Chemical Reactions Chapter 3 3.7-3.9

3.7 Gas-Forming Reactions Formation of a gas: gases may form directly in a double replacement reaction or can form from the decomposition of a product such as carbonic acid or sulfurous acid (table on page 123) often metal carbonates and acids Ex. Excess hydrochloric acid solution is added to a solution of potassium sulfite. H+ + SO3-2  H2O(l) + SO2(g)

Classifying Exchange Reactions CuCO3(s) + H2SO4(aq)  Ba(OH)2(s) + HNO3(aq)  CuCl2(aq) + (NH4)2S(aq) 

3.8 Oxidation-Reduction Reactions These reactions involve the transfer for electrons. The oxidation numbers of at least two elements must change. These may be single replacement, combination, and decomposition reactions. When a problem mentions an acidic or basic solution, it is probably redox.

Oxidation-Reduction Reactions A substance that loses electrons is oxidized and is the reducing agent. A substance that gains electrons is reduced and is the oxidizing agent. LEO goes GER If one substance is oxidized, another substance must be reduced. Hence, redox reactions!

Oxidation-Reduction Reactions 2Ag+ + Cu  2Ag + Cu+2 Oxygen may not be present; but a transfer of electrons results in a charge reduction. Silver ion accepts electrons from copper and is reduced to silver (silver ion is the oxidizing agent). Copper donates electrons to silver ion and is oxidized to copper (II) ion and is the reducing agent.

Assigning Oxidation States The oxidation state of… an atom in an element is zero (Na, O2) monatomic ion is same as charge (Na+) fluorine always has an oxidation number of -1 oxygen is usually -2 except for peroxides/fluorine Cl, Br, and I are -1 except with oxygen or fluorine hydrogen is +1 in covalent compounds, -1 in binary hydrides, such as NaH In a compound, the sum must be zero, KMnO4 For ions, sum must equal overall charge, Cr2O7-2

Determining Oxidation Numbers Assign an oxidation number to the underlined atom in each ion or molecule. Fe2O3 H2SO4 CO32- NO2-

3.9 Classifying Reactions in Aqueous Solution Precipitation Reactions: Ions combine in solution to form an insoluble reaction product. Acid-Base Reactions: Water is a product of many acid-base reactions, and the cation of the base and the anion of the acid form a salt. Gas-Forming Reactions: often involve metal carbonates and acids Oxidation-Reduction: NOT ion exchange; electrons are transferred

Practice Problem Classify each of the following as a precipitation, acid-base, gas-forming, or redox reaction. Write the net ionic for each. CuCO3(s) + H2SO4(aq)  Ga(s) + O2(g)  Ba(OH)2(s) + HNO3(aq)  CuCl2(aq) + (NH4)2S(aq) 

Homework After reading Sections 3.7-3.9, you should be able to do the following… P. 137c-d (45,49-52,81-85)