Electrolytes At a young age, you are taught not to bring electrical devices into the bathtub to avoid electric shock Although pure water is not electrically conductive, substances dissolved in water can make it conductive. These substances are called electrolytes, species that dissociate in water to create ions – Electrolytes can be classified as strong or weak Strong electrolytes dissociate completely Weak electrolytes only dissociate partially

Strong Electrolytes All strong electrolytes are salts (e.g. NaCl) – When NaCl is added to water, it produces hydrated Na + and Cl -. Virtually no intact NaCl will remain Basic Rules for determining strong electrolytes 1.All salts containing group 1 and ammonium cations are strong electrolytes 2.All salts containing ammonium cations are strong electrolytes 3.All salts containing nitrate, perchlorate, and acetate anions are strong electrolytes 4.All strong acids and strong bases*

Weak Electrolytes and Non-electrolytes Compounds containing ionic bonds that do not fall under the classifications written in the previous slide are weak electrolytes. Keep in mind that weak electrolytes still dissolve readily in water, they just don’t dissociate as well as strong electrolytes. – ex. Silver (I) sulfate Non-electrolytes have no ionic bonds, but can dissolve readily in water (ex. Ethanol) 99.28% 0.72%

Strong Acids and Bases: Arrhenius Definition The term “acid” comes from Latin and translates to “sour”. The term was used to describe the sour taste of citrus fruit. Arrhenius was the first to recognize the nature of acids. In his studies, he found that certain acids, all of which possessing an H + ion, behaved as strong electrolytes in water. – He proposed that acids were substances that produced H+ (aq) when dissolved in solution (Arrhenius definition). These solutions are characterized by low pH values (< 7). We will revisit this concept in Ch. 20. Acids that are strong electrolytes are called STRONG ACIDS. These are listed below: HCl HBr HI HClO 4 (perchloric acid) HNO 3 (nitric acid) H 2 SO 4 (sulfuric acid)

Bases are known physically for their slippery, soapy consistency. Chemically, Arrhenius defines bases as substances that produce OH - (hydroxide) in solution. Basic solutions are characterized by their high pH values (> 7). Bases that are strong electrolytes are called STRONG BASES. These include hydroxides of all group 1 metals, and the hydroxides of Ca 2+, Sr 2+, and Ba 2+. Strong Acids and Bases: Arrhenius Definition

Double Replacement / Acid-Base Neutralization Reactions When present simultaneously in solution, electrolytes will undergo a double replacement reaction in which the cations and anions are swapped. With acids and bases, there are H + and OH - ions present. These ions will combine to produce covalent H-O-H bonds (H 2 O), as is evidenced by the phase change (aq  L). The by-product of this reaction is salt. Thus, acids and bases will neutralize each other, creating pH-neutral salt water (neutralization reaction). H + Cl - (aq) + Na + OH - (aq) H 2 O(L) + Na + Cl - (aq)

Net-Ionic Equations In the previous reaction, we saw that H + and OH - underwent a chemical reaction to yield a new substance, H 2 O(L). There is no H + (aq) or OH - (aq) remaining in the system at the end of the reaction. The Na + (aq) and Cl - (aq) ions, however, do not react, as their phase does not change. These ions are called spectator ions because they are not chemically involved in the process. A net-ionic reaction is written by omitting the spectators from the reaction. H + (aq) + OH - (aq) + Na + (aq) + Cl - (aq) H 2 O(L) + Na + (aq) + Cl - (aq) H + (aq) + OH - (aq) H 2 O(L)

Titrations Knowing that acids and bases neutralize each other, lets imagine that we have an acid or base of unknown concentration. How can we find the concentration? Perform a titration

Titrations In a titration, an indicator is added to the base solution. In the example to the right, as long as the pH is above 7 (basic) the indicator will make the solution pink. An exact volume of an acid solution is added to a buret. The acid solution is added drop-by-drop until the solution just turns clear (neutralized, pH =7 ). At this point, you have a stoichiometric equivalent of acid and base. Therefore, if you know the concentration of one, you can immediately determine the concentration of the other.

Say we have 100 mL of a basic NaOH solution of an unknown concentration. We titrate with 5 mL of 1.0 M HCl, and the solution just turns clear. Titrations Before titration After titration NaOH(aq) + HCl(aq)  H 2 O(L) + NaCl(aq) We know that the acid and base are completely neutralized, and none is left in solution. Moles of acid added = Stoichiometric equivalent of base Concentration of base solution =

Solubility As a solute dissolves in a solvent, the molarity of the solution goes up. At a certain point, the dissolve ions/molecules have a high probability of colliding with undissolved solute and reattaching. This process, the opposite of dissolving, is called crystallization. dissolve crystallize When the rates of these processes become equal, no further dissolving can occur, and the solution is said to be saturated. The amount of solute required to form a saturated solution is known as the solubility of that solute.

Factors Affecting Solubility Solute Solvent Interactions – Intermolecular forces between solute and solvent – Stronger attractions means greater solubility – Polar likes polar, nonpolar likes nonpolar Temperature – In general, the solubility of solids dissolve in liquid increases with temperature; solubility of gases in solution decreases.