Science league topic 9: General properties of aqueous solution

1. Science league topic 9: General properties of aqueous solution

2. Solutions: • Homogeneous mixtures of two or more pure substances. • The solvent is present in greatest abundance. • All other substances are solutes.

3. One of the most valuable properties of water is its ability to dissolve. An individual water molecule has a bent shape with a H-O-H bond angle of approximately 105 degrees. Water is polar thus having positive & negative partial charges on its ends. Water: The Universal Solvent

4. Ionic Compounds in Water • When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them. • The positive ends of a water molecule are attracted to negative cations and the negative ends are attracted to positive cations in an ionic compound – this is called dissociation or ionization. • The ions become hydrated & move around independently.

5. SOLUBLEIONIC COMPOUNDS “DISSOCIATE” NaCl(s)  Na+(aq) + Cl-(aq) AgNO3(s)  Ag+(aq) + NO3-(aq) MgCl2(s)  Mg2+(aq) + 2 Cl-(aq) Na2SO4(s)  2 Na+(aq) + SO42-(aq) AlCl3(s)  Al3+(aq) + 3 Cl-(aq)

6. Some covalent compounds IONIZE in solution Covalent acids form ions in solution, with the help of the water molecules. For instance, hydrogen chloride molecules, which are polar, give up their hydrogens to water, forming chloride ions (Cl-) and hydronium ions (H3O+). Strong acids are completely ionize when dissolved in water.

7. Covalent Compounds in Water • Water also dissolves many nonionic substances such as ethanol (C2H5OH). • The reason for this is that ethanol is also polar. • Polar dissolves polar – “like dissolves like”. • This is the reason water will not dissolve oil.

8. “Like Dissolves Like” Nonpolar solutesdissolve best in nonpolar solvents Polar and ionic solutesdissolve best in polar solvents. But there are exceptions.

9. Definition of Electrolytes and Nonelectrolytes An electrolyte is: • A substance whose aqueous solution conducts an electric current. A nonelectrolyte is: • A substance whose aqueous solution does not conduct an electric current.

10. How to tell if the solution is Electrolyte The ammeter measures the flow of electrons (current) through the circuit. If the ammeter measures a current, and the bulb glows, then the solution conducts. If the ammeter fails to measure a current, and the bulb does not glow, the solution is non-conducting.

11. Solutions as Electrolytes • Strong conductor = strong electrolyte, completely ionized in water • Weak conductor = weak electrolyte, partially ionized • No conductor - nonelectrolyte

12. Electrolytes and Nonelectrolytes Solution of molecular compounds tend to be nonelectrolytes • except for acids and bases (NH3). Dissolving rxn?

13. Strong Electrolytes Are… • Strong acids,completely ionized in solution. • Strong bases • Soluble ionic salts

14. Solubility rules • Check the handout for complete list.

15. Weak acids such as lactic acid usually ionize less than 5% of the time. • HF is weak acid, but strongly active. It is used for itching glass. • Many of these weaker acids are “organic” acids that contain a “carboxyl” group. • The carboxyl group does not easily give up its hydrogen .

16. Because of the carboxyl group, organic acids are sometimes called “carboxylic acids”. Other organic acids and their sources include: • Citric acid – citrus fruit • Malic acid – apples • Butyric acid – rancid butter • Amino acids – protein • Nucleic acids – DNA and RNA • Ascorbic acid – Vitamin C This is an enormous group of compounds; these are only a few examples.

17. Answers to Electrolytes NONELECTROLYTES: ELECTROLYTES: • Pure water • Sugar solution • Ethanol solution • Pure NaCl • Tap water (weak) • NaCl solution • HCl solution • Lactate solution (weak)

18. Ionic equation

19. The molecular equation lists the reactants and products in their molecular form. AgNO3 (aq) + KCl(aq) AgCl(s) + KNO3 (aq) Molecular Equation

20. In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions. This more accurately reflects the species that are found in the reaction mixture. Ag+(aq) + NO3- (aq) + K+ (aq) + Cl- (aq)  AgCl(s) + K+(aq) + NO3- (aq) Ionic Equation

21. To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl(s) + K+(aq) + NO3-(aq) Net Ionic Equation

22. To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Ag+(aq) + Cl-(aq) AgCl(s) Net Ionic Equation

23. To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right. The only things left in the equation are those things that change (i.e., react) during the course of the reaction. Those things that didn’t change (and were deleted from the net ionic equation) are called spectator ions. Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) AgCl(s) + K+(aq) + NO3-(aq) Net Ionic Equation

24. Writing Net Ionic Equations • Write a balanced molecular equation. • Dissociate all strong electrolytes (aq). • Cross out anything that remains unchanged from the left side to the right side of the equation. • Write the net ionic equation with the species that remain.

25. Writing Net Ionic Equations

26. Homework • Page 177: 25, 31, 43a, 63, 64, 65, 66, 71, 73