ACIDS and BASES Acid – Base theories Naming acids and bases Oxides Reactions and properties of acids and bases Strengths of acids and bases

Acid and Base Theories 1) Arrhenius Theory An acid is a substance that gives H+ ion, when dissolved in water. For example, hydrochloric acid reacts with water to form hydrogen ions which are transferred to a water molecule to form a hydronium ion (H3O+). But simply the reaction is: HCl H+ + Cl-

Acids which have one ionizable hydrogen atom per molecule are called monoprotic acids. Example: HNO3 H+ + NO3- Acids which have two ionizable hydrogen atom per molecule are called diprotic acids. H2SO4 H+ + HSO4− HSO4− ⇌ H+ + SO42− Acids which have three ionizable hydrogen atom per molecule are called triprotic acids. H3PO4   ⇌ H+ + H2PO4– H2PO4– ⇌ H+ + HPO42–       HPO42– ⇌ H+ +  PO43–       

A base is a substance that gives OH- ion, when dissolved in water. NaOH → Na+ + OH− Ca(OH)2 → Ca2+ + 2OH- Reaction of NH3 produce OH-: NH3 + H2O → NH4+ + OH- so it is a base.

Limitations of the Arrhenius theory Hydrochloric acid is neutralized by both sodium hydroxide solution and ammonia solution. In both cases, you get a colourless solution which you can crystallize to get a white salt - either sodium chloride or ammonium chloride. These are clearly very similar reactions. The full equations are: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) NH3(aq) + HCl(aq) NH4Cl(s) In the sodium hydroxide case, hydrogen ions from the acid are reacting with hydroxide ions from the sodium hydroxide - in line with the Arrhenius theory. However, in the ammonia case, there don't appear to be any hydroxide ions!

You can get around this by saying that the ammonia reacts with the water, it is dissolved in to produce ammonium ions and hydroxide ions: NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) This is a reversible reaction, and in a typical dilute ammonia solution, about 99% of the ammonia remains as ammonia molecules. Nevertheless, there are hydroxide ions there, and we can squeeze this into the Arrhenius theory. However, this same reaction also happens between ammonia gas and hydrogen chloride gas. NH3(g) + HCl(g) NH4Cl(s) In this case, there aren't any hydrogen ions or hydroxide ions in solution - because there isn't any solution. The Arrhenius theory wouldn't count this as an acid-base reaction, despite the fact that it is producing the same product as when the two substances were in solution.

Naming Acids and Bases A. Naming Acids: The name of the acid is determined based on the name of the anion, specifically, based on the ending of the anion name.  The three possibilities are listed here: Anion Name Acid Name -ide Hydro-ic acid -ite -ous acid -ate -ic acid

Common Anions Fluoride F- Chloride Cl- Bromide Br- Iodide I- Sulfide Nitride N3- Sulfite SO32- Nitrite NO2- Chlorite ClO2- Hypochlorite OCl- Phosphate PO43- Hydrogen phosphate HPO42- Dihydrogen phosphate H2PO4- Nitrate NO3- Sulfate SO42- Hydrogen sulfate HSO4- Perchlorate ClO4- Chlorate ClO3- Carbonate CO32-

B. Naming Bases Simply use the normal rules for naming compounds; ionic or covalent depending on the elements in the compound. Example: NaOH: Sodium hydroxide Ca(OH)2: Calcium hydroxide NH3: Ammonia

a) Name the following acids and bases: NaOH: H2SO3: H2S : H3PO4: NH3: Example: a) Name the following acids and bases: NaOH: H2SO3: H2S : H3PO4: NH3: HCN: Ca(OH)2: Fe(OH)3: H3P: Sodium hydroxide Sulfurous acid Hydrosulfuric acid Phosphoric acid Ammonia Hydrocyanic acid Calcium hydroxide Iron (III) hydroxide Hydrophosphoric acid

b) Write the formulas of the following acids and bases: Hydrofluoric acid: Hydroselenic acid: Carbonic acid: Lithium hydroxide: Nitrous acid: Cobalt (II) hydroxide: Sulfuric acid: Beryllium hydroxide: Hydrobromic acid: HF H2Se H2CO3 LiOH HNO2 Co(OH)2 H2SO4 Be(OH)2 HBr

