1Everything Science www.everythingscience.co.za13. Types of reactionsChemistryGrade 11
2Everything Science www.everythingscience.co.zaAcids and basesThe Arrhenius definition of acids and bases defines an acid as a substance that increases theconcentration of hydronium ions (H3O+) in a solution. A base is a substance that increases theconcentration of hydroxide ions (OH−) in a solution. However this description only applies tosubstances that are in water.The Bronsted-Lowry model defines acids and bases in terms of their ability to donate or acceptprotons.A Bronsted-Lowry acid is a substance that gives away protons (hydrogen cations H+), and istherefore called a proton donor.A Bronsted-Lowry base is a substance that takes up protons (hydrogen cations H+), and istherefore called a proton acceptor.
3Everything Science www.everythingscience.co.zaAmphoteric and polyprotic substancesAn amphoteric substance is one that can react as either an acid or base. Water (H2O) isan example of an amphoteric substance.An amphiprotic is one that can react as either a proton donor (Bronsted-Lowry acid) or asa proton acceptor (Bronsted-Lowry base). HCO3−and HSO4−are examples of amphiproticsubstances.A polyprotic (many protons) acid is an acid that has more than one proton that it candonate.
4Everything Science www.everythingscience.co.zaConjugate acid-base pairsA conjugate acid-base pair refers to two compounds in a reaction (one reactant and oneproduct) that transform or change into the other through the loss or gain of a proton.
5Everything Science www.everythingscience.co.zaAcid-base reactionsThe reaction between an acid and a base is a neutralisation reaction.Acids and bases are used in domestic uses (for example calcium carbonate on acidic soil)in biology (for example in antacids for stomach ulcers) and in industry (for example inabsorbing harmful SO2gas). These reactions are very important to us and without themmany aspects of daily life would be very different.Indicators are chemical compounds that change colour depending on whether they are inan acid or in a base.
6Everything Science www.everythingscience.co.zaAcid-base reactions contd...● When an acid reacts with a metal hydroxide a salt and water are formed. The salt is madeup of a cation from the base and an anion from the acid. An example of a salt is potassiumchloride (KCl), which is the product of the reaction between potassium hydroxide (KOH) andhydrochloric acid (HCl).● When an acid reacts with a metal oxide a salt and water are formed. An example is thereaction between magnesium oxide (MgO) and hydrochloric acid (HCl).● When an acid reacts with a metal carbonate a salt, water and carbon dioxide are formed. Anexample is the reaction between calcium carbonate (CaCO3 ) and hydrochloric acid (HCl).
7Everything Science www.everythingscience.co.zaOxidation and reductionOxidation is the loss of electrons by a molecule, atom or ion.Reduction is the gain of electrons by a molecule, atom or ion.A redox reaction is one involving oxidation and reduction, where there is always a change inthe oxidation numbers of the elements involved. Redox reactions involve the transfer ofelectrons from one compound to another.An oxidation number is the charge an atom would have if it was in a compound composed ofions.If the oxidation number of a species becomes more positive, the species has been oxidisedand if the oxidation number of a species becomes more negative, the species has beenreduced.
8Everything Science www.everythingscience.co.zaOxidation numbersWe use the following rules to assign oxidation numbers to elements in compounds:1. A molecule consisting of only one element always has an oxidation number of zero, sinceit is neutral.2. Monatomic ions (ions with only one element or type of atom) have an oxidation numberthat is equal to the charge on the ion.3. In a molecule or compound, the sum of the oxidation numbers for each element in themolecule or compound will be zero.4. In a polyatomic ion the sum of the oxidation numbers is equal to the charge.5. An oxygen atom usually has an oxidation number of −2. One exception is in peroxides(e.g. hydrogen peroxide) when oxygen has an oxidation number of −1.6. The oxidation number of hydrogen is often +1. One exception is in the metal hydrideswhere the oxidation number is −1.7. The oxidation number of fluorine is −1.
9Everything Science www.everythingscience.co.zaRedox reactionsA half reaction is either the oxidation or reduction reaction part of a redox reaction. In thetwo half-reactions for a redox reaction the number of electrons donated is exactly the sameas the number of electrons accepted.An element that is oxidised is called a reducing agent, while an element that is reduced iscalled an oxidising agent.A redox reaction is one involving oxidation and reduction, where there is always a changein the oxidation numbers of the elements involved. Redox reactions involve the transfer ofelectrons from one compound to another.
10Everything Science www.everythingscience.co.zaFor more practice see:www.everythingscience.co.zaShortcode: ESBR7