Chapter 9 pH

pH: Acid-Base Concentration The relative concentration of hydrogen ions is measured in concentration units called pH units Expressed in terms of moles per liter, or molarity –The greater the concentration of hydrogen ions in a solution, the more acidic the solution –The greater the concentration of hydroxyl ions, the more basic, or alkaline, the solution –The pH scale extends from 0-14 and is logarithmic (each successive change of one pH unit represents a tenfold change in hydrogen ion concentration) The pH of a solution is thus defined as the negative logarithm of the hydrogen ion concentration (H + ) in moles per liter or –log[H + ] –A pH of 7 is neutral (at which [H + ] is M) »The number of hydrogen ions exactly equals the number of hydroxyl ions (pH=pOH) A pH below 7 is acidic A pH above 7 is basic or alkaline

pH SCALE

pH pH SCALE *measure of the hydronium ion ( H3O+) concentration (hydrogen ion H+ ion concentration) *hydronium ion is formed by the attraction between a hydrogen ion ( H+ ) from an acid and a water molecule ( H2O ) *indicates HOW ACIDIC a solution is *series of numbers 0 to 14 *middle is 7 (neutral point) *below 7 (acid) *above 7 (base)

pH 10-7 MOLES OF H3O+ IONS IN 1 LITER OF H2O pH moles H3O+ / liter H2O MOLES H3O+ / LITER H2O pH 1 ( 0.1 moles/liter ) ( moles/l) pH 2 ( 0.01 moles/liter ) ( moles/l) pH 3 ( moles/liter ) ( moles/l) pH 4 ( moles/liter ) ( moles/l) pH 5 ( moles/liter ) ( moles/l) pH 6 ( moles/liter ) ( moles/l) pH 7 ( moles/liter) ( moles/l) pH 8 ( moles/liter ) ( moles/l) pH 9 ( moles/liter ) ( moles/l) pH 10 ( moles/liter ) ( moles/l) pH 11 ( moles/liter ) ( moles/l) pH 12 ( moles/liter ) ( moles/l) pH 13 ( moles/liter )( moles/l) pH 14 ( moles/liter ) ( moles/l)

Neutralization Neutralization occurs when an acid and a base are mixed together –They react with each other in displacement reactions to form a salt and water –Example: when hydrochloric acid and sodium hydroxide interact, sodium chloride (a salt) and water are formed –HCl + NaOH → NaCl + H 2 O Called a neutralization reaction, because the joining of H + and OH - to form water neutralizes the solution Although the salt produced is written in molecular form (NaCl), remember that it actually exists as dissociated sodium and chloride ions when dissolved in water

Acids, Bases, and Salts in the Home Water does not mix with grease or oil. Cleaning products improve water’s ability to clean because they help water mix with oily substances –Soap allows water and oil to mix Soaps are salts of sodium or potassium and fatty acids, which have long hydrocarbon chains. –The hydrocarbon chains of the soap are nonpolar, so they can mix with oils –The ionic parts of the soap molecules can mix with water Animal fats or vegetable oils react with sodium hydroxide or potassium hydroxide to make soap –The products of the reaction are soap and an alcohol called glycerol

Acids, Bases, and Salts in the Home Detergents have replaced soap in many uses –As useful as soap is for cleaning, it does not work well in hard water (contains dissolved Mg 2+, Ca 2+, and Fe 3+ cations that cause insoluble salts called scum) Settles on clothing, dishes, skin, hair, and ring around bathtub or sink Many shampoos, liquid soaps, and body washes contain detergents, not soap

Acids, Bases, and Salts in the Home Detergents are salts of sodium, potassium, and sometimes ammonium –Like anions in soaps, the anions in detergents are composed of long hydrocarbon chains that have negatively charged ends BUT. The charged end of a detergent is a sulfonate group (- SO 3 - ), not a carboxylate group (-CO 2 - ) –Sulfonate ions do not form insoluble salts with the ions in hard water –The hydrocarbon chains in detergents come from petroleum products rather than animal fats or plant oils

Acids, Bases, and Salts in the Home Soaps and detergents act in the same way. The long hydrocarbon chains are soluble in oil and grease. The charged ends are soluble in water. Water molecules attract the charged sulfonate group in detergent molecules and keep the oil droplet suspended among the water molecules

Acids, Bases, and Salts in the Home A water softener works through a process known as “ion exchange,” which means that as it removes calcium and magnesium (elements that make water “hard”), it replaces those ions on a one for one basis, usually with sodium. This means that the amount of sodium added to the water is directly proportional to how hard your water is. In most instances the amount being added is so small that it is not a significant contributor to the amount of sodium in your diet.

Acids, Bases, and Salts in the Home Dissolved calcium and magnesium precipitate out of hard water as scale, which builds up on the insides of pipes, water heaters, tea kettles, coffee makers and industrial machinery. Scale reduces flow through pipes and is a poor conductor of heat. Eventually, pipes can become completely clogged.water heatersteapipes Hard water reduces soap's ability to lather, whether in the shower, sink, dishwasher or washing machine, and reacts with soap to form a sticky scum.

Acids, Bases, and Salts in the Home

Many household cleaners contain ammonia –Ammonia is a solution of ammonia gas in water. It is a weak base because it ionizes only slightly in water to form ammonium ions and hydroxide ions which make it basic – –NH 3 + H 2 O   NH OH - –Many ammonia cleaners contain alcohols, detergents, and other agents

Acids, Bases, and Salts in the Home Bleach is a basic solution of sodium hypochlorite (NaOCl) –It is a very strong disinfectant (kills viruses and bacteria) –Removes colors and stains Unlike soaps and detergents it does not remove the substance causing the stain. It changes the stain to a colorless form When mixed with water, the hypochlorite ion (ClO - ), reacts to release an oxygen atom. The oxygen atom reacts with the colored molecule and bleaches it. –Mixed with acids/bases will produce deadly gases Acid (vinegar) produces chlorine gas (Cl 2 ) Bases (ammonia) produces toxic chloramine gas (NH 2 Cl)

Acids, Bases, and Salts in the Home Home products: –OJ: acid –Antaacids: neutralizes acidity –Clothing: acidic dyes –Shampoos are adjusted for pH Slightly acidic: clean less but retain keratin shine, protect dryness Basic: clean oil from hair, hair swells, less shine, dry hair

Acids, Bases, and Salts in the Home Antioxidants: prevents oxygen from reacting with molecules –Example: Lemon juice on freshly cut fruit (apple) Vitamin C in OJ

Acids, Bases, and Salts in the Home Kitchen: –Acids: marinades (unravel protein), bacteria used to produce yogurt (curdles milk- thick gel) –Salts: baking soda (sodium hydrogen carbonate) used in baking (releases CO 2 ) –Bases: drain cleaners