Strength of Acids and Bases The strength of an acid (or base) is determined by the amount of IONIZATION This is not the concentration If the acid (or base) completely dissociates to produce H+ (or OH-) ions in solution, STRONG ACID (or BASE) If the acid (or base) dissociates to a slight extent in aqueous solution, WEAK ACID (or BASE)

STRONG ACIDS A strong acid contains a relatively weak conjugate base One that has a low attraction for protons HNO 3, HCl, H 2 SO 4 and HClO 4 are among the common strong acids

Weak acids are much less than 100% ionized in water Contains a relatively strong conjugate base – High attraction for protons One of the best known is acetic acid (CH 3 CO 2 H) Weak Acids

STRENGTH OF ACIDS AND THEIR CONJUGATE BASES

Strong Base – 100% dissociated in water NaOH (aq) ---> Na + (aq) + OH - (aq) Weak base – Less than 100% ionized in water – One of the best known weak bases is ammonia NH 3 (aq) + H 2 O (l)  NH 4 + (aq) + OH - (aq) Strong and Weak Bases

COMMON ACIDS AND BASES

WATER WILL IONIZE ITSELF

[H + ] is critical in many chemical reactions Because [H + ] in an aqueous solution is typically quite small, a logarithmic pH scale is used! By definition: pH = – log [H + ], [H + ] = 10 -pH The pH scale, similar to the Richter scale, describes a wide range of values – Because the pH scale is a log scale based on 10, the pH changes by 1 for every power-of-10 change in [H + ] – An earthquake of “6” is 10 times as violent as a “5” Thus, the pH scale condenses possible values of [H + ] to a 14 point scale Also, it is easier to say pH = 7 vs. [H + ] = 1 x 10 –7 DETERMINING THE ACIDICITY OF A SOLUTION

How to Read the pH Scale Under 7 = acid 7 = neutral Over7 = base

pH = - log [H+]; [H+] = 10 -pH (Remember that the [ ] mean Molarity) What is the pH if [H + ]= 6.3 x 10 –5 ? CALCULATIONS WITH PH Ans: x 10 –8 M What is the [H + ] if pH = 7.4?

Calculate the pH of each of the following: A 0.15 M solution of hydrochloric acid A 3.00 X M solution of nitric acid MORE PRACTICE

Since acids and bases are opposites, pH and pOH are opposites! pOH does not really exist, but it is useful for changing bases to pH pOH looks at the perspective of a base: pOH = - log [OH - ] Since pH and pOH are on opposite ends: pH + pOH = 14 Determining the Basicity of a Solution pOH

pH [H + ] [OH - ] pOH

PRACTICE! The pH of rainwater collected in a certain region of the northeastern United States on a particular day was What is the H + ion concentration of the rainwater? The OH - ion concentration of a blood sample is 2.5 x M. What is the pH of the blood?

Calculating [H 3 O + ], pH, [OH - ], and pOH A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) M. Calculate the [H 3 O + ], pH, [OH - ], and pOH of the two solutions at 25°C. What is the [H 3 O + ], [OH - ], and pOH of a solution with pH = 3.67? Is this an acid, base, or neutral? Problem above with pH = 8.05?

There are several ways to test pH – Blue litmus paper red = acid – Red litmus paper blue = basic – pH paper multi-colored – pH meter 7 is neutral, 7 base – Universal indicator multi-colored – Indicators Phenolphthalein – Natural indicators like red cabbage, radishes pH TESTING

pH Indicators Indicators are dyes that can be added that will change color in the presence of an acid or base Some indicators only work in a specific range of pH Once the drops are added, the sample is ruined Some dyes are natural, like radish skin or red cabbage

Paper Testing Paper tests like litmus paper and pH paper – Put a stirring rod into the solution and stir – Take the stirring rod out – Place a drop of the solution from the end of the stirring rod onto a piece of the paper – Read and record the color change – Note what the color indicates – You should only use a small portion of the paper You can use one piece of paper for several tests

pH Meter Remember that acids and bases are electrolytes – Strong acids/bases are STRONG electrolytes – Weak acids/bases are WEAK electrolytes Tests the voltage of the electrolyte Converts the voltage to pH Very cheap, accurate Must be calibrated with a buffer solution

To analyze the acid or base content of a solution, chemists often perform a TITRATION – A titration involves the delivery of a measured volume of a solution of known concentration (TITRANT) from a buret into the solution being analyzed (ANALYTE) – Titrant is added slowly to the analyte until exactly enough has been added to just react with all of the analyte EQUIVALENCE POINT – [H+] = [OH-] – Indicator or pH meter is used to determine equivalence point – Neutralization is said to occur when the acid and base have same numbers of moles present in the combined solution ACID-BASE REACTIONS TITRATIONS

Setup for Titrating an Acid with a Base

Based on stoichiometry – Write a balanced chemical equation – Calculate the number of moles of the standard solution present in the solution Standard solution is the solution in the buret and has a known concentration Will always be given a volume and concentration of the standard solution – Use the molar relationship from the equation (STOICHIOMETRY) to convert from moles of standard solution into moles of unknown solution – Calculate the unknown quantity (molarity or volume) of the unknown solution by using the relationship M = n/V using the number of moles found in step 3 – Sometimes, you will need to convert from moles to other quantities such as volume of a gas (L) and grams Remember your stoichiometry road map! HOW TO SOLVE A TITRATION-TYPE PROBLEM

Boric acid (H3BO3) is a solid acid that is good for controlling ants. It also neutralizes bases. How many grams of boric acid would be needed to neutralize grams of calcium hydroxide? NEUTRALIZATION (TITRATION) EXAMPLE PROBLEM 1

35.62 mL of NaOH is neutralized with 25.2 mL of M HCl by titration to an equivalence point. What is the concentration of the NaOH? NEUTRALIZATION (TITRATION) EXAMPLE PROBLEM 2

25 drops of citric acid are titrated with.112 M KOH until the indicator changes from yellow to blue. 48 drops of KOH are added before a color change occurs. What is the concentration of the citric acid? What is the pH of the citric acid? **20 drops = 1 mL** NEUTRALIZATION (TITRATION) EXAMPLE PROBLEM 3