CSEC Chemistry (May 13, 2020)
Acids, Bases & Salts: The Missing Piece- Concentration vs Strength
If you really have been seriously reviewing this topic, even if you are looking at it for the first time, you would have come upon the term, “strong” and “weak” acids and alkalis.
As a child, when tasked with making tea or drinks by my mother, I was admonished not to make it “too sweet.” I barely added any sugar once and she asked, “Where yuh stay and throw in the sugar, Charlton?” (Charlton was miles away. That was her way of saying it was fresh.
Too fresh or too sweet speaks of concentration, that is, how much of a substance is present in a given volume.
Think of concentration like population density-number of persons living in an area (square units). Take it closer to your classroom (assuming each student is identical), concentration would be the number of students occupying that space (but the space is in terms of volume-(l*w*h). We could say a classroom measures 10 metres by 10 metres by height of 10 metres. Its volume would be 1000 cubic metres. The more students in that standard space, the more densely populated the classroom would be or the more concentrated. So, 40 students in a class would be 40 student “in that” given volume or 40 students “PER” cubic metre. (Not really going into the units of concentration used in Chemistry just yet)
As it relates to strength of an acid, imagine that each student from the above scenario were a molecule of HCl in the classroom filled with water. (That’s the only way they wouldn’t drown; just think of them as acid molecule). If all (or most) the 40 students do well and get a “1” in Chemistry, the pass rate is 100%. That is what happens with a strong acid: all the “units” of acid actually break down and form MANY ions in aqueous medium. We normally use a single headed arrow to show this.
HCl (aq) –> H+(aq) + Cl-(aq) (Many H+(aq) are formed)
Many H+(aq) would be formed as a result.
Take another class of the same size (concentration), 40 students in the same space. If one or very few students pass then we would call this acid solution a weak one as only very few units of the acid break down to form ions.
For a reaction like this where few ions are liberated from the dissociation of an acid, reversible or double headed arrows are used. Take ethanoic acid as an example.
CH3COOH(aq) <—->CH3COO-(aq) + H+(aq) (Very few H+(aq) are formed)
(The concept is similar to perhaps what used to be observed where stronger/ “brighter” students would end up in some schools and the not so “strong” ones end up in another.)
The concept applies to alkalis in the same way. Strong alkalis (soluble bases) ionize completely in aqueous medium to yield many OH-(aq) while weak alkalis ionize partially or incompletely to yield few OH-(aq).
1. Differentiate between the concentration and strength of an acid.
2. Use your text to copy some strong and weak acids and alkalis and write the equations showing how they dissociate.
You may find it “shocking” that strong acids and alkali are inherently strong electrolytes and would conduct electricity well. The same cannot be said about weak acids and alkalis. (Everything is connected)
Contributed by: Kemil Walford