8.4 Strong and Weak Acids and Bases TOPIC 8 ACIDS AND BASES 8.4 Strong and Weak Acids and Bases

ESSENTIAL IDEA The pH depends on the concentration of the solution. The strength of acids or bases depends on the extent to which they dissociate in aqueous solution. NATURE OF SCIENCE (1.8) Improved instrumentation – the use of advanced analytical techniques has allowed the relative strength of different acids and bases to be quantified. NATURE OF SCIENCE (3.1) Looking for trends and discrepancies – patterns and anomalies in relative strengths of acids and bases can be explained at the molecular level. NATURE OF SCIENCE (1.9) The outcomes of experiments or models may be used as further evidence for a claim – data for a particular type of reaction supports the idea that weak acids exist in equilibrium.

THEORY OF KNOWLEDGE The strength of an acid can be determined by the use of pH and conductivity probes. In what ways do technologies, which extend our senses, change or reinforce our view of the world?

UNDERSTANDING/KEY IDEA 8.4.A Strong and weak acids and bases differ in the extent of ionization.

UNDERSTANDING/KEY IDEA 8.4.B A strong acid is a good proton donor and has a weak conjugate base.

UNDERSTANDING/KEY IDEA 8.4.C A strong base is a good proton acceptor and has a weak conjugate acid.

A strong acid completely ionizes or dissociates and the equilibrium lies far to the right. A strong acid has a weak conjugate base. Its conjugate base is weaker than water because the water molecules win the competition for the H+ ions.

A weak acid ionizes or dissociates only to a small extent A weak acid ionizes or dissociates only to a small extent. Its equilibrium lies far to the left and most of the acid is still present as HA at equilibrium. A weak acid has a strong conjugate base which is much stronger than water. This means the water is not very successful in pulling off the H+ ions.

UNDERSTANDING/KEY IDEA 8.4.D Strong acids and bases of equal concentrations have higher conductivities than weak acids and bases.

Strong acids and bases are high electrical conductors because they dissociate almost completely into ions. Weak acids and bases only dissociate to a small extent into ions so they are weak electrical conductors.

SIX STRONG ACIDS HCl HBr % ionization = amount ionized x 100 HI initial concentration HNO3 H2SO4 HClO4 A strong acid completely dissociates or ionizes in water. If the acid is not listed above, it is then considered a weak acid.

SEVEN STRONG BASES Strong bases completely ionize in water. LiOH Sr(OH)2 NaOH Ba(OH)2 KOH Ca(OH)2*** RbOH calcium hydroxide is only CsOH slightly soluble Strong bases completely ionize in water. Bases not listed above are considered weak.

APPLICATIONS/SKILLS Be able to distinguish between strong and weak acids and bases in terms of the rates of their reactions with metals, metal oxides, metal hydroxides, metal hydrogen carbonates and metal carbonates and their electrical conductivities for solutions of equal concentrations.

Strong acids and bases contain a higher concentration of ions which can be used as a means of distinguishing between them. The more ions present, the higher the electrical conductivity. Stronger acids react faster because more H+ ions are present. (qualitative only) The pH scale can be used to determine acid/base strength.

The reactions outlined in ppt 8 The reactions outlined in ppt 8.2 are the ones referenced in the Applications/Skills slide. Strong acids and bases dissociate into more ions than their weak counterparts so the reactions using strong acids and bases will be more vigorous thus allowing you to qualitatively distinguish between a strong and a weak acid or base.

Citations International Baccalaureate Organization. Chemistry Guide, First assessment 2016. Updated 2015. Brown, Catrin, and Mike Ford. Higher Level Chemistry. 2nd ed. N.p.: Pearson Baccalaureate, 2014. Print. Most of the information found in this power point comes directly from this textbook. The power point has been made to directly complement the Higher Level Chemistry textbook by Catrin and Brown and is used for direct instructional purposes only.