UNIT 5 Aqueous Reactions and Solution Stoichiometry Acid-Base Neutralization

Writing Equations for Reactions Involving Ions There are three ways to write equations for reactions involving ions. Molecular equation: HCl (aq) + AgNO 3 (aq)  AgCl (s) + HNO 3 (aq) Here no ions are shown but all reactants and products are identified and their physical states given. Complete or Total Ionic Equation: H + (aq) + Cl - (aq) + Ag + (aq) + NO 3 - (aq)  AgCl (s) + H + (aq) + NO 3 - (aq) Here ALL ions that exist in significant amounts in solution are shown. Spectator ions do not participate in the reaction but are shown. Net ionic equation: Cl - (aq) + Ag + (aq)  AgCl (s) Here only ions which participate in the reaction are shown. Spectator ions are NOT shown.

Writing Equations for Reactions Involving Ions Write the three equations for the reaction between solutions of barium nitrate and sodium phosphate. Molecular equation: 2Na 3 PO 4 (aq) + 3Ba(NO 3 ) 2 (aq)  Ba 3 (PO 4 ) 2 (s) + 6NaNO 3 (aq) Complete or Total Ionic Equation: 6Na + (aq) + 2PO 4 3- (aq) + 3Ba 2+ (aq) + 6NO 3 - (aq)  Ba 3 (PO 4 ) 2 (s) + 6Na + (aq) + 6NO 3 - (aq) Net ionic equation: 2PO 4 3- (aq) + 3Ba 2+ (aq)  Ba 3 (PO 4 ) 2 (s) The net ionic equation says that ANY soluble compound with phosphate and ANY soluble compound with barium ion will react to form the barium phosphate precipitate.

Acid-Base Neutralization Metathesis reactions are reactions of the form: AX + BY  AY + BX If either of the POSSIBLE new compounds AY or BX is: insoluble in water (a precipitate) a weak electrolyte a gas then the metathesis reaction will occur. Acid-base neutralization is a metathesis reaction. The product of the neutralization is either a weak electrolyte (usually water) or a gas.

Acid-Base Neutralization Acid-base neutralization is a metathesis reaction between an acid and a base. HX + BOH  BX + HOH ( = H 2 O ) Definitions of acids and bases Acids produce H + in water (Arrhenius) are proton donors (proton = H + ) (Brønsted- Lowry) Bases produce OH - in water (Arrhenius) are proton acceptors (Brønsted-Lowry)

Acid-Base Neutralization Acid-base neutralization is a metathesis reaction between an acid and a base. HX + BOH  BX + H 2 O acid + base  water (a weak electrolyte) Acids are proton donors (proton = H + ). Bases are proton acceptors and are metal hydroxides, ammonia, the amines and alcohols: HCl + NH 3  NH Cl -  NH 4 Cl(s) acid + ammonia  salt

Acid-Base Neutralization In a neutralization, the acid and the base are reactants. Identify the acid and the base in the following reactions (hint: look for the H + ): CH 3 OH + HBr  CH 3 OH Br - CH 3 NH 2 + H 2 O  CH 3 NH OH - Here, the acids had the H at the left of the formula (HBr, HOH). (It is helpful to know the Lewis structures of ammonia and of water.)

Acid-Base Neutralization Identify the acid and the base in the following reactions: CH 3 CO 2 H + KOH  CH 3 CO 2 - K + + H 2 O CH 3 COOH + CH 3 Li  CH 3 CO 2 - Li + + CH 4 Organic acids (CO 2 H or COOH) have the acid H at the right of the formula. Here are two more: CH 3 OH + NaH  CH 3 O - Na + + H 2 C 4 H 9 Li + H 2 O  C 4 H 10 + LiOH (Li + and OH - )

Acid-Base Neutralization KNOW THE STRONG ACIDS: HCl, HBr, HI, HClO 3, HClO 4, HNO 3, H 2 SO 4 by name AND formula. These ionize completely in water. HCl(aq)  H + (aq) + Cl - (aq) KNOW THE STRONG BASES: LiOH, NaOH, KOH, RbOH, CsOH (these are Group 1A metal hydroxides), Ca(OH) 2, Sr(OH) 2, and Ba(OH) 2, by name AND formula. These ionize completely in water. Ba(OH) 2 (aq)  Ba 2+ (aq) + 2OH - (aq) 2HCl(aq) + Ba(OH) 2 (aq)  2H 2 O(l) + BaCl 2 (aq) NET IONIC REACTION BETWEEN A STRONG ACID AND A STRONG BASE: H + (aq) + OH - (aq)  H 2 O(l)

Acid-Base Neutralization - Examples Acetic acid is neutralized by a solution of potassium hydroxide: M.E.: HC 2 H 3 O 2 (aq) + KOH (aq)  KC 2 H 3 O 2 (aq) + H 2 O (l) Complete ionic equation: HC 2 H 3 O 2 (aq) + K + (aq) + OH - (aq)  K + (aq) + C 2 H 3 O 2 - (aq) + H 2 O (l) Net ionic equation: HC 2 H 3 O 2 (aq) + OH - (aq)  C 2 H 3 O 2 - (aq) + H 2 O(l)

Acid-Base Neutralization - Examples Solid magnesium hydroxide is neutralized by hydroiodic acid: M.E.: Mg(OH) 2 (s) + 2HI (aq)  MgI 2 (aq) + 2H 2 O (l) Complete ionic equation: Mg(OH) 2 (s) + 2H + (aq) + 2I - (aq)  Mg 2+ (aq) + 2I - (aq) + 2H 2 O (l) Net ionic equation: Mg(OH) 2 (s) + 2H + (aq)  Mg 2+ (aq) + 2H 2 O (l)

Acid-Base Neutralization with Gas Formation Hydrochloric acid reacts with a solution of sodium sulfide. One of the products is a gas. Molecular Equation: 2HCl (aq) + Na 2 S (aq)  2NaCl (aq) + H 2 S (g) acid base salt gas The sulfide ion acts as a proton acceptor and is therefore the base. Complete ionic equation: 2H + (aq) + 2Cl - (aq) + 2Na + (aq) + S 2- (aq)  2Cl - (aq) + 2Na + (aq) + H 2 S (g) Net ionic equation: 2H + (aq) + S 2- (aq)  H 2 S (g)

Acid-Base Neutralization Recognize the “gases in disguise:” H 2 SO 3, H 2 CO 3 and NH 4 OH. These compounds are unstable in water and decompose to form a gas and water. H 2 SO 3 (aq)  H 2 O (l) + SO 2 (g) H 2 CO 3 (aq)  H 2 O (l) + CO 2 (g) NH 4 OH (aq)  H 2 O (l) + NH 3 (g)

Acid-Base Neutralization Hydrochloric acid is neutralized by a solution of sodium sulfite. 2HCl (aq) + Na 2 SO 3 (aq)  2NaCl (aq) + H 2 SO 3 (aq) Molecular equation: 2HCl (aq) + Na 2 SO 3 (aq)  2NaCl (aq) + H 2 O (l) + SO 2 (g) The sulfite ion acts as a proton acceptor and is therefore the base. Complete ionic equation: 2H + (aq) + 2Cl - (aq) + 2Na + (aq) + SO 3 2- (aq)  2Cl - (aq) + 2Na + (aq) + H 2 O(l) + SO 2 (g) Net ionic equation: 2H + (aq) + SO 3 2- (aq)  H 2 O (l) + SO 2 (g)