121010SCH4U_acidbase October 15, 2012

SNC2D

Acids & BasesChemical Reactions

What are acids & bases?

Back in Gr.10 we talked about Acids and Bases!

121010SCH4U_acidbase October 15, 2012

Acetic Acid Hydrochloric Acid

StrongWeakConcentrated

Diluted

Let's look at a couple acids:

Concentrated

Diluted

Misconception!

Strong

Weak

Concentrated

Diluted

Concentrated vs Diluted

This is refer to the amount of acid dissolved in water.

The concentration of the acid in terms of moles per litre

Molar

2 Molar HCl

Strong vs Weak

This is refer to the extent of dissociation when the acid/base is dissolve in water.

HA⇌ H+ + A-

121010SCH4U_acidbase October 15, 2012

Hydrochloric acid is a strong acid because the hydrogen chloride molecules completely dissociate to form hydrogen ions and chloride ions. This is represented by the one-way arrow ‘→’.

Hydronium ion

99%1%

STRONG ACID

Ethanoic acid is a weak acid as it only partially dissociates in water to form hydrogen ions and ethanoate ions. This reaction exists as an equilibrium, as shown by the two-way arrow.

1%99%

WEAK ACID

A strong base is one that readily accepts a proton. The oxide ion (O2−) and hydroxide ion (OH−) are examples of strong bases, although the hydroxide ion is the strongest base that can exist in aqueous solution. When mixed with water, hydroxide ions in compounds such as the group 1 hydroxides gain an H+ ion, and the water loses an H+ ion forming OH−(aq).

Weak bases are substances in which only a small proportion of the molecules or ions react with water to form hydroxide ions in aqueous solution. The weak base ammonia reacts with water according to the equation:

Only a small fraction of ammonia molecules react.

proton donor

How can we tell if an acid/base is strong/weak?

?

One, memorize ;)

121010SCH4U_acidbase October 15, 2012

Two, test it! How reactive is it?

How good is it in terms of conducting electricity?

So, in summary, we can have the following:What about pH?

Can we tell if an acid is strong or weak by looking at its pH?

What does pH stand for?The term pH was invented by Danish chemist Søren Sørenson in 1909, and stands for pondus hydrogenii, or ‘potential hydrogen’. The phrase refers to the potential of a solution to produce hydrogen ions, as Sørenson recognized that the hydrogen ions were critical to the behaviour of enzymes that he was investigating. Strictly speaking, pH is a measure of what is known as the ‘activity’ of hydrogen ions, though for the types of problems that we will encounter, the concentration of hydrogen ions is quite sufficient. When Sørenson was recording the results of his experiments, he needed to record the exact hydrogen ion concentration every time. He soon tired of writing, for example,10−6.4 mol dm−3, and slipped into the habit of simply writing the exponent of the concentration, which in the example given is 6.4. His colleagues quickly followed suit and, as a result, a convenient and practical way of recording the relative acidity of solutions came into widespread use and acceptance.

121010SCH4U_acidbase October 15, 2012

At 25°C, solutions with a pH less than 7 are describedas acidic, those with a pH greater than 7 are basic, and solutions with a pH of exactly 7 are neutral.

So, for water, the concentration of hydrogen ion in 1.0 x 10-7 M. What is the pH?

Calculate [H+] from pH

121010SCH4U_acidbase October 15, 2012

HCl 6M

10 molecules of HCl ➔ 10 molecules of H+

6M of HCl ➔ 6 M of H+

pH = - log [6]= - 0.79

What is the pH of 6 M HCl?