Transcript Slide 1
The difference between the impossible and the possible lies in a person's determination. Tommy Lasorda WATER IS AN EXCELLENT SOLVENT FOR MANY DIFFERENT SUBSTANCES. IONIC SUBSTANCES CAN DISSOLVE IN WATER. WHEN AN IONIC SUBSTANCE DISSOLVES IN WATER, THE INDIVIDUAL IONS ARE DISPERSED, THAT IS, THEY ARE COMPLETELY SEPARATED FROM EACH OTHER. SINCE WATER IS A POLAR MOLECULE, THE NEGATIVE ENDS OF WATER MOLECULES CAN SURROUND POSITIVE IONS (CATIONS), AND THE POSITIVE ENDS OF WATER MOLECULES CAN SURROUNG NEGATIVE IONS (ANIONS). HOW SOLUBLE A SALT IS DEPENDS ON THE STRENGTH OF ATTRACTION BETWEEN THE NEGATIVE AND POSITIVE IONS IN THE SOLID AND THE STRENGTH OF ATTRACTION OF THE WATER MOLECULES FOR THE INDIVIDUAL SALT IONS. FOR EXAMPLE, SODIUM CHLORIDE, NaCl, IS VERY SOLUBLE, BUT SILVER CHLORIDE, AgCl, IS NOT VERY SOLUBLE. A SUBSTANCE THAT PRODUCES IONS WHEN DISSOLVED IN WATER IS CALLED AN ELECTROLYTE, THAT IS, IT CONDUCTS ELECTRICITY. WE HAVE SUBSTANCES THAT ARE WEAK ELECTROLYTES (ONLY PRODUCE A FEW IONS IN SOLUTION) AND WE HAVE STRONG ELECTROLYTES (PRODUCE LOTS OF IONS IN SOLUTION). THERE ARE THREE KINDS OF STRONG ELECTROLYTES: 1)SALTS 2)STRONG ACIDS 3)STRONG BASES LET’S CONSIDER STRONG ACIDS FIRST. AN ACID IS A SUBSTANCE THAT PRODUCES HYDROGEN IONS ON SOLUTION. OF COURSE, A HYDROGEN ION WOULD ACTUALLY BE JUST A PROTON, AND AS SUCH, IT WOULD BE ASSOCIATED WITH OTHER WATER MOLECULES (HYDRATED). HA + H2O H3O+ + A- THIS HYDRATION OF THE HYDROGEN ION (PROTON) IS USUALLY UNDERSTOON, SO WE WOULD WRITE THE DISSOCIATION OF A STRONG ACID AS: HA (aq) H+ (aq) + A- (aq) IN THE CASE OF A STRONG ACID, IT IS 100% DISSOCIATED IN SOLUTION TO HYDROGEN IONS AND THE ACID ANIONS. THE HYDROGEN IONS ARE VERY REACTIVE, AND THIS IS WHAT MAKES STRONG ACIDS SO DANGEROUS. THEY WILL EVEN DISSOLVE METALS. THE ANCIENT ALCHEMISTS USED TO SEARCH FOR THE UNIVERSAL SOLVENT. CERTAIN STRONG ACIDS COME CLOSE. COMMON STRONG ACIDS INCLUDE: HCl - HYDROCHLORIC ACID HNO3 - NITRIC ACID H2SO4 - SULFURIC ACID THEY WOULD ALL BE 100% DISSOCIATED IN AN AQUEOUS SOLUTION. SULFURIC ACID IS UNUSUAL IN THAT IT HAS TWO REPLACABLE HYDROGEN IONS. THE FIRST ONE WOULD DISSOCIATE 100% IN AQUEOUS SOLUTION. H2SO4 H+ + HSO4THE SECOND WOULD ONLY PARTIALLY DISSOCIATE. HSO4- = H+ + SO4-2 THERE