Aqueous Solutions

Soluble and Insoluble Soluble generally means that more than 1 g of solute will dissolve in 100 mL of water at room temperature. Insoluble means that the solubility is less than 0.1 g per 100 mL. While many ionic compounds are soluble in water, many other compounds are not. It is important to know which compounds are soluble and which are not.

Nearly all Alkali Metals (group 1) are soluble in water. Sulfide and phosphide compounds tend to be insoluble. Sodium sulfide and potassium phosphide are soluble. Iron sulfide and calcium phosphide are insoluble. Why do some ions form soluble compounds, while other form insoluble compounds?

Factors that Affect the Solubility of Ionic Substances Two effects on solubility are: ion charge ion size

Effect of Ion charge on Solubility Compounds with small ion charges tend to be soluble. Compounds with large ion charges tend to be insoluble. Increasing the charge, increases the force that holds the ions together. Na + SOLUBLE (single charge, less hold) PO 4 3- INSOLUBLE (large charge, more hold)

Effect of Ion Size on Solubility When an atom gives up or gains an electron, the size of the ion that results is different from the size of the original atom. In general, metal ions tend to be smaller than their respective atoms. Non-metal ions tend to be larger than their corresponding atom.

Small ions bond more closely together so the bond between them is stronger than the bond between large ions. As a result, compounds with small ions tend to be less soluble than larger ones because they are more tightly held together.

Soluble or Insoluble: General Solubility Guidelines Many factors affect solubility so predicting solubility is neither straightforward nor simple. The General Solubility Guidelines are a useful summary of ionic-compound interactions with water. To use the table, remember that the higher the guideline number, always takes precedence over a lower guideline number.

General Solubility Guidelines GuidelineCationAnionResultExceptions 1Li +, Na +, K +, Rb +, Cs +, NH 4 + NO 3 -, ClO 3 -, CH 3 COO - SolubleCa(ClO 3 ) 2 is insoluble 2Ag +, Pb 2+, Hg + CO 3 2-, PO 4 3-, O 2- S 2-, OH - Insoluble BaO and Ba(OH) 2 are soluble. Group 2 sulfides decompose 3Cl -, Br -, I - Soluble 4Ba 2+, Ca 2+, Sr 2+ Insoluble 5Mg 2+, Cu 2+, Zn 2+, Fe 2+, Fe 3+, Al 3+ SO 4 2- Soluble

BaCl 2 is a white crystalline powder. Is it soluble or insoluble? The Ba 2+ ion is insoluble. It is in guideline 4. The Cl - ion is soluble. It is listed in guidelie 3. The higher the guideline number takes precedence. BaCl 2 is soluble.

Reactions in Aqueous Solutions When you mix two aqueous ionic compounds together, there are two possible outcomes. Either the compounds will remain in solution without reacting, or one aqueous ionic compounds will chemically react with the other. How can you predict which outcome will occur?

Lead (II) Nitrate and Sodium Iodide As Lead II nitrate is added to potassium iodide, a yellow solid (a precipitate) is forming. This is a double replacement reaction, a reaction that involves a change of ions to form two new compounds. It has the general equation: AB + CD  AD + CB In a double replacement reaction, the cations exchange anions.

Double Displacement Reactions That Produce a Precipitate A double replacement reaction that produces an insoluble substance, called a precipitate, is called a precipitate reaction. If you didn ’ t have the chemicals handy, you could still predict that mixing Pb(NO 3 ) (aq) with KI (aq) would result in an insoluble compound. Start by writing and balancing the equation (you can predict the products).

Pb(NO 3 ) (aq) + 2 KI (aq)  PbI K(NO 3 ) Then use the solubility guidelines. Pb 2+ is guideline 2, insoluble (NO 3 ) - is guideline 1, soluble » These two together make a soluble compound because of the NO 3 - being higher and soluble. K + is guideline 1, soluble I - is guideline 3, soluble » These two together make a soluble compound because they are both soluble!

