CHM 101 Acids, Bases and Salts. Redox Reactions PDF

CHM 101 ACIDS, BASES AND SALTS. REDOX REACTIONS BY MR ADENIYI TAYO  Acids generally have a sour taste and react strongly with a wide variety of substances such as metals. Bases are known to react with acids. Early chemists found it difficult to decide a reason why acids had a...

CHM 101 ACIDS, BASES AND SALTS. REDOX REACTIONS BY MR ADENIYI TAYO  Acids generally have a sour taste and react strongly with a wide variety of substances such as metals. Bases are known to react with acids. Early chemists found it difficult to decide a reason why acids had a set of properties in common According to the idea from a French chemist “Antoine Lavoisier” he described “acids as substances that contain oxygen” This was valid for a number of common acids such as sulphuric acid [H2SO4] and nitric acid [HNO3] but does not fit hydrochloric acid [HCl] The search for common factor in acids came to a head during 1830’s when a german Chemist “Justus Von Liebig” wrote that “Acids are hydrogen compounds in which the hydrogen may be replaced by metals. All the work on acids has led to the following standard definition of an acid Acid will; Give out hydrogen with a metal; a derivative of salt Neutralize a base to give a salt and water only Give carbon-dioxide with carbonate This summary gives a great information about the properties of acids but does not tell us anything about the chemical structure except that they contain hydrogen as one of their elements. In the study of acids; it was noted that when acids was found in solutions, it conducted electricity extremely well. This experimental evidence tells us that acid contains ions “Acids give hydrogen ions in solution” When a molecule like hydrogen chloride dissolves in water, it dissociates into ions; HCl + H2O → H+ (aq) + Cl- (aq) Experimental evidence showed that the active ingredient of an acidic solution is not a simple hydrogen ion, H + (aq) rather, it is the oxonium ion, H3O+ (aq) {oxonium ions are also known as hydronium ion} Hence when hydrogen chloride dissolves in water, there is a chemical reaction; HCl(aq) + H2O(l) → H3O+ (aq) + Cl-(aq) H2SO4 + 2H2O → SO42- + 2H3O+ HNO3 + H2O →NO3- + H3O+ HClO4 + H2O → ClO4- + H3O+ {Chloric (vii) acid} The Bronsted theory of acids and bases Due to the restriction of the previous definition of acids, bronsted brought out his own theory having noted that some chemist carry out reacton in non-aqueous solvent such as liquid ammonia and liquid sulphur dioxide. He stated that; Acids are proton donors Bases are proton acceptors Example HCl(a) + NH3(a) → NH4+(a) + Cl-(a) “a” – shows that different ions and molecules are surrounded by solvent molecules, ammonia rather than water. Another reaction is the neutralization of acids with base HNO3(aq) + NaOH(aq) → NaNO3(aq) + H2O(l) Nitric acid solution is a mixture of oxonium ions and nitrate ions; HNO3(aq) + H2O(aq) → H3O+(aq) + NO3-(aq) Sodium hydrogen solution is a micture of sodium ions and hydroxide ions. NaOH(aq) → Na+ (aq) + OH-(aq) H3O+ + OH- (aq) → H2O + H2O Oxonium ion donates a proton to the hydroxide ion In the reaction of hydride ion and water H-(aq) + H2O(l) → OH- (aq) + H2 (g) H2O (l) loses proton; so it is bronsted acid The Lewis theory of acids and bases The American chemist decided that more general theory of acids and bases was possible He stated that; Acids are electron pair acceptors Bases are electron pair donors This theory was seen to fit with a reaction of ammonia with boron trifluoride in which ammonia function as lewis base NH3 + BF3 → NH3---->BF3 NH3 donates a lone pair of electron BF3 has empty 2p orbital 4NH3 + CU2+ → [CU(NH3)4]2+ NH3 donate lone to the CU2+ ion [CU(NH3)4]2+ Tetraammine copper(ii) The terms used in describing acids Strong acids are acids that completely dissociate into ions Weak acids are acids that partially dissociate into ions Concentrated acids are acids that have many moles of acid in a litre of solution Dilute acids are acids that have few moles of acid in a litre of solution Note;- For acid to be strong, the ion it makes must be energetically stable that is it must not easily change back into molecule from which it came. The pH of a solution