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9.1 Introduction to Oxidation and Reduction Assessment Statements 9.1.1 Define oxidation and reduction (electron loss and gain) 9.1.2 Deduce the oxidation number of an element in a compound. 9.1.3 State the names of compounds using oxidation numbers. 9.1.4 Deduce whether an element undergoes oxidation or reduction in reactions using oxidation numbers, History Modern chemistry comes from John Dalton’s atomic theory proposed in 1805 Chemistry really begin when humans used fire Roman historian Pliny (27-79) said glass was first discovered by accident by Phoenician sailors while using fire to cook their food around 5000 BCE Saltpeter, KNO3, may have been used to balance their cooking pots and the combination formed glass Glass beads were found in Egypt and Mesopotamia (Iraq) around 2500 BCE Heat made by burning charcoal was used to alloy copper and tin during the Bronze Age, 4500 BCE to 1200 BCE Copper & tin occur naturally, but this was the 1st evidence for the use of heat to reduce metallic ores, like iron oxides Evidence of iron oxides are found in India dating 1800 BCE, start of Iron Age Combustion wasn’t fully understood until after oxygen was discovered by Joseph Priestley (17331804) Quickly realized that oxygen forms oxides so the word oxidation was created to describe the addition of oxygen When oxygen is removed “reduction” is used Now oxidation and reduction refer to transfer of electrons Assessment Statements 9.1.1 Define oxidation and reduction (electron loss and gain) REDOX Zn (S) + I2 (aq) Zn2+ (aq) + 2I-(aq) Lets look at this reaction: Zinc atoms are becoming positively charged. Iodine atoms are becoming negatively charged. Half Reactions Zn (S) Zn2+ (aq) + 2e- Zinc is losing electrons to become positively charged OXIDATION I2 (aq) + 2e- 2I-(aq) Iodine is gaining electrons to become negatively charged REDUCTION LEO LEO says GER! LEO says GER! Loss of Electrons = Oxidation Gain of Electrons = Reduction Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons) OIL RIG Assessment Statements 9.1.2 Deduce the oxidation number of an element in