Chapter 6 Chemical Reactions Lecture Notes PDF

# Faculty of Biotechnology - General Chemistry - lecture 6 ## Chemical Reactions ### Valence - The valence of an element can be defined as a measure of its combining power or the number of electrons an atom must gain, lose, or share to have a full or stable outer electron shell. - Elements someti...

# Faculty of Biotechnology - General Chemistry - lecture 6 ## Chemical Reactions ### Valence - The valence of an element can be defined as a measure of its combining power or the number of electrons an atom must gain, lose, or share to have a full or stable outer electron shell. - Elements sometimes gain, lose, or share electrons in order to have eight electrons in their outer shell. - This observation led to the development of the octet rule, which states that outer electron shells prefer to have eight electrons, even though the shell may not be full (octet means a group of eight). ### Oxidation Number **HOW** you can calculate the oxidation number - To determine the oxidation state of an element in a molecule, follow the following steps: - Determine the oxidation state of all elements forming the molecule (from the guiding table) except the element of interest. - The sum of all oxidation state (including the unknown oxidation state) must be equal to zero or to the net charge on the molecule. ### All Chemical Equations - Have two parts: - Reactants - the substances you start with - Products - the substances you end up with Reactants -> Products - The reactants turn into the products. ### Symbols Used in Equations - The arrow separates the reactants from the products. - The + sign = "and" - (s) after the formula - solid - (g) after the formula - gas - (l) after the formula - liquid - (aq) after the formula - dissolved in water, an aqueous solution. - ↑ used after a product indicates a gas (same as g) - ↓ used after a product indicates a solid (same as s) - <-> indicates a reversible reaction - Δ heat, shows that heat is supplied to the reaction - Pt, is used to indicate a catalyst used supplied, in this case, platinum. ### What is a Catalyst? - A substance that speeds up a reaction without being changed by the reaction. - Enzymes are biological or protein catalysts. ### Definite Composition - When atoms combine, they do so in definite ratios of intact atoms to produce compounds with definite composition. ### Types of Chemical Reactions #### #1 Combination (Synthesis) Reactions - Combine - put together - Two elements, or compounds combine to make one compound. Examples: - Ca + O2 -> CaO - SO3 + H2O -> H2SO4 #### #2 Decomposition Reactions - Decompose = fall apart - One reactant falls apart into two or more elements or compounds. Examples: - NaCl electricity -> Na + Cl2 - CaCO3 Δ -> CaO + CO2 #### #3 Single Replacement - One element replaces another. - Reactants must be an element and a compound. - Products will be a different element and a different compound. Examples: - k + NaCl -> Na + KCl - F2 + LiCl -> LiF + Cl2 - Metals replace metals (and hydrogen): - K + AlN -> K3N - Zn + HCl -> ZnCl2 - Think of water as HOH. - Metals replace one of the H, combine with hydroxide: - Na + HOH -> NaOH **Reactivity Series of Metals** | Element | | |---|---| | Potassium | Most reactive | | Sodium | | | Calcium | | | Magnesium | | | Aluminium | | | Carbon | | | Zinc | | | Iron | | | Tin | | | Lead | | | Hydrogen | | | Copper | Least reactive | | Silver | | | Gold | | | Platinum | | | CH | added for comparison | #### #4 Double Replacement - Two things replace each other. - Reactants must be two ionic compounds or acids. - Usually in aqueous solution. - Will only happen if one of the products: - Doesn't dissolve in water and forms a solid - Or is a gas that bubbles out. - Or is a water molecule formed. Examples: - AgNO3 + NaCl -> AgCl + NaNO3 (Precipitate) - NaOH + HCl -> NaCl + H2O (water molecule) - Na2CO3 + HCl -> NaCl + CO2 + H2O (gas evolved) ### Combustion Reaction - Usually in a combustion reaction oxygen combines with another compound to form carbon dioxide and water. - An example of a combustion reaction is the burning of naphthalene: C10H8 + 12 O2 -> 10 CO2 + 4H2O ### Acid and Bases - **Acids:** Arrhenius defined an acid as