Aqueous Chemistry

Recap To date, we have learned about: Chemical elements and their classifications Valence electron configurations and their effect on chemical properties and reactivity Predicting ionic charges Now, we will begin to learn about chemical reactions involving ionic compounds

Ionic Reactions Simple chemical reactions can be classified as one of the following: combination reactions decomposition reactions combustion reactions single replacement reactions double replacement reactions

Combination Reactions In a combination reaction, multiple reactants combine to form a single product The reaction may occur between two elements Or between an element and a compound Or between two compounds 3Li(s) + P(g)  Li3P(s) Ca(s) + Cl2(g)  CaCl2(s) MgO(s) + CO2(g)  MgCO3 (s)

Decomposition Reactions 2HgO(s) 2Hg(l) + O2(g) 2KClO3(s) 2KCl(s) + 3O2(g)

Combustion Reactions Combustion reactions are reactions that consume O2(g) as a reactant and produce oxide compounds as products. These reactions release a lot of heat. Fires are the result of combustion reactions. Ex. Metal Oxide formation 2Ca(s) + O2(g)  2CaO(s) 4Fe(s) + 3O2(g)  2Fe2O3 (s) When hydrocarbons (CxHy) and carbohydrates (CxHyOx) undergo combustion, the products are always carbon dioxide and water. C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O (L)

Single Replacement Reactions In a single replacement reaction, a free element replaces a bound ion/element. When the reaction involves metals, it can also be referred to as a transmetallation reaction. Zn(s) + 2AgCl (aq)  ZnCl2(aq) + 2Ag(s)

Zn(s) + 2AgCl (aq) ZnCl2(aq) + 2Ag(s) Single Replacement Transmetallation reactions occur because one metal is less stable in its elemental state (0 charge) than the other. In the reaction below, Zn displaces Ag because Zn0 is less stable than Ag0: Zn(s) + 2AgCl (aq) ZnCl2(aq) + 2Ag(s) Less stable metals prefer to exist as ions instead of elements, especially in aqueous solution. These unstable elements will react and become highly stable ions. In order to know what can displace what, an activity series can be used.

Group Work As shown in the table, elemental Li (s) is the least stable metal, and is the most likely to react. It will displace any metal ion. K (s) will displace any metal ion except Li. And so on. Predict the products. Na (s) + ZnSO4(aq) Al (s) + KNO3 (aq) Na2SO4(aq) + Zn (s) No reaction

Single Replacement Reactions Involving Acids When a metal reacts with a binary acid (HX), the metal replaces the hydrogen ion to yield an ionic compound and hydrogen gas. Zn(s) + 2HCl (aq) ZnCl2(aq) + H2(g)

Redox Reactions Single replacement reactions are examples of red-ox (reduction-oxidation) reactions A reduction reaction occurs when the oxidation state (charge) of an element/ion becomes more negative the course of a reaction (i.e. the species gains electrons) In an oxidation process, the oxidation state of an element/ion becomes more positive during a reaction (i.e the species loses electrons)

Redox Reactions Consider the following single replacement reaction: Zn(s) + Cu SO4 (aq) Zn SO4 (aq) + Cu (s) On the reactant side, we have elemental Zn. The charge on any pure element is 0 On the product side, we have a Zn2+ ion. Since the charge of Zn has gone from 0 to 2+, Zn has undergone an oxidation. Zn loses 2 electrons. Where did they go??? On the product side, we have elemental Cu, so Cu has undergone a reduction from 2+ to 0 by taking 2 electrons from Zn. On the reactant side, we have a Cu2+ ion.

Oxidizing and Reducing Agents Zn(s) + Cu SO4 (aq) Zn SO4 (aq) + Cu (s) We have identified the reduction and oxidation processes in the reaction above O: Zn0  Zn2+ + 2e- R: Cu2+ + 2e-  Cu0 RED-OX REACTIONS Because Cu2+ gets reduced, it is the oxidizing agent (it oxidizes Zn). Because Zn gets oxidized, it is the reducing agent (it reduces Cu2+)

Oxidizing and Reducing Agents Zn(s) + Cu SO4 (aq) Zn SO4 (aq) + Cu (s)

(C.I.R.L) Rust Formation Reduced 4Fe(s) + 3O2(g) 2Fe2O3(s) Oxidized

Double Replacement Reactions In a double replacement result, two salts react, and the anions exchange places AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq) ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g)

Examples Balance the following double replacement reactions A. CaBr2 (aq) + K2CO3(aq) B. NH4Cl (aq) + MgSO4 (aq)

Double Replacement Reactions Yield Physical Changes An easy way to identify a chemical reaction is if there is a change in phase. In a Precipitation Reaction, an insoluble (solid, does not dissolve) ionic product is formed. In the figure to the left, Na2S (aq) and Cd(NO3)2 (aq) undergo double replacement to form CdS and NaNO3 . CdS is insoluble

Net Ionic Equations It is proper practice to use NET IONIC EQUATIONS when there is a change in phase Ex. Na2S(aq) + Cd(NO3)2(aq) 2NaNO3(aq) + CdS(s) Since we know that ionic solutions dissociate in water, we can rewrite the equation above in ionic form: 2Na+(aq) + S2-(aq) + Cd2+(aq) + 2NO3-(aq) CdS(s) + 2Na+(aq) + 2NO3-(aq) The ions in red undergo a chemical reaction, as indicated by the change in phase. The remaining ions are called SPECTATOR IONS because they are not involved in the reaction in any way.

Net Ionic Equations Na+(aq) + S2-(aq) + Cd2+(aq) + NO3-(aq) CdS(s) + Na+(aq) + NO3-(aq) The spectators ions cancel out. The remaining reactants and products comprise the net ionic equation. In order to write a net ionic equation, you must know which ionic compounds are insoluble. The solubility rules enable this. Cd2+(aq) + S2-(aq) CdS(s) NET IONIC EQUATION

Solubility Rules All group 1 and ammonium salts are water soluble! All nitrates, acetates, and perchlorates are soluble With the exception of all anions mentioned in #2, Ag, Pb, and Hg(I) salts are not soluble With the exception of those cations mentioned in #1, carbonates, sulfides, oxides, and phosphates are not soluble With the exception of Ca, Sr, and Ba, and those cations mentioned in #1, all hydroxides are not soluble Most sulfates are soluble EXCEPT for those of Ca, Sr, Ba, and of those cations mentioned in #3.

Examples Use solubility rules to predict the products of the following double replacement reactions. Write the net ionic reaction. If there is no reaction, write ‘no reaction’: MgBr2 (aq) + K2CO3 (aq) NaCH3COO (aq) + CaBr2 (aq)