Ionic compounds tend to be strong electrolytes. QUESTION: Which of the following is a strong electrolyte? A. CH3OH, B. KOH, C. HF, C. NH3 An electrolyte is a substances that, when dissolved in water, produces a mixture that conducts electricity. (Pure water is a very poor electrical conductor.) Ionic compounds tend to be strong electrolytes. Solid KOH is a not an electrical conductor. Motion of K+ and OH- ions are restricted in solid. Liquid KOH is an electrical conductor; ions are less restricted. In aqueous solution, ions are also more free to move, KOH  K+(aq) + OH-(aq) SCRIPT: Which of the following is a strong electrolyte? A. CH3OH, B. KOH, C. HF, C. NH3 PAUSE CLICK An electrolyte is a substances that, when dissolved in water, produces a mixture that conducts electricity. Water is a very poor electrical conductor. Ionic compounds tend to be strong electrolytes. So all we have to do is find the ionic compound among the choices given. The easiest way to tell is by looking for the first element in the formula. If it’s a metallic element, then you’re looking at the formula of an ionic compound. Or if you see an NH4 grouping, you’re looking at the an ionic compound whose cation is ammonium. Among the choices given here, only choice B is an ionic compound. So the correct answer is B. Potassium is a metallic element. So, the compound KOH is ionic. The cations are potassium ions. CALLOUT “K+”, pointing to K in KOH Let’s clarify a few things about electrical condutivity…. Solid potassium hydroxide is NOT an electrical conductor. That’s because the motion of ions is restricted in a solid. However, liquid potassium hydroxide is an electrical conductor. The ions have more freedom of movement in a liquid. The same is true when you dissolve an ionic compound in water. HIGHLIGHT KOH  K+ + OH- The ions will spread out and are free to move in response to an electric potential. CONTINUED ON NEXT SLIDE

QUESTION: Which of the following is a strong electrolyte? A. CH3OH, B. KOH, C. HF, C. NH3 Molecular compounds are nonelectrolytes, except molecular acids and molecular bases. Most acids, like HF, are weak electrolytes. HF  H+(aq) + F-(aq) A small fraction of molecules are ionized. Exceptions: the six strong acids: HClO4, HNO3, H2SO4, HCl, HBr, HI Molecular bases are weak electrolytes. Example: NH3(aq) + H2O(aq)  NH4+(aq) + OH-(aq) CLICK In general, molecular compounds are nonelectrolytes. Choice A for example, methanol HIGHLIGHT CH3OH Is a nonelectrolyte. If you add it to water, the resulting mixture will still not conduct electricity. However, there are some exceptions. Molecular acids and bases are mostly weak electrolytes. A molecular acid is made up of molecules that have hydrogen and have a tendency to break up into ions when dissolved in water. An example is HF. When we dissolve HF in water, a small fraction of HF molecules will break up into hydrogen ions HIGHLIGHT H+ in chemical equation and fluoride ions. One thing all molecular acids have in common is that they produce hydrogen ions when dissolved in water. However, most of the HF molecules remain intact, so HF is considered a weak electrolyte. The resulting mixture is not a very good electrical conductor CLICK You should know that there are only six acids known to ionize completely in water. These acids are Strong electrolytes and are called strong acids. You should memorize these: HIGHLIGHT formulas as acids are mentioned perchloric acid, nitric acid, sulfuric acid, hydrochloric acid, hydrobromic acid, and hydriodic acid. Molecular bases consitute another class of molecular compounds that are weak electrolytes. The most common example you’ll encounter is ammonia. HIGHLIGHT NH3 When dissolved in water, a small fraction of ammonia molecules react with water to produce Ammonium HIGHLIGHT ammonium And hydroxide ions. HIGHLIGHT hydroxide CONTINUED ON NEXT SLIDE

QUESTION: Which of the following is a strong electrolyte? A. CH3OH, B. KOH, C. HF, C. NH3 NOTE: an ionic compound would not be a strong electrolyte if it is insoluble in water. One final note, an ionic compound would not be considered a strong electrolyte if it does not dissolve in water. If you get a question like this on a test and you have not been taught how to determine if an ionic compound is soluble or insoluble in water, then just assume that any ionic compound given is soluble. CLICK PAUSE END RECORDING

Video ID: 1-25-2 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08