ARE ACIDS THAT WE REFER TO AS WEAK ACIDS THAT DO NOT DISSOCIATE 100% IN AQUEOUS SOLUTION. ONE OF THE MORE COMMON IS ACETIC ACID. IT IS USUALLY WRITTEN AS: HC2H3O2 OR AS HAc WHERE Ac- REPRESENTS THE ACETATE ION C2H3O2THE STRUCTURE IS REALLY REPLACABLE HYDROGEN THE –CO2H GROUP IS AN ORGANIC (CARBON BASED) ACID GROUP. THERE ARE MANY ORGANIC ACIDS WITH THIS FUNCTIONAL GROUP. VINEGAR IS 3% ACETIC ACID. ONLY 1% OF THE ACETIC ACID MOLECULES DISSOCIATE IN WATER SOLUTION. ACIDS ARE CHARACTRIZED BY A SOUR TASTE. FOR EXAMPLE, CITRUS FRUITS CONTAIN CITRIC ACID. TWO REPRESENTATIONS OF CITRIC ACID, C6H8O7 ANOTHER EXAMPLE OF A STRONG ELECTROLYTE WOULD BE A STRONG BASE. A BASE IS A SUBSTANCE THAT PRODUCES HYDROXIDE IONS, OH-, WHEN DISSOLVED IN WATER. EXAMPLES OF STRONG BASES WOULD INCLUDE NaOH - SODIUM HYDROXIDE KOH POTASSIUM HYDROXIDE - WHEN DISSOLVED IN WATER THEY WOULD DISSOCIATE 100% INTO THE METAL CATION AND THE HYDROXIDE ION. NaOH Na+ (aq) + OH- (aq) BASES ARE CHARACTERIZED BY A BITTER TASTE. THEY ALSO HAVE A SLIPPERY FEEL. WEAK BASES EXIST, WHICH DO NOT DISSOCIATE 100%. THE MOST COMMON IS AMMONIUM HYDROXIDE, NH4OH. NH4OH NH4+ (aq) + OH- (aq) AMMONIUM HYDROXIDE DISSOCIATES ONLY ~1% IN AQUEOUS SOLUTION. SO, IN CLASSIFYING SUBSTANCES ON THE BASIS OF ELECTRICAL CONDUCTIVITY IN WATER SOLUTIONS, WE WOULD HAVE STRONG ELECTROLYTES - IONIC SUBSTANCES - STRONG ACIDS - STRONG BASES WEAK ELECTROLYTES - WEAK ACIDS - WEAK BASES TO THESE CATEGORIES, WE WOULD HAVE TO ADD NONELECTROLYTES. THESE ARE SUBSTANCES THAT WOULD DISSOLVE IN WATER, BUT WOULD NOT CONDUCT ELECTRICITY. NONELECTROLYTES WOULD BE POLAR COMPOUNDS IN ORDER TO DISSOLVE IN WATER, BUT THEY WOULD NOT IONIZE, SO THEY WOULD NOT CONDUCT ELECTRICITY. EXAMPLES WOULD INCLUDE ALCOHOLS AND SUGARS. AN ALCOHOL IS CHARACTERIZED BY AN –OH GROUP ATTACHED TO A CARBON. Ethyl alcohol, C2H5OH Alcohol group THE –OH GROUP CAN HYDROGEN BOND JUST LIKE WATER. YOU CAN THINK OF A SUGAR AS A POLYALCOHOL. SO, WATER SOLUBLE NONELECTROLYTES HAVE SOME SORT OF POLAR GROUP(S) ON THE MOLECULE THAT ALLOW THE MOLECULE TO DISSOLVE IN WATER – TO HYDROGEN BOND WITH WATER. ALCOHOLS AND SUGARS ARE BUT TWO EXAMPLES, AND IN THESE TWO EXAMPLES IT IS THE PRESENCE OF THE –COH GROUP (ALCOHOL FUNCTIONAL GROUP) THAT GIVES THEM WATER SOLUTILITY.