Acids and Bases

Acid Acids produce H+ when dissolved in water. Acids taste sour. Acids sting or burn to the touch. Acids are very reactive—strong acids will dissolve metals and burn organic matter (living or formerly living things)

Bases (Alkalis) Bases produce OH- ions when dissolved in water. Bases taste bitter. Bases feel slippery. Bases are very reactive—strong bases will burn organic matter (living or formerly living things).

Strength of Acids and Bases Not all acids are equal. The acid in a car battery will burn skin very quickly. Lemon juice is safe to eat, but still burns if you get it into your eyes or a cut. What determines how dangerous an acid is? The concentration of hydrogen ions in the acid.

Concentration Concentration tells how much of something is in one place. One way to talk about concentration is with a percentage. Vinegar is about 5% acetic acid (the other 95% is mostly water). A solution of 5% HCl (hydrochloric acid) contains about 250 times as many H+ ions as vinegar. HCl is less massive than acetic acid and dissolves more completely. They have the same concentration by mass, but 5% HCl is 250 times stronger.

Concentration We need a way to measure concentration by the number of particles in the solution. The problem is there are lots of particles to count. In 1 microliter of vinegar there are more hydrogen ions than all the grains of rice grown in Thailand last year. We need an easy way to figure out how many particles and to count them at the same time.

Moles Moles? A mole is a number like a dozen. 1 dozen = 12 1 mole = 6.022x1023 1 mole of a substance is 6.022x1023 atoms or molecules of that substance. 1 mole of a substance has a mass equal to the atomic mass of the substance Moles?

Molarity Molarity is a measure of how concentrated a solution is. It tells how many moles of solute are in one liter of a solution. Abbreviated “M” If one liter of solution contains one mole of solute the solution is said to be “one molar” (1M) M = mol/L

pH The strength of acids and bases is measured by its pH. pH is a measure of the molarity of H+ in a solution. It is (very roughly) the number of decimal places in the molarity of H+. Lower numbers represent strong acids; higher numbers, strong bases.

Neutrality Water can be thought of as hydrogen hydroxide—HOH. Sometimes a water molecule will split into H+ and OH-. This split doesn’t last long, so at any one time about one molecule in 555,000,000 will be split. That’s a molarity of 0.0000001 or a pH of 7. H20 → H+ + OH- H+ + OH- → H20 0.0000001mol/L 0.0000001M 0.0000001 = 10-7 -log10 (0.0000001) = 7 pH = 7 Substances with a pH of 7 are neither acid nor base. They are neutral.

Neutralization + If H+ and OH- are mixed, they form H2O. This means that when acids and bases of similar strength are mixed they cancel each other out—they neutralize. The products of a neutralization reaction are water and a salt (not always NaCl). +

Indicators Indicators are chemicals that change color depending on pH. Litmus paper is a common example—litmus is a purple dye that turns red in acid and blue in a base. Universal indicators turn a variety of colors to give a better indication of pH. Indicators can also be liquids, such as phenolphthalein and red cabbage juice.

Buffers Buffers are weak acids or weak bases that prevent changes in pH. In blood, carbonic acid (H2CO3) and bicarbonate (HCO3-) act as buffers keeping pH between 7.35 and 7.45. MgO is used to buffer aspirin and prevent stomachaches that can result from taking aspirin.