Strengths of Acids and Bases

What does it mean to be strong? In regards to an acid or base: The strength of an acid or base has nothing to do with Molarity or concentration. Rather, it is how completely the substance breaks apart into its ions. A strong acid will almost completely dissociate leaving no undissolved molecules in solution. A weak acid only partially dissociates in solutions.

Strengths of Acids and Bases Some acids are better proton donors than others, and some bases are better proton acceptors than others. There is an inverse relationship between the two. The easier a substance donates a proton, the harder it is for the conjugate base to accept a proton. In other words: the stronger an acid, the weaker its conjugate base the stronger a base, the weaker its conjugate acid

Strong Acids There are several acids that are known to be strong – what does that mean again? Not an obvious way to recognize them, so need to memorize them: HCl – HydrochloricHNO 3 – Nitric HI – HydroiodicHClO 3 – Chloric HBr – Hydrobromic HClO 4 – Perchloric H 2 SO 4 – Sulfuric

Strong Bases Same thing with Bases. There are several strong bases that will almost completely dissociate. They are hydroxides of Alkali and Alkaline Earth Metals. LiOH - lithium hydroxide NaOH - sodium hydroxide KOH- potassium hydroxide Ca(OH) 2 - calcium hydroxide Sr(OH) 2 - strontium hydroxide Ba(OH) 2 - barium hydroxide

Why is strength important? The strength of an acid or base determines what direction an equation will favor, to the left or to the right. Identify the two acids in the below reaction. (Hint, one is the conjugate) HCl (aq) + H 2 O(l)  H 3 O + (aq) + Cl - (aq) The reaction favors the transfer of the proton from the strongest acid to the weakest base. Which direction will this reaction favor? Which is the stronger acid? ACID 

Acid and Base Strength This chart shows the strength of some acids and bases. The top represents the strongest acid and their weak conjugate bases. The bottom represents the strong bases and their weak conjugate acids.

Acid and Base Strength Think about the above equation. Which direction will be favored? To the Left or To the Right? Use the chart to determine which is the stronger Acid. The Hydronium Ion is higher than acetic acid and acts as the stronger acid. The reaction will favor….. HC 2 H 3 O 2 (aq) + H 2 OH 3 O + (aq) + C 2 H 3 O 2 – (aq) 

Practice Problems: Identify the Acid, Base, and their Conjugates. Then use the chart to determine which direction the reaction favors, to the products or reactants: HCO 3 – + OH -  H 2 O + CO 3 2– H 2 O + HC 2 H 3 O 2  H 3 O + + C 2 H 3 O 2 -