CH305 Chapter 5 notes part 2

CH305 Chapter 5 notes part 2 - CH 305 Chapter 5 part 2: The...

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CH 305 Chapter 5 part 2: The Water We Drink Back to Water as a Solvent Consider an IONIC substance: ions in the crystal attract polar water molecules - these begin to pull a crystal apart. Free ions are surrounded by water molecules: See Fig 5.15 in your book. Not ALL ionic compounds are soluble! Sometimes ions are too strongly held to each other for the water molecules to be able to pull them out of the crystal. How do you know which ones dissolve? See Table 5.9 and the one in the HW. Solubility: Precipitation Mix two soluble substances:- the positive ion of one compound swaps for the positive ion of the other - sort of ‘square dance’ One of these new compounds may not be soluble and forms a precipitate . To decide if something will precipitate : Swap the ions of the original compounds and look up the new substances in the solubility table. Examples: Silver Nitrate and Sodium Chloride Lead Nitrate and Sodium Sulfide: Solubility (also see Table 5.10) Your Turn 5.23 page 240 Solubility in water has environmental implications: Examples: Calcium Carbonate is insoluble (see Chapter 6 - it reacts and form soluble compounds with acids ) Many other rocks are made of insoluble minerals. All nitrates are soluble, and are used heavily in fertilizer Lead and Mercury are insoluble as metals / compounds. IF a reaction turns them into soluble compounds (e.g., nitrates), they can contaminate drinking water. PCB’s (coolants in transformers), DDT, perc (dry cleaning solvent) are insoluble but have escaped into the environment, and concentrate in the body fat of fish and prey animals. So why does sugar Dissolve? Hydrogen Bonding! Molecular compounds which contain O-H bonds are able to hydrogen bond to water molecules, which enables them to dissolve and mix with water. A few examples of molecules with O-H bonds that dissolve: Table sugar (sucrose) (Fig 5.16), Ethanol (booze) (Fig 5.17), Methanol (don’t drink this) Ethylene Glycol (antifreeze) (fig 5.17) Can you now see why oil and water (or gasoline and water. .) do not mix? Remember this: "Like dissolves Like" Nonpolar solutes dissolve in non-polar solvents; polar solutes dissolve in polar solvents. Comparing Ionic and Covalent Compounds The difference between ionic and covalent bonding leads to a great difference in the general properties of ionic and covalently bonded compounds. Learn these: hint: think of salt and sugar! IONIC COMPOUNDS •High MP and BP (solids) •Soluble in polar solvents like water ( often ) •Insoluble in nonpolar solvents like hexane, toluene •Molten and/or aqueous solutions conduct electricity well •Formed from elements with very different electronegativities
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COVALENT COMPOUNDS •Lower MP and BP (solid, liquid or gas) •Often insoluble in polar solvents (exception: compounds which can H-bond) •Many soluble in nonpolar solvents ••Molten and/or aqueous solutions poor conductors of electricity •Formed from elements with similar electronegativities Legislature: 1974 Safe Drinking Water Act (Table 5.11 page 243)
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CH305 Chapter 5 notes part 2 - CH 305 Chapter 5 part 2: The...

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