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3.4: Aqueous Solutions

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    Learning Objectives 

    • Outline the differences between strong electrolyte, weak electrolyte, and a nonelectrolyte 

    • Predict the solubility of ionic compounds in water using solubility rules 

    • Memorize the six strong acids that are strong electrolytes 

    Introduction

    Many reactions that we deal with in General Chemistry involve water. We discussed solutions in Chapter 1 in which the solute is  dissolved in a solvent  to make a solution. We will be studying reactions in which the solvent is water,  known as aqueous solutions. Before we explore the different types of aqueous reactions, we must first understand what happens to ionic compounds once they dissolve in water. To dissolve an ionic compound, you must separate the cations from the anions. Water is particularly good at the task because it is a polar molecule (more on this later) in which the hydrogen end is partially positive (stabilizing the anions) and the oxygen end is partially negative (stabilizing the cations). Typically, cations and anions are dispersed evenly in the solvent. If you place an electrode, a conductor of electricity such as a copper wire, in the solution and you connect it to a battery, you are able to conduct electricity due the flow of charged particles. Let us discuss the possibilities of what might occur when ionic compounds are placed in water. 

    Electrolytes

    Pure water does not conduct electricity, and it has been observed that when a substance dissolves in water, it may produce mobile ions that allow the water to conduct electricity, and we call that compound an electrolyte, or it may not, in which case we call it a nonelectrolyte.

    This diagram shows three separate beakers. Each has a wire plugged into a wall outlet. In each case, the wire leads from the wall to the beaker and is split resulting in two ends. One end leads to a light bulb and continues on to a rectangle labeled with a plus sign. The other end leads to a rectangle labeled with a minus sign. The rectangles are in a solution. In the first beaker, labeled “Ethanol No Conductivity,” four pairs of linked small green spheres suspended in the solution between the rectangles. In the second beaker, labeled “K C l Strong Conductivity,” six individual green spheres, three labeled plus and three labeled minus are suspended in the solution. Each of the six spheres has an arrow extending from it pointing to the rectangle labeled with the opposite sign. In the third beaker, labeled “Acetic acid solution Weak conductivity,” two pairs of joined green spheres and two individual spheres, one labeled plus and one labeled minus are shown suspended between the two rectangles. The plus labeled sphere has an arrow pointing to the rectangle labeled minus and the minus labeled sphere has an arrow pointing to the rectangle labeled plus.

    Figure \(\PageIndex{1}\) Ethanol on the left is a nonelectrolyte and does not conduct electricity. KCl is a strong electrolyte and the bulb is very bright. Acetic acid is a weak electrolyte, and although the image may not show it, if the concentrations are the same, the light is dimmer than for the KCl

    There are two basic ways an aqueous compound can be an electrolyte.

    Soluble Ionic Compounds

    If ionic compounds dissolve and form a solution, the ions separate and are free to move about and conduct the electricity. But not all ionic compounds dissolve, and so they can be weak, strong or even nonelectrolytes. Typically, the undissolved ionic compound forms a solid that falls to the bottom as a precipitate. 
    image.png
    Figure \(\PageIndex{2}\) Figure \(\PageIndex{2}\)In the above image, the solid KCl is being surrounded by water molecules which cause the ions to leave the crystal and enter the solution. Once they enter the solution they are mobile and can conduct electricity, so KCl is an electrolyte (it is actually a strong electrolyte)

    Covalent Compound that React with Water

    The second way to produce an electrolyte happens with certain types of covalent molecules that react with the water. Acids give a proton to the water and so form ions as in the image below where HCl reacts with water to from chloride and hydronium ions. Some bases will extract a proton from the water and form ions, as in the case below of ammonia, which grabs a proton from the water forming the weak electrolyte ammonium hydroxide. Just as in the case with ionic compounds, covalent compounds can be weak, strong, or nonelectrolytes.