Oxides Nonmetal Oxides Metal Oxides CO2, SO2, SO3 etc. show acidic properties CO, NO, N2O are neutral Na2O, BaO etc. show basic properties Amphoteric metals show both basic and acidic properties such as Al and Zn

Acidic Property of Nonmetal Oxides The oxides of nonmetals are usually acidic except NO, N2O and CO (They are neutral) CO2 + H2O H2CO3 SO2 + H2O H2SO3 SO3 + H2O H2SO4 N2O5 + H2O 2HNO3 Cl2O + H2O 2HOCl P4O10 + H2O 4H3PO4 Monoxides of halogens are acidic such as Cl2O, Br2O. Oxides of some metals at high oxidation states show acidic properties such as Mn2O7, CrO3. Acidic nonmetal oxides react with bases to form salts. SO3 + 2KOH K2SO4 + H2O

Basic Properties of Metal Oxides Oxides of metals are usually basic. Na2O + H2O 2NaOH BaO + H2O Ba(OH)2 Some metal oxides can not dissolve in water but they can dissolve in acidic solutions. MnO + 2HCl MnCl2 + H2O CrO + 2HCl CrCl2 + H2O Basic oxides react with acids to form salts. CaO + H2SO4 CaSO4 + H2O

Amphoteric Oxides Oxides amphoteric metals are also amphoteric. Al2O3 + HCl AlCl3 + H2O Al2O3 + 2NaOH + 3H2O 2NaAl(OH)4 (sodium tetrahydroxoaluminate)

Properties and Reactions of Acids and Bases A. Properties of Acids: They taste sour They turn litmus dye from blue to red They conduct electricity (electrolyte) They react with active metals to form H2 gas. Mg + 2HCl MgCl2 + H2

They react with carbonates to form CO2 gas. The acids which do not contain oxygen in their structures can not react with semi noble metals Cu, Hg, Ag.The oxy acids react with these metals producing gases other than H2. Cu + 2H2SO4 CuSO4 + SO2 + 2H2O 3Ag + 4HNO3 3AgNO3 + NO + 2H2O They react with carbonates to form CO2 gas. Na2CO3 + 2HCl NaCl + H2O + CO2 They react with bases to form salts and water. HCl + NaOH NaCl + H2O (neutralization)

B. Properties of Bases They have bitter taste They turn the litmus dye from red to blue They react with fats in the skin to form soaps They conduct electricity (electrolyte) They only react with amphoteric metals: Zn, Al Zn + 2NaOH Na2ZnO2 + H2 2Al + 6 NaOH 2Na3AlO3 + 3H2

Amphoteric metals have both acidic and basic properties such as Al, Zn, Sn, Pb, Cr Al + 6HCl AlCl3 + 3H2 2Al + 6NaOH 2Na3AlO3 + 3H2 Oxides and hydroxides of amphoteric metals are also amphoteric. Al2O3 + HCl AlCl3 + H2O Al2O3 + 2NaOH + 3H2O 2NaAl(OH)4 ZnO + 2 HCl ZnCl2 + H2O ZnO + 2NaOH + H2O Na2(Zn(OH)4)

2.Bronsted-Lowry Theory A Bronsted-Lowry (BL) acid is defined as any substance that can donate a hydrogen ion (proton) and a Bronsted-Lowry base is any substance that can accept a hydrogen ion (proton). Thus, according to the BL definition, acids and bases must come in conjugate pairs. For example, consider acetic acid dissolved in water: CH3COOH + H2O CH3COO- + H3O+ Conjugate acid-base pairs: CH3COOH and CH3COO- H2O and H3O+ Act as an acid Act as a base Act as a base Act as an acid

Label Bronsted-Lowry acids and bases in the following reactions and show the direction of proton transfer. H2O + H2O OH- + H3O+ H+ Acid Base Base Acid H+ 2. NH3 + H2O NH4+ + OH- Base Acid Acid Base When a Bronsted-Lowry acid has given up its proton, it is capable of getting back that proton and acting as a base. Conjugate base is what is left after an acid gives up a proton. The stronger the acid, the weaker the conjugate base. The stronger the base, the weaker the conjugate acid.

