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Predicting Whether a Reaction Will Occur

Predicting Whether a Reaction Will Occur. “Forces” that drive a reaction: Formation of a solid Formation of water Transfer of electrons Formation of a gas When chemicals (dissolved in water) are mixed and one of these four things can occur, the reaction will generally happen. Dissociation.

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Predicting Whether a Reaction Will Occur

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  1. Predicting Whether a Reaction Will Occur “Forces” that drive a reaction: • Formation of a solid • Formation of water • Transfer of electrons • Formation of a gas When chemicals (dissolved in water) are mixed and one of these four things can occur, the reaction will generally happen.

  2. Dissociation • Ionic compounds • Metal + nonmetal (Type I & II) • Metal + polyatomic anion • Polyatomic cation + anion • Dissociation: When ionic compounds dissolve in water the anions and cations are separated from each other. • We know that ionic compounds dissociate when they dissolve in water because the solution conducts electricity.

  3. K Cl K+ Cl- Cu SO4 Cu+2 SO42- Dissociation (cont.) • Potassium chloride dissociates in water into potassium cations and chloride anions. KCl(aq) = K+ (aq) + Cl- (aq) • Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions. CuSO4(aq) = Cu+2(aq) + SO42-(aq)

  4. K+ Ag+ K+ Ag I I- NO3- NO3- Precipitation Reactions • In all precipitation reactions, the ions of one substance are exchanged with the ions of another substance when their aqueous solutions are mixed. • At least one of the products formed is insoluble in water. KI(aq) + AgNO3(aq) KNO3(aq) + AgIs

  5. Process for Predicting the Products of a Precipitation Reaction • Determine what ions each aqueous reactant has. • Exchange ions. • (+) ion from one reactant with (-) ion from other • Balance charges of combined ions to get formulaof each product. • Balance the equation. • Count atoms. • Determine solubility of each product in water. • Use solubility rules. • If product is insoluble or slightly soluble, it will precipitate.

  6. Solubility Rules • Most compounds that contain NO3- ions are soluble. • Most compounds that contain Na+, K+, or NH4+ ions are soluble. • Most compounds that contain Cl- ions are soluble, except AgCl, PbCl2, and Hg2Cl2 • Most compounds that contain SO42- ions are soluble, except BaSO4, PbSO4, CaSO4

  7. Solubility Rules (cont.) • Most compounds that contain OH- ions are slightly soluble (will precipitate), except NaOH and KOH, which are soluble, and Ba(OH)2, Ca(OH)2, which are moderately soluble. • Most compounds that contain S2-, CO32-, or PO43- ions are slightly soluble (will precipitate).

  8. Predict what will happen when the following solutions are mixed: • AgNO3(aq) and NH4Cl(aq) • Na3PO4(aq) and Co(NO2)2(aq) • K2CrO4(aq) and Pb(C2H3O2)2(aq)

  9. Describing Reactions in Aqueous Solutions • Molecular equations: equations that show the complete formulas of all reactants and products Example: • Ionic equations: All substances that are strong electrolytes are represented as ions. Solids are not written as separate ions. Example:

  10. Describing Reactions in Aqueous Solutions (cont.) • Spectator ions: ions that do not participate in the reaction Example: • Net ionic equation: Only the components directly involved the reaction are written. Example:

  11. Write molecular, complete ionic, and net ionic equations for the following reaction: • nickel(II) sulfate and barium chloride

  12. Write molecular, complete ionic, and net ionic equations for the following reaction: • iron(III) nitrate and sodium hydroxide

  13. Reactions that Form Water: Acids and Bases • When acids dissociate in water they release H+ ions and their anions. • When bases dissociate in water they release or form OH- ions and their cations.

  14. Acid-Base Reactions • In the reaction of an acid with a base, the H+ from the acid combines with the OH- from the base to make water. • The cation from the base combines with the anion from the acid to make the salt. acid + base salt + water H2SO4(aq) + Ca(OH)2(aq)  CaSO4(aq) + 2 H2O(l) • The net ionic equation for a strong acid-strong base reaction is always: H+ (aq) + OH- (aq)  H2O(l)

  15. Erucic Acid (a weak acid from marsh marigold)

  16. Complete the following acid-base reactions: HCl(aq) + RbOH(aq) → HNO3(aq) + LiOH(aq) →

  17. Oxidation-Reduction Reactions • Oxidation-reduction reactions: reactions that involve a transfer of one or more electrons • The substance that loses electrons in the reaction is oxidized. The substance that gains electrons in the reaction is reduced.

  18. Reactions of Metals withNonmetals (Oxidation-Reduction) • The metal loses electrons and becomes a cation (oxidation) • The nonmetal gains electrons and becomes an anion (reduction) • In the reaction, electrons are transferred from the metal to the nonmetal. 2K(s) + F2(g) → 2KF(s) metal nonmetal

  19. Characteristics of Oxidation-Reduction Reactions • Metal + nonmetal  ionic compound (solid) • In the ionic compound the metal is now a cation and the nonmetal is an anion. • Two nonmetals can undergo an oxidation-reduction reaction. Look for O2 as a reactant or product. In this case the products are not ionic. • Examples:

  20. Ways to Classify Reactions • Precipitation reactions: reactions that involve solid formation. Example: • Double displacement reactions: ion exchange reactions AB + CD → AD + CB Example: • Acid-base reactions: reactions that involve water formation Example: • Both precipitation reactions and acid-base reactions involve compounds exchanging ions.

  21. Ways to Classify Reactions (cont.) • Oxidation-reduction reactions: reactions that involve electron transfer Example: Also classified as a single displacement reaction: A + BC → B + AC • Combustion reactions: reactions in which O2(g) is reacted with a carbon compound • Release a lot of energy • Subclass of oxidation-reduction reactions • Example:

  22. Other Ways to Classify Reactions • Synthesis reactions: reactions in which chemicals combine to make one product • Metal + nonmetal reactions can be classified as synthesis reactions. Example: • Reactions of metals or monmetals with O2 can be classified as synthesis reactions. Example: • These two types of synthesis reactions are also subclasses of oxidation-reduction reactions.

  23. Other Ways to Classify Reactions • Decomposition reactions: reactions in which one reactant breaks down into simpler compounds or elements • Generally initiated by addition of electric current or heat Example: • Opposite of a synthesis reaction

  24. Summary of Classes of Reactions

  25. Classify the following reactions in as many ways as possible: C(s) + O2(g) → CO2(g) Zn(s) + 2HBr(aq) → H2(g) + ZnBr(aq) 2KClO3(s) → 2KCl(s) + 3O2(g) H2SO4(aq) + Ba(OH)2(aq) → 2H2O(l) + BaSO4(s)

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