Acid-Base Reactions and Gas-Forming Reactions

Outline

• More Reaction Types
  • Acid-Base Reactions
  • Evaluating Acid-Base Reactions
  • Gas Reactions
• Practice with Concepts
• Discussion Questions
• Optional Website Reading

More Reaction Types

Among the major classes of reaction types are acid-base reactions, which may also be gas-producing, precipitation, or oxidation-reduction reactions. It is important to realize that these are not mutually exclusive categories!

Acids and Bases

Acid-base reactions, not surprisingly, involve the combination of acids and bases in solution. Remember that

• acids added to water donate a proton (H⁺)
• bases donate a hydroxide ion (OH^-)
• A strong acid HA dissociates completely in water, leaving only ions: HA --> H⁺ + A^-. At the end of the reaction, no molecular HA is left.
• A strong base BOH dissociates completely in water, leaving only ions: BOH --> B⁺ + OH^-. At the end of the reaction, no molecular BOH is left.
• A weak acid dissociates partially in water, reaching an equilibrium state in a reversible reactions: HA <--> H⁺ + A^-. When the reaction is at equilibrium (concentrations are no longer changing, although individual molecules of HA may be breaking up or reforming), HA, H⁺, and A^- are all present in the solution.
• A weak base combines with water, reaching an equilibrium state in a reversible reactions: BOH + H₂O <--> BH₂⁺ + OH^-. When the reaction is at equilibrium, all the products and reactants are present in some concentration.
• Strong acids form when H⁺ combines with elements from Group 17 (on the right side of the periodic chart, next to the noble gases), specifically Cl, Br, and I; or with oxoanions (oxygen containing negatively-charged ions), like NO₃^-, SO₄^2-, and ClO₄^-.
• Strong bases form when the hydroxide ion OH^- combines with elements from Group I and Group II (on the left side of the periodic chart), namely Li, Na, K, Ca, Sr, and Ba.

What happens when acids and bases mix? It depends on whether the acid or base is strong or weak.

	Strong Base	Weak Base
Strong Acid	HA → H+ + A- complete BOH → B+ + OH- complete H+ + OH- → H2O Net reaction is neutralization	HA → H+ + A- complete B + H2O ↔ BH+ + OH- partial H+ + OH- ↔ H2O partial B + H+ → BH+ Net reaction is formation of the conjugate acid from the base: the solution is acidic
Weak Acid	BOH → B+ + OH- complete HA ↔ H+ + A- partial H+ + OH- ↔ H2O partial HA + OH- → A- + H2O Net reaction is formation of the conjugate base from the acid: the solution is basic.	Check precipitation table!

Evaluating acid/base reactions

To figure out what is going on in an acid/base reaction (for example, HNO₂ + NaOH), follow these steps:

1. Determine whether the reactants involved are weak or strong. NaOH is a strong base (lye). HNO₂ is a weak acid (nitrous acid: note the subscript is 2, not 3).
2. Write the base dissociation equation: NaOH --> Na⁺ + OH^-.
3. Write the acid dissociation equation: HNO₂ --> H⁺ + NO₂^-.
4. Write the net equation using the chart pattern for weak acid/strong base reactions, substituting the base for B and the acid for A: HNO₂ + OH^- --> NO₂^- + H₂O.

Remember that the net equation for a strong acid/strong base reaction is simply the formula for the creation of water from its ions! HCl + Ca(OH)₂ --> H⁺ + Cl^- + Ca²⁺ + 2(OH)^- --> H₂O + Cl^- + Ca²⁺. The ions Cl^- and Ca²⁺ are spectator ions, which do not participate in the final reaction, and which can be cancelled out of net ionic equation for the reaction: H⁺ + 2(OH)^- --> H₂O

Gas Reactions

Some reactions form gases instead of precipitates or new solutes. The major difference in writing balanced equations for gas-formation involves including the (g) notation on the gas product.

Practice with the Concepts

Evaluating a reaction

Bubbling CO₂ through water produces H₂CO₃, which dissociates to for H₃O+ and HCO₃-. Is this solution acidic or basic?

Tossing calcium oxide (lime) into water forms calcium hydroxide, which dissociates to calcium ions and hydroxide. Is this solution acidic or basic?

mycroft (whose proper name is Never Capitalized) performs the exceedingly dangerous experiment combining hydrochloric acid with lye (sodium hydroxide). [DO NOT perform this experiment without supervision!]. What is the net ionic equation?

Discussion Questions

• Why is the reaction of a strong acid with a strong base called neutralization?
• What is an electrolyte?

Optional Readings

Tony St. John's YouTube Video on predicting the outcome of acid-base reactions and writing net ionic equations gives a number of examples and a consistent procedure.

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