    This figure shows two flasks, labeled a and b. The flasks are both sealed with stoppers and are nearly three-quarters full of a liquid. Flask a is labeled H C l followed by g in parentheses. In the liquid there are approximately twenty space-filling molecular models composed of one red sphere and two smaller attached white spheres. The label H subscript 2 O followed by a q in parentheses is connected with a line to one of these models. In the space above the liquid in the flask, four space filling molecular models composed of one larger green sphere to which a smaller white sphere is bonded are shown. To one of these models, the label H C l followed by g in parentheses is attached with a line segment. An arrow is drawn from the space above the liquid pointing down into the liquid below. Flask b is labeled H subscript 3 O superscript positive sign followed by a q in parentheses. This is followed by a plus sign and C l superscript negative sign which is also followed by a q in parentheses. In this flask, no molecules are shown in the open space above the liquid. A label, C l superscript negative sign followed by a q in parentheses, is connected with a line segment to a green sphere. This sphere is surrounded by four molecules composed each of one red sphere and two white smaller spheres. A few of these same molecules appear separate from the green spheres in the liquid. A line segment connects one of them to the label H subscript 2 O which is followed by l in parentheses. There are a few molecules formed from one central larger red sphere to which three smaller white spheres are bonded. A line segment is drawn from one of these to the label H subscript 3 O superscript positive sign, followed by a q in parentheses.

    Figure \(\PageIndex{3}\) In the above image the gaseous molecule HCl dissolves in water (a), where it then gives a proton to the water and forms the electrolyte with chloride and hydronium ions.

     

    \[\text{NH}_{3} + \text{H}_{2}\text{O} \rightleftharpoons \text{NH}_{4}^{+} + \text{OH}^{-}\]

    11.10_ammonia.PNG

    Figure \(\PageIndex{4}\) In the above image, ammonia grabs a proton from water forming ammonium hydroxide

     

    Types of Electrolytes

    Compounds can be Strong, Weak, or Nonelectrolytes

    1. Strong Electrolytes – strong conductors of electricity due to formation of a large number of mobile ions
    2. Weak Electrolytes -Weak conductors of electricity due to formation of a few mobile ions
    3. NonElectrolytes – nonconductors of electricity as they do not form ions in aqueous solutions

    Strong Electrolytes

    Ionic - Soluble Salts and Strong Bases
    NaCl(aq) --> Na+ (aq) + Cl- (aq)
    NaOH(aq) --> Na+ (aq) + OH- (aq)

    Covalent - Strong Acids (protonate water)
    HCl(aq) + H2O --> H3O+ (aq) + Cl- (aq)
    H2SO4 (aq) + H2O --> H3O+ (aq) + HSO4- (aq)

    Note

    There are six strong acids that you must know that are strong electrolytes: 

    • 1. HCl (hydrochloric acid)
    • 2. H2SO4 (sulfuric acid)
    • 3. HNO3 (nitric acid)
    • 4. HBr (hydrobromic acid)
    • 5. HClO4 (perchloric acid)
    • 6. HI (hydroiodic acid)

    Any other acid can be deduced to be a weak acid, and therefore, a weak electrolyte. 

     

    •  

    Weak Electrolytes

    Ionic - Slightly Soluble Salts
    CoCl2 (s) <==> Co+2(aq) + 2Cl- (aq)

    Covalent - Weak Acids & Amine Bases (hydrolyze water)
    HF(aq) + H2O <==> H3O+ (aq) + F- (aq)
    NH3 (aq) + H2O <==> NH4+ (aq) + OH-

    NonElectrolytes

    Ionic - Insoluble Salts
    CoS(aq) <=--> Co+2(aq) + S-2 (aq)

    Covalent - Molecules which do not hydrolyze or protonate water
    C12H22O11(s) + H2O --> C12H22O11(aq)

    The following animations gives an atomic scale visualization of strong, weak and nonelectrolytes

    Video 3.4a: 2'37" Youtube animation giving an explaination for how some compounds can be weak, strong or non-electrolytes.

    Quick 1'29" video showing conductivty of various solutes. Note, the solid ionic compounds do not conduct because their ions are not mobile. You should pause the video before the add the light bulbs and try and predict if the bulb will come on.

     

    Solubility Rules

     

    We saw that some compounds are soluble, other are partially soluble, and some are are completely insoluble. How do we know determine this? We will start off with the simplest types, which are ionic compounds, and we will base these on the nature of the ions. There are sort of two opposing processes going on. Are the attractions of the ions in the crystal stronger than their attraction towards the water, or weaker? If the ionic attractions within the crystal are stronger, they do not dissolve and they form a precipitate. If on the other hand, the ions are more attracted to the water, they leave the crystal and the compound is soluble. We will use the solubility rules to determine if a salt is soluble or not. 