The relationship between the Bronsted-Lowry theory and the Arrhenius theory The Bronsted-Lowry theory doesn't go against the Arrhenius theory in any way - it just adds to it. Hydroxide ions are still bases because they accept hydrogen ions from acids and form water. An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton to them.

The hydrogen chloride / ammonia problem This is no longer a problem using the Bronsted-Lowry theory. Whether you are talking about the reaction in solution or in the gas state, ammonia is a base because it accepts a proton (a hydrogen ion). If it is in solution, the ammonia accepts a proton from a hydronium ion: NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) If the reaction is happening in the gas state, the ammonia accepts a proton directly from the hydrogen chloride: NH3(g) + HCl(g) NH4Cl(s)

3. Lewis Theory A Lewis acid is a chemical compound, A, that can accept a pair of electrons from a Lewis base, B, that acts as an electron-pair donor, forming an adduct, AB. A + :B → A—B A Lewis base is also a Brønsted-Lowry base.

The Bronsted-Lowry theory says that they are acting as bases because they are combining with hydrogen ions. The reason they are combining with hydrogen ions is that they have lone pairs of electrons - which is what the Lewis theory says. The two are entirely consistent. But what about other similar reactions of ammonia or water, for example?

Ammonia reacts with BF3 by using its lone pair to form a co-ordinate bond with the empty orbital on the boron.

Co-ordinate (dative covalent) bonding A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei. In the formation of a simple covalent bond, each atom supplies one electron to the bond. A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

The reaction between ammonia and hydrogen chloride

Representing co-ordinate bonds In simple diagrams, a co-ordinate bond is shown by an arrow. The arrow points from the atom donating the lone pair to the atom accepting it.

Dissolving hydrogen chloride in water to make hydrochloric acid

Lewis acids Lewis acids are electron pair acceptors. In the above example, the BF3 is acting as the Lewis acid by accepting the nitrogen's lone pair. On the Bronsted-Lowry theory, the BF3 has nothing about it. Then what makes HCl a Lewis acid? Chlorine is more electronegative than hydrogen, and that means that the hydrogen chloride will be a polar molecule. The electrons in the hydrogen-chlorine bond will be attracted towards the chlorine end, leaving the hydrogen slightly positive and the chlorine slightly negative.

The lone pair on the nitrogen of an ammonia molecule is attracted to the slightly positive hydrogen atom in the HCl. As it approaches it, the electrons in the hydrogen-chlorine bond are repelled still further towards the chlorine. Eventually, a co-ordinate bond is formed between the nitrogen and the hydrogen, and the chlorine breaks away as a chloride ion.

Relative Strengths of acids and Bases The strength of an acid depends on how easily the proton H+ is lost or removed from an acid Two factors determine the acid strength: The polarity of H atom: The more polarized the bond is, the more easily the proton is removed and greater the acid strength. The size of the atom X (in HX): The greater the atom X, the weaker is the bond and greater the acid strength.

Periodic Trends for Binary Acids: Down a group: Sizes of the atoms increase. HF HCl Acidity increases HBr HI Across a period: Polarity of the bond increases. CH4 NH3 H2O HF Acidity inreases.

Oxyacids: HOF HOCl Acidity decreases. H-O bond HOBr ionizes more easily when the HOI oxygen atom is bonded to a more electronegative atom.

For a series of oxyacids: HClO HClO2 HClO3 HClO4 Acidity increases As the number of oxygen atoms increases, The oxidation number of central atom (Cl) increases. This increases the ionization of O-H bond. Therefore, acid strength increases.

Polyprotic Acids and Their Anions: H3PO4 H2PO4- HPO42- H2CO3 HCO3- Acidity decreases H2SO4 HSO4-

Organic Acids Organic acids have carboxyl group (COOH). They are weak acids. Example: HCOOH: Formic acid CH3COOH: Acetic acid