the universal solvent

Transcription

1 Chapter 7: Acids, Bases, and Solutions Solution a homogeneous mixture Solutions have the same properties throughout, containing solute particles (molecules or ions) that are too small to see Solvent the part of the solution that is present in the largest amount and dissolves the other substances Solute the substance that is present in a solution in the smaller amount and is dissolved by the solvent Water the universal solvent Water is the most common solvent and can dissolve so many things that it is known as the universal solvent Solutions without water Not only can solutions be made without water, solutions can be made in any combination of solids, liquids, and gases Examples of Common Solutions Solute Solvent Sample Solution Gas Gas Air (oxygen and other gases dissolved in nitrogen) Gas Liquid Soda water (carbon dioxide in water) Liquid Liquid Antifreeze (ethylene glycol in water) Solid Liquid Dental filling (silver in mercury) Solid Liquid Ocean water (sodium chloride and other compounds in water) Solid Solid Brass (zinc in copper) or bronze (tin in copper) Colloids and suspensions Colloid a mixture containing small, undissolved particles that do not settle out Colloids contain particles too small to see but that are large enough to scatter light beams Tyndall effect the scattering of light by the very small particles in a colloid Examples of a colloid Fog, gelatin, milk, mayonnaise, whipped cream, and ball point pen ink Suspension a mixture in which the particles are easily seen and large enough to be separated by settling or filtration Unlike solutions, suspensions do not have the same properties throughout, they are heterogeneous Particle Size Types of Mixtures Heterogeneous Homogeneous Simple Suspension Colloid Solution Visible Greater than 1 µm Microscopic 1 µm down to 1 nm Molecular / Ionic 1 nm down to 62 pm Settles Out Yes No No Key Feature Easily Separated by Filtering Tyndall Effect and Brownian Motion Clear

2 Particles in a solution Ionic solutes Water molecules are polar (they have one slightly positive end, δ+, and one slightly negative end, δ ) and so are attracted to the opposing charges of the ions in ionic solutes Note that ions in solution can conduct electricity Molecular solutes Polar molecules, like sugar or methanol, are also easily dissolved in water because the oppositely charged ends can attract each other Effects of solutes on solvents Solutes lower the freezing point and raise the boiling point of a solvent Pure water freezes at 0 C and boils at 100 C Brine (saturated NaCl) freezes at about -23 C and boils at about 107 C Adding a solute to water reduces the percentage of water molecules available to either nucleate (form a solid crystal) or vaporize so the water must be made colder to freeze or hotter to boil More solute particles raises the boiling point more (or lowers the freezing point more) Common examples Salt is put on roads to lower the freezing point of water (thus melting the snow) Ethylene glycol is added to radiators in cars to prevent the water from freezing in the winter and also raises the boiling point so water will evaporate more slowly Concentration and solubility Concentration a comparison of the amount of solute to the amount of solvent (or to the total amount of solution) Concentrated solutions have a large amount of dissolved solute Dilute solutions have a small amount of dissolved solute Solubility a measure of how much solute can be dissolved in a solvent at a given temperature Saturated solution a condition when the solvent cannot dissolve any more solute Unsaturated solution a condition when the solvent can dissolve more solute Supersaturated solution a condition in which more solute is dissolved than is predicted by solubility at the given temperature Solubility is a characteristic property of a solute and can be used to identify substances water at 0 C can dissolve 35.7 g of table salt (NaCl) but only 6.9 g of baking soda (NaHCO 3 ) and 180 g of sugar (C 12 H 22 O 11 )

3 Factors affecting solubility Pressure Solubility of gases is especially affected by pressure: that s why gas bubbles escape from soda when the cap is removed CO 2 is put into the bottle under pressure so water dissolved more of the gas Scuba divers can get bends if they dive too deep the added pressure can cause nitrogen (which does not dissolve appreciably at normal pressures) in the blood and when the divers return to the surface bubbles of nitrogen form in their blood which can be fatal Solvent Alcohol (a polar molecule) or salt (which forms charged ions) easily dissolve in water but oil (a nonpolar molecule) does not dissolve in water Scientists use the expression like dissolves like to describe the observation that nonpolar solvents are required for nonpolar solutes and polar solvents are required for polar or ionic solutes Temperature For most solids, solubility increases as the temperature increases At 0 C 180 grams of sugar dissolves in 100 g of water but at 100 C 487 g of sugar will dissolve in 100 g of water By increasing the temperature of the water more sugar can be dissolved and when the temperature is lowered, sometimes the sugar will remain dissolved (supersaturation) If a string or seed crystal of sugar is added to the supersaturated solution the excess sugar will quickly form crystals known as rock candy Scuba divers can get bends if they dive too deep the added pressure can cause nitrogen (which does not dissolve appreciably at normal pressures) in the blood and when the divers return to the surface bubbles of nitrogen form in their blood which can be fatal Describing acids and bases Acids a substance that tastes sour, reacts with metals and carbonates, and turns blue litmus paper red Sour taste some foods have naturally occurring acids and therefore they taste very sour Lemons and grapefruits have citric and ascorbic acids Caution: chemists do not use taste to identify acids in the laboratory many foods have naturally occurring acids which are safe to eat but many acids found in the lab are toxic Reactions with metals acids are described as corrosive and will react with metals eating them away Some metals do react with acids: zinc, magnesium, and iron Other metals do not react with acids: copper, silver, and gold will react with very few acids Reactions with carbonates carbonates contain the polyatomic anion CO 2 3 which reacts with acids to form carbonic acid which then decomposes to form water and CO 2 Example: geologists use hydrochloric acid, HCl (aq), to test for limestone (calcium carbonate) 2 HCl + CaCO 3 CaCl 2 + H 2 CO 3 H 2 CO 3 H 2 O + CO 2 Reactions with indicators acids turn blue litmus red Indicator a compound that changes color when in contact with an acid or a base Bases a substance that tastes bitter, feels slippery, and turns red litmus paper blue Bitter taste quinine is a base that gives tonic water its bitter taste Soaps, shampoos, and detergents contain bases and would be bitter to the taste but no one should identify them by tasting