     

    What are the Solubility Rules?

    These are what I am calling a "rule of thumb," and allow us to roughly predict if a salt will dissolve or not. It must be understood that the concept is relative, for example, table salt is considered a soluble salt and if you add table salt to water it will dissolve a lot, up to 359g per liter, but at that point it becomes saturated, and any more will form a precipitate. On the other hand silver chloride is an insoluble salt, and you can only dissolve 0.0019g into 1 liter, but any more will fall to the bottom as a precipitate.

    Note, some textbooks give slightly different rules, and this set is incomplete. If your text is different, please discuss this with your instructor. When you get to general chemistry 2 you will learn a different approach, where we can quantify the amount dissolved for an insoluble salt, like the 1.9 mg/liter for silver chloride.

     

    Is there a strategy to using the solubility rules?


    Yes, for the ones I have set up below.

    • We first look at the [+] cation, and ask if it is any one of the cations listed in step IA. If yes, we say it is soluble, and the question is answered. After this step we focus on the [-] anions. If it is not soluble from step 1A, we go to 1B, and if the anion is from this list, it is soluble.
    • We now go to the compounds that are usually soluble, step II, and you need to memorize the exceptions, which are insoluble.
    • We now go to the compounds that are usually insoluble, step III, and you need to memorize the exceptions, which are soluble.

    NOTE: the exceptions in steps II and III have opposite meanings.

    Solubility Rules

    I. Soluble
    a. Group 1A & Ammonium (the only cations in this list)
    b. NO3-, ClO4-, ClO3-, CH3CO2-

    II. Usually Soluble
    a. Cl-, Br-, I-, (Except those with Ag+, Hg2+2, & Pb+2)
    b. F- (Except Mg+2, Ca+2, Sr+2, Ba+2 & Pb+2)
    b. SO4-2 (Except those with Ca+2, Sr+2, Ba+2, Ag+ & Pb+2)

    III Insoluble (Except with cations from I.A)
    a. OH- (Except those with Sr+2& Ba+2)
    b Everything else (this is not true, but will work in this class)

     

    Exercise \(\PageIndex{1}\)

    Use the solubility rules to determine if the following compounds are soluble or insoluble. Indicate answer by writing formula followed by (aq) for soluble and (s) for insoluble. (You may also want to write the names of the species)

    A) PbSO4 

    B) NaClO4 

    Answer

    A) Insoluble. Sulfate ions are typically soluble, but lead(II) is an exception, so lead(II) sulfate is an insoluble solid. This means that PbSO4 (s) is not broken up into ions in water, but is a precipitate.

    B) Soluble. perchlorate ions and alkali metals are soluble, so sodium perchlorate is  soluble, meaning that it breaks up into ions, Na+(aq) + ClO-4 (aq) in water. 

     

    Exercise \(\PageIndex{2}\)

    Identify the following as a strong electrolyte, a weak electrolyte, or a non-electroylte. 

    A) Ba(NO3)2 

    B) H3PO4

    C) C6H12O6  

    D) HNO3 

    E) AgBr

    Answer

    A) strong electrolyte - soluble in water, ions are produced 

    B) weak electrolyte - weak acid (not one of the six) 

    C) non-electrolyte - molecular compound, not ionic 

    D) strong electrolyte- strong acid (one of the six) 

    E) non-electrolyte - insoluble precipitate - no ions formed 

    Exercise \(\PageIndex{3}\)

    Both salt (NaCl) and table sugar, glucose (C6H12O6) dissolve in water. Why is salt water a strong electrolyte, while sugar water is a non-electrolyte? 

    Answer

    NaCl is an ionic compound that dissociates into ions that conduct electricity. C6H12O6 is a molecular compound that does not break up into ions, and therefore, does not conduct electricity. The sugar molecule remains intact, but each sugar molecule is separated from the other when added to water. The reason is dissolves in water is because of the term "the like dissolves the like", meaning both sugar and water are polar molecules. We will discuss this in more depth later in the text. 

    Practice Worksheet: Identify if the following compounds are soluble or insoluble. 

     Worksheet Key- Check your work.

     


     

     

     

    Contributors

    • Bob Belford (UALR) and November Palmer (UALR)
    • Modified by Ronia Kattoum (UA of Little Rock) 

    3.4: Aqueous Solutions is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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