2H 2 O H 3 O + + OH Hydronium ion hydroxide ion Q: But how often does this happen? This is the fundamental concept of all acid-base chemistry In pure water, how much of it is water and how much is ions? 2H 2 O H 3 O + + OH A: In pure water, the That s s only 0.0000001 M! In pure water, the concentration of OH - is also 1.0 x M. In pure water, [H 3 O + ] = [OH - ] Hydronium Ion Concentration = Hydroxide Ion Concentration Let s s increase the concentration to 1.0 x M that s s 0.000001 M H 3 O +.. now the ph = 6.00 Now, Let s s increase the H 3 O + concentration to 1.0 x 10 (that s s 0.00001 M H 3 O + ).. now the ph = 5.00 10-5 M Let s s increase the H 3 O + concentration to 1.0 x 10-4 M (that s s 0.0001 M H 3 O + ).. now the ph = 4.00 1
ph = a measure of how acidic or basic a substance is ACIDS BASES Let s s increase the H 3 O + concentration to 1.0 x 10 (that s s 0.001 M H 3 O + ).. now the ph = 3.00 10-3 M The Nature of Acids and Bases ph = - log [H 3 O + ] where [H 3 O + ] means the concentration of Hydronium Ions. ph ph = - log [H + ] where [H + ] is a simplification of what really exists, [H 3 O + ] ions. ph and hydronium ions ph means the power of hydronium The amount of H 3 O +1 in a substance determines the acidity. The amount of OH -1 determines the basisity. To calculate ph: ph = - log[h 3 O +1 ] ph equation What is a logarithm? On the Ti-83, push -log,then the number and you will get the ph. Round to.1 place. NO UNITS!! 2
Indicators There are many kinds of acid/base indicators. One of these is phenolphthalein which turns at about a ph of 8. How Indicators Work Indicators are substances that reflect one color of light in acidic solutions and a different color in basic solutions. It is CLEAR in acids and pink in bases. Various indicators, basic solutions on left and acidic solutions on right. Indicators, Continued Different indicators react to different levels of acidity. Different indicators display different colors. Not all indicators react the same way to identical acid or base solutions. Acids or bases cause the indicator molecule to change shape and reflect a different color of light. Titration Titration = A procedure for determining the concentration of a solution. Acid-Base Titration = A titration that uses an acid-base neutralization reaction. How does it work? 3
Acid - Base Titration We neutralize a solution of unknown concentration with a solution of known concentration. Add acid to base, or base to acid Use an indicator or a ph meter to find the endpoint. When a solution is neutral [H 3 O + ] = [OH - ] Hydronium Ion Concentration = Hydroxide Ion Concentration When a solution is neutral moles of acid = moles of base this occurs at the endpoint or equivalence point in a titration. Neutralization has occurred, and so Can be used to calculate the concentration of the unknown. Acid-Base Titration: Find the Molarity of 50.0 ml an unknown acid solution if only 75.0 ml of 0.125 M NaOH is needed for neutralization. If it takes 18.7 ml of a 1.0 M NaOH solution to neutralize 10.0 ml of HCl, what is the concentration of the HCl? How much of a 0.275 M HCl will be needed to neutralize 25.0 ml of 0.15 M NaOH? 4
Another one.05 moles of HCl is dissolved in 500 ml of water. If 25 ml of the acid is added to a.05 molar solution of NaOH. How many ml of base will be needed to titrate the acid to the end point? Strong and Weak Acids Strong acids are strong electrolytes they completely ionize in solution There are 6 strong acids: HCl hydrochloric acid HNO 3 nitric acid H 2 SO 4 sulfuric acid HBr hydrobromic acid HI hydroiodic acid HClO 4 perchloric acid Weak acids are weak electrolytes they only partially ionize in solution Examples: any other acid acetic acid vinegar Strong and Weak Bases Strong bases are strong electrolytes they dissociate completely into ions in solution. LiOH lithium hydroxide NaOH sodium hydroxide KOH potassium hydroxide RbOH rubidium hydroxide CsOH cesium hydroxide Weak bases are weak electrolytes they only partially ionize in solution Ex. NH 3, H 2 O Strength and Concentration The strength and concentration of an acid or base are not the same thing! The ph of a solution depends on its strength and its concentration. A weak acid can have a dangerously low ph if it is highly concentrated A strong acid can be diluted so that the ph of the solution is not too low. A weak base can have a dangerously high ph if it is highly concentrated A strong base can be diluted so that the ph of the solution is not too high. 5