Introduction to Acids and Bases IB Chemistry Power Points Topic 08 Acids and Bases

In aqueous solutions, a proportion of the water molecules dissociate; The ions formed are H + or positively charged hydrogen ions and negatively charged hydroxide ions (OH - ) Technically 2 H 2 O (l)  H 3 O + (aq) + OH - (aq) K w = [H + ][OH − ] = 1 x

Some chemical compounds contribute additional H + to make the solution more acidic. Other compounds remove H + ions. A compound that increases H + is called an acid Examples: HCl, H 2 SO 4, HNO 3, CH 3 COOH

A compound that removes H + ions from an aqueous solution is called a base. This reaction is called a neutralization. Often this is done by adding OH - ions for example NaOH, KOH, Ca(OH) 2. Soluble bases are called alkalis.

Neutralization Reactions hydroxides acid + base  water + salt HCl + NaOH  H 2 O + NaCl (aq) metal oxides acid + base  water + salt 2 HCl + Cu 2 O  H 2 O + CuCl 2 (aq) ammonia acid + base  salt HCl + NH 3  NH 4 Cl (aq)

Three theories of acids Arrhenius (most common) Bronsted-Lowry Lewis

Arrhenius (most common): an acid dissociates to yield H + and a base dissociates to yield OH - Hydrochloric acid H + + Cl - Sodium hydroxide Na + + OH -

Bronsted-Lowry: an acid is a proton (H + ) donor and a base is a proton acceptor

amphiprotic

Lewis: An acid is an electron pair acceptor and a base is an electron pair donor A dative covalent bond is formed

Example of Lewis Acid Lewis Acid Lewis Base

This is a common example that is not an obvious acid/base rxn Boron trifluoride acts as a Lewis Acid. The boron has only 6 electron in valence shell so the lone pair of electrons forms a dative bond and fills up the valence shell of the boron

Indicators Acids and bases are substances with specific physical and chemical properties. We can determine if substances are acidic or basic by testing their reaction with indicators.

Indicators are organic substances that change color in the presence of an acid or a base. Some common indicators in acidin base Litmusredblue Phenolphthaleincolorlesspink Methyl orangeredyellow

Reactions of acids React with active metals (above copper in reactivity series) 2 HCl + Ca  CaCl 2 + H 2 Reaction with carbonates H 2 SO 4 + Na 2 CO 3  Na 2 SO 4 + CO 2 + H 2 O Reaction with bicarbonates HNO 3 + NaHCO 3  NaNO 3 + CO 2 + H 2 O

Acid/base properties of Period 3 oxides (topic 3) Metal oxides Na 2 O and MgO react with water to form hydroxides (basic solutions) Na 2 O + H 2 O  2 NaOH (aq) Aluminum oxide is amphoteric (will react as a base with an acid or vice versa) Al 2 O HCl  2 AlCl H 2 O Other period 3 oxides (non-metal S, P, Cl oxides) react with water to form acidic solutions SO 3 + H 2 O  H 2 SO 4 (aq) see page 15 in study guide

Acid/base properties of Period 3 chlorides (topic 13) Chlorides across Period 3 become more acidic across the period NaCl (aq) is neutral MgCl 2 (aq) is weakly acidic Chlorides of Al, Si, P, S and Cl 2 react with water to produce HCl (aq) solutions see Study guide page 16

Strong Acids vs Weak Acids The strength of an acid or base depends on how easily it dissociates in water. The dissociation of an acid or base is an equilibrium. HA (aq)  H + (aq) + A - (aq) BOH (aq)  B + (aq) + OH - (aq) Strong acids or bases dissociate (ionize) easily – the equilibrium favors the ionic products : k c >> 1

Strong vs Weak When the strength of an acid or base is discussed, it is very important NOT to confuse “strength” with “concentration” A 5M acid solution contains 5 mol of acid per dm 3 but its strength is determined by how much of that acid is ionized. Strong acids : HCl, H 2 SO 4, HNO 3 (mono vs diprotic) Strong bases : NaOH, KOH, Ba(OH) 2 Weak acids: CH 3 COOH, H 2 CO 3, carbonic acid CO 2(aq) Weak bases: NH 3, ethylamine CH 3 CH 2 NH 2

Strong vs Weak How to tell Strong acids and bases are mostly ionized and therefore solutions are good electrolytes (high conductivity). The pH of the solution can also be measured.

What is the pH scale? pH is a measurement of hydrogen ion concentration It tells you how acids or basic (or alkaline) something is Ranges from 0 (most acidic) to 14 (most basic

How does scale work? The scale is logarithmic. As you go up or down, the concentration is changed by a power of ten Example pH 3 is 100 times more concentrated than pH 5 neutral pH 10 is 100 times less concentrated than pH 8

Strong Acid example HCl HCl (aq)  H + (aq) + Cl - (aq) completely dissociated pH of 0.1 M soln = 1 strong electrolyte reacts vigorously note simplified “net ionic” equation

Weak Acid example CH 3 COOH CH 3 COOH (aq)  H + (aq) + CH 3 COO - (aq) partially dissociated pH of 0.1 M soln = 2.9 weak electrolyte reacts slowly