does not necessarily determine the strength of an acid because a solution of high pH may be highly diluted. pH = -Log10 [H+]  Salts Salts composed of small ions, and typically have high melting and boiling points, and are hard and brittle. They are usually derivatives if acids Salts are compounds because they contain atleast two elements chemically combined together. They are ionic compound i.e they are made of ions which are atoms or group of atoms that have either a positive or negative charge. Ions bear a charge due to either loss or gain of one or more of their valence electrons. Negatively charged ions have gained additional electros in their outer shell; they are called anions Positively charged ions have lost electrons; they are called cations Metals such as sodium, magnesium and calcium have tendency to donate their electrons which makes them to be cations. Non metal like chlorine and most of the halogens readily accept electrons which makes them to be anions In order to derive any salt chemical formula, the cation and anion must be identified. For example; table salt NaCl; the salt molecule is comprised of a sodium cation Na+ and a chloride anion Cl- The sodium cation and chloride anion are combined together through an ionic bond. Salt has a very high solubility in water which is due to high polarity. Water molecules pull apart the salt cations and anions breaking their ionic bonds Equation Properties of salt Salt has a crystalline structure and often appears in different colours Calcium is white Copper sulphate is blue Sodium chloride ranges from transparent to white The melting and boiling points of salt are extremely high and excess amount of energy is required to break the ionic bonds of salt Salts are polar which makes them water soluble and excellent electricity conductors. They are good electrolytes because they completely dissociate in water forming ions swimming in water solution. Salts taste well and they don’t have any odor.  Salts often form crystal structure or crystal lattice i.e a highly ordered formation of molecules. Gases Gas is one of the three classical states of matter Properties of gases A gas has no definite shape or volume of it own i.e it acquires the shape of the container and intermolecular attraction is weakest in gases A gas has no surface of its own A gas is not rigid and is easily compressed A gas can diffuse into another gas A gas on cooling changes into liquid state A gas can flow in all directions The molecules are very loosely packed in gases Gases are not visible  Some gases are air; carbon-dioxide, oxygen etc. The state of a gas can be completely defined by specifying its temperature,volume,number of moles and pressure. Redox Reaction A redox reaction is a reaction in which both oxidation and reduction occur. It involves the transfer of electrons. The number of electrons lost in oxidation is equal to the nmber of electrons gained in reduction Example; Cu2+(aq) + Zn(s) → Zn2+(aq) + Cu(s) Reduction half reaction;- Cu2+(aq) + 2e- → Cu(s) Oxidation half reaction;- Zn(s) - 2e- → Zn2+ (aq) Oxidation is therefore the addition of oxygen and loss of electron Reduction is therefore the addition of hydrogen and gain of electron Oxidizing agent [oxidant] is a substance which bring about oxidation; acceptor of electrons and become reduced Reducing agent [reductant] is a substance which brings about reduction; donor of electrons and become oxidized Cu2+(aq) + Zn(s) → Zn2+(aq) + Cu(s) Cu2+ ;- oxidizing agent Zn(s) ;- reducing agent Balancing redox reactions in acidic medium H2O is added to the side that is deficient in oxygen Then H+ is used to make it up Example 1 C2O42- + MnO4- → Mn2+ + CO2 Balanced equation 5C2O42- + 2MnO4- + 16H+ → 2Mn2+ + 10CO2 + 8H2O Example 2 MnO4- + Fe2+ → Mn2+ + Fe3+ Balanced equation MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ +4H2O Balancing redox reactions in basic medium H2O is added to the side that is rich in oxygen OH- is used to make it up Example 1 N2O4 (g) + Br- (aq) → NO2-(aq) + BrO3-(aq) Balanced equation 3N2O4 + Br- + 6OH- → 6NO2- + BrO3- + 3H2O Example 2 CN-(aq) + MnO4- → CNO(aq) + MnO2(s) Balanced equation 3CN- + 2MnO4- + H2O → 3CNO- + 2MnO2 + 2OH- Thank you for listening