace the atoms of another element in a compound

◦ 3 types of single-replacement reactions:  Metal replaces a hydrogen in a water molecule  One metal replaces another metal in a compound dissolved in water  Metals differ in reactivity – see Fig  Replacement of a nonmetal in a compound by another nonmetal  Halogens are frequently involved and differ in reactivity – see Fig ◦ Double-replacement reaction – exchange of ions between 2 compounds

 Double-replacement reactions produce either a precipitate, gas, or water  Precipitate – solid produced during a chemical reaction in a solution Predicting products of chemical reactions – see Table 10-3

Reactions in Aqueous Solutions Solute – substance dissolved in water Solvent – most plentiful substance in a solution Aqueous Solutions - solutions in which the solvent is water Reactions That Form Precipitates – some reactions that occur in aqueous solutions produce precipitates

◦ Chemists use ionic equations to show the details of reactions that involve ions in aqueous solutions ◦ Complete ionic equation – ionic equation that shows all of the particles in a solution as they realistically exist. 2Na + (aq) + 2OH - (aq) + Cu 2+ (aq) + 2Cl - (aq) 2Na + (aq) + 2Cl - (aq) + Cu(OH) 2 (s) ◦ Spectator ions – ions that do not participate in a reaction 2Na + (aq) + 2OH - (aq) + Cu 2+ (aq) + 2Cl - (aq) 2Na + (aq) + 2Cl - (aq) + Cu(OH) 2 (s)

◦ Net ionic equation – ionic equations that include only the particles that participate in the reaction 2OH - (aq) + Cu 2+ (aq) Cu(OH) 2 (s) Reactions That Form Water – water molecules produced in the reaction increase number of solvent particles ◦ No evidence of chemical reaction observable b/c water is colorless/odorless HBr(aq) + NaOH(aq) H 2 O(l) + NaBr(aq) H + (aq) + Br - (aq) + Na + (aq) + OH - (aq) H 2 O(l) + Na + (aq) + Br - (aq) H + (aq) + OH - (aq)H 2 O(l)

Reactions That Form Gases – carbon dioxide, hydrogen cyanide, and hydrogen sulfide are commonly produced ◦ Some breads and other baked goods rise during baking b/c of trapped CO 2 gas produced by the reaction between baking powder and water ◦ Overall equation – p. 298