a compound that donates protons (H+) in solution: - HCl HOH -> H+ + Cl- - H2SO4 HOH -> H+ + HSO4- - **Bases:** Arrhenius defined a base as any compound that donates hydroxyl (OH-) ions in solution: - NaOH HOH -> Na+ + OH- - KOH HOH -> K+ + OH- ### Properties of Acids - Acids change blue litmus paper to red. - Acids have a sour taste. Like lemon taste. - Acids react with metals to release hydrogen gas. For example: Zn + 2H+ -> Zn++ + H2 - Acids react with carbonates and bicarbonates to form carbon dioxide. For example: CaCO3 + 2HCl -> CaCl2 + H2O + CO2 ↑ - Acids react with bases to form salts and water (neutralization reaction). For example: HCl + NaOH -> NaCl + H2O ### Properties of Bases - Bases change red litmus paper to blue. - Bases possess a bitter taste and feel soapy when in contact with the skin. - Bases react with acids to form salts and water (neutralization reaction). ### Modern Acid-Base Theory - **Acid** is any compound (charged or uncharged) capable of donating a proton. - **A Base** is defined as a charged or uncharged substance capable of accepting a proton. NH3 + H2O -> NH3 + H+ + OH- -> NH4 + OH - By accepting a proton from water, ammonia has effectively increased the concentration of hydroxyl ions in the solution. ### Classification of Acids and Bases - **Strong acids** are acids that are completely or nearly 100% ionized in their solutions. - **Strong bases** are bases completely ionized in solution. KOH = K+ + OH- HA = H+ + A- - **Weak acids and bases** are only partially ionized in their solutions. ### Strong Acids | Type | Formula | |---|---| | Hydrogen halides | HCl HBr HI | | Oxyacids of halogens | HCLO3 HBrO3 HIO3 HCLO4 HBrO4 HIO4 | | Sulfuric acid | H2SO4 | | Nitric acid | HNO3 | ### Strong Bases | Name | Formula | |---|---| | Sodium hydroxide | NaOH | | Potassium hydroxide | KOH | | Cesium hydroxide | CSOH | | Calcium hydroxide | Ca(OH)2 | ### Dissociation Constant (K) - For dissociation reaction of acid and base it is the equilibrium between dissociated molecules and undissociated molecules. CH3COOH -> CH3COO- + H+ Ka = [CH3COO-(aq)][H+(aq)] / [CH3COOH(aq)] - So it is very high in strong acid and very low in weak acid. pKa = -log Ka CH3COOH = CH3COO- + H+ 0.100 0.0042 0.0042 Kb is the dissociation constant of base ### pH - Acids donate protons (hydrogen ions, H+) in solution. Thus, the acidity of a solution must be related to this property. ### Measurement of pH - **Litmus paper:** Litmus paper is a paper coated with a dye, which is red in an acid pH or blue in a basic pH. - **pH paper:** pH paper works on the same principle as litmus paper but uses several different dyes. - **pH meter:** The most accurate tool for pH measurement is the pH meter. This makes use of an electrode dipped into solution and is accurate to about 0.01 pH unit. ### Salts - It is an ionic compound that contains a positive ion other than hydrogen and a negative ion other than hydroxyl. - (1) **Strong acid and strong base:** Neutral salt. HCl + NaOH -> NaCl + H2O - (2) **Weak acid and weak base:** Neutral salt. 2H2CO3 + Fe(OH)2 -> Fe(HCO3)2 + 2H2O - (3) **Strong acid and weak base:** Acidic salt. 2HCl + Fe(OH)2 -> FeCl2 + 2H2O - (4) **Weak acid and strong base:** Basic salt. H2CO3 + NaOH -> NaHCO3 + H2O ### Determination of Salt Type - **Na Cl**: - Add OH -> NaOH St (Base part) - Add H -> HCl St (Acid part) - Result: (Neutral) - **Fe Cl2**: - Add OH -> - Add H -> ### Important Notes To calculate the "e" value in equivalent weight - Acidic salts react with base like NaOH. - Basic slats react with acid like HCL. ### Oxidation-Reduction Reactions - **Oxidation:** is defined as the loss of electrons or an increase in the valence of an element. Consider, for example, the oxidation of elemental iron: - FeO-2e -> Fe+2 - **Reduction** is defined as the gain of electrons or a decrease in the valence of an element. Consider the reduction of elemental oxygen: - O2 + 4e -> 2 O -2 - The oxidation of one element and the reduction of another always occur simultaneously. ### Reducing Agent - (1) Gives up electrons. - (2) Oxidized during reaction - (3) Unusually low valence. ### Oxidizing Agent - (1) Gains electrons - (2) Reduced during reaction - (3) Unusually high valence.