4 Slippery feel bases feel slippery like soap because they turn your skin into soap therefore it is not safe to identify bases by their slippery feel Reactions with indicators bases turn red litmus blue Other reactions of bases there are many useful reactions of bases Bases react with fat or tallow to create castile soap Bases react with acids to form salt and water Uses of Acids and Bases Acids and bases have many uses around the home and in industry Uses of acids Vitamins ascorbic acid and folic acid Body processes lactic acid is a waste product in cells that causes muscles to feel tired Sulfuric acid is used in car batteries Dilute acids are used to clean cement driveways and bricks and mortar Nitric and phosphoric acids are used to make fertilizers Uses of bases Mortar and cement are made from calcium oxide and calcium hydroxide Baking soda is a base that reacts with acids in foods to make bread and cookies rise Bases are used as drain cleaners and to make soaps and shampoos Acids and bases in solution Svante Arrhenius defined acids as substances that produce hydrogen ions (H + ) in water solutions Example: HCl H + + Cl Svante Arrhenius defined bases as substances that produce hydroxyl ions (OH ) in water solutions Example: NaOH Na + + OH Not all bases contain a hydroxyl anion, but they still produce one in water Example: NH 3 + H 2 O NH OH Common Acids and Bases Acid Formula Base Formula hydrochloric acid HCl sodium hydroxide NaOH nitric acid HNO 3 potassium hydroxide KOH sulfuric acid H 2 SO 4 calcium hydroxide Ca(OH) 2 carbonic acid H 2 CO 3 aluminum hydroxide Al(OH) 3 acetic acid HC 2 H 3 O 2 ammonia NH 3 phosphoric acid H 3 PO 4 calcium oxide CaO Strength of acids and bases Many people confuse concentrated and dilute with terms that scientists call strong and weak acids and bases Concentrated means there is a large amount of solute dissolved in the solvent and dilute means there is a small amount of solute dissolved in the solvent

5 Strong acids and bases ionize 100% in solution while weak acids and bases ionize only a small amount Strong acid: HCl H + + Cl 100% of the HCl forms H + ions Weak acid: HC 2 H 3 O 2 H + + C 2 H 3 O 2 < 0.1% of the HC 2 H 3 O 2 forms H + ions Using the ph scale Notice that some acids and bases form very small amounts of H + or OH ions The ph scale was developed by scientists to measure very small amounts of ions ph means the negative of the power of 10 of the concentration of the H + ion ph 12 means the concentration of the H+ ion is 1 x molar a basic solution ph 7 means the concentration of the H+ ion is 1 x 10 7 molar a neutral solution ph 2 means the concentration of the H+ ion is 1 x 10 2 molar an acidic solution Numbers like 12, 7, or 2 are much easier to use than numbers like (or 10 7 ) Small numbers, like ph = 2, indicate a much higher concentration of H + (0.01) than larger numbers, like ph = 12, which equals an H+ of Therefore, low ph values represent high H + concentrations or acidic solutions and high ph values represent low H + concentrations or basic solutions Lemon juice has a ph of about 2 (fairly acidic) Ammonia has a ph of about 11.5 (fairly basic) Acid base reactions Acid base reactions are often called neutralization reactions by chemists Mixing an acid solution with a base solution will result in a final solution with a ph that is somewhere between the original or reactant solutions If the volumes and concentrations of the acid and base reactant solutions are chosen carefully, the result will be a neutral (ph = 7) solution of salt and water The reactants of a neutralization reaction are an acid and a base The products of a neutralization reaction are a salt and water HCl + NaOH H 2 O + Na + + Cl HNO 3 + KOH H 2 O + K + + NO 3 Digestion Mouth saliva in the mouth is nearly neutral, ph = 7, (no sour and no bitter taste) which is the perfect ph for amylase (which breaks starches into sugars) Stomach the stomach is quite acidic, ph = 2, where the acids and other enzymes break food down Small intestine here, bacteria help break down food at a slightly basic ph of about 8