Now use the solubility guidelines for the products. Pb 2+ is guideline 2, insoluble I - is guideline 3, soluble » These two together make an insoluble compound because of the lead is higher and insoluble. K + is guideline 1, soluble (NO 3 ) - is guideline 1, soluble » These two together make a soluble compound because they are both soluble!

Predicting the Formation of a Precipitate Example Will the following reaction form a precipitate? Potassium carbonate and copper (II) sulfate

K 2 CO 3 + CuSO 4  K 2 SO 4 + CuCO 3 K + and CO 3 2-  K 2 CO 3 Potassium is guideline 1, soluble Carbonate is guideline 2, insoluble » Soluble because potassium is higher Cu 2+ and SO 4 2-  CuSO 4 Copper is guideline 5, soluble Sulfate is guideline 5, soluble » Soluble

K + and SO 4 2-  K 2 SO 4 Potassium is guideline 1, soluble Sulfate is guideline 5, soluble » Soluble Cu 2+ and CO 3 2-  CuCO 3 Copper is guideline 5, soluble Carbonate is guideline 2, insoluble » Insoluble because carbonate is higher There will be a precipitate. It will be copper II carbonate.

Double Displacement Reactions that Produce a Gas Double displacement reactions are responsible for producing a number of gases, which include: – Hydrogen (H 2 ) – Hydrogen sulfide (H 2 S) – Sulfur dioxide (SO 2 ) – Carbon dioxide (CO 2 ) – Ammonia (NH 3 )

Reactions that Produce Hydrogen Gas Alkali metals form bonds with hydrogen to produce hydrides. Hydrides react readily with water to produce hydrogen gas. For example » LiH (s) + H 2 O (l)  LiOH (aq) + H 2 (g)

Reactions that Produce Hydrogen Sulfide Gas Sulfides react with certain acids to produce hydrogen sulfide. For example » K 2 S (aq) + 2 HCl (aq)  2 KCl (aq) + H 2 S (g)

Reactions that Produce Sulfur Dioxide Gas Some reactions produce a compound that, afterward, decomposes into a gas and water. Sodium sulfite reacts with hydrochloric acid to form sulfurous acid, which then breaks down into sulfur dioxide and water. The net reaction is the sum of both changes. If the same compound appears on both sides of the equation, it can be eliminated. For example » Na 2 SO 3 (aq) + 2 HCl (aq)  2 NaCl (aq) + H 2 SO 3 (aq) » H 2 SO 3 (aq)  SO 2 (g) + H 2 O (l) » Net equation Na 2 SO 3 (aq) + 2 HCl (aq)  2 NaCl (aq) + SO 2 (g) + H 2 O (l)

Reactions that Produce Carbon Dioxide Gas The reaction of carbonate with an acid produces carbonic acid, which decomposes rapidly into carbon dioxide and water. Na 2 CO 3 (aq) + 2 HCl (aq)  2 NaCl (aq) + H 2 CO 3 (aq) H 2 CO 3 (aq)  CO 2 (g) + H 2 O (l) Net equation » Na 2 CO 3 (aq) + 2 HCl (aq)  2 NaCl (aq) + CO 2 (g) + H 2 O (l)

Reactions that Produce Ammonia Ammonia gas is very soluble in water. You can detect it by its sharp and pungent smell. NH 4 Cl (aq) + NaOH (aq)  NaCl (aq) + NH 3 (aq) + H 2 O (l)

Double Displacement Reactions That Produce Water The neutralization reaction between an acid and a base is a very important double replacement reaction. It is called a neutralization reaction. In a neutralization reaction, water result from the H + ion from the acid, bonding with the OH - ion from the base. H 2 SO 4 (aq) + 2 NaOH (aq)  Na 2 SO 4 (aq) + 2 H 2 O (l)

Most metal oxides (metal mixed with oxygen) are bases, so they will react with an acid to produce water and a salt. MgO (s) + 2 HNO 3 (aq)  Mg(NO 3 ) 2 (aq) + H 2 O (l) Non-metal oxides are acidic and will mix with a base to produce water and salt. CO 2 (g) + 2 LiOH (s)  Li 2 CO 3 (aq) + H 2 O (l)