Strong Acids. Learning Objective Calculate pH for solutions of strong acids. Key Points Strong acids can catalyze chemical reactions. Strong acids are defined by their pKa. The acid must be stronger in aqueous solution than a hydronium ion, so its pKa must be lower than that of a hydronium ion.
Strong acids can be organic or inorganic. Strong acids must be handled carefully because they can cause severe chemical burns. Strong acids are essential for catalyzing some reactions, including the synthesis and hydrolysis of carbonyl compounds. The further to the left it lies, the weaker the acid is. You can get a measure of the position of an equilibrium by writing an equilibrium constant for the reaction. The lower the value for the constant, the more the equilibrium lies to the left.
The dissociation ionization of an acid is an example of a homogeneous reaction. Everything is present in the same phase - in this case, in solution in water. You can therefore write a simple expression for the equilibrium constant, K c. Here is the equilibrium again:. At the bottom of the expression, you have a term for the concentration of the water in the solution.
That's not a problem - except that the number is going to be very large compared with all the other numbers. In other words, if the acid is weak the concentration of the water is virtually constant.
In that case, there isn't a lot of point in including it in the expression as if it were a variable. Instead, a new equilibrium constant is defined which leaves it out. This new equilibrium constant is called K a. You may find the K a expression written differently if you work from the simplified version of the equilibrium reaction:. It is actually exactly the same as the previous expression for K a! This second version of the K a expression is not as precise as the first one.
To take a specific common example, the equilibrium for the dissociation of ethanoic acid is properly written as:. These are all weak acids because the values for K a are very small.
They are listed in order of decreasing acid strength - the K a values get smaller as you go down the table. However, if you aren't very happy with numbers, that isn't immediately obvious. Because the numbers are in two parts, there is too much to think about quickly! To avoid this, the numbers are often converted into a new, easier form, called pK a. For example, hydrochloric acid, HCl, as a strong acid it donates a proton to water, H2O, to form the hydronium ion, H3O plus, and the conjugate base to HCl which is the chloride anion, Cl minus.
In reality, this reaction reaches an equilibrium. However, the equilibrium lies so far to the right and favors the product so much that we don't draw an equilibrium arrow, we simply draw an arrow going to the right, indicating the reaction essentially goes to completion. It's also acceptable to completely leave water out of the equation and to show hydrochloric acid, HCl, turning into H plus and Cl minus. And since there's only one water molecule difference between H plus and H3O plus, H plus and H3O plus are used interchangeably.
Hydrochloric acid is an example of a monoprotic strong acid. Monoprotic means, hydrochloric acid has one proton that it can donate in solution. Sulfuric acid is H2SO4 and it's a strong acid, but it's a diprotic acid, meaning it has two protons that it can donate, however, only the first ionization for sulfuric acid is strong. Let's calculate the pH of a strong acid solution.
In this case, we're gonna do a 0. Because nitric acid is a strong acid, we assume the reaction goes to completion. Therefore, if the initial concentration of nitric acid is 0. Therefore, if the reaction goes to completion, the concentration of hydronium would also be 0. Since our goal is to calculate the pH of this solution, we know that the equation for pH is pH is equal to the negative log of the concentration of hydronium ions.
Therefore, we just need to plug in the concentration of hydronium ions into our equation. This gives us the pH is equal to the negative log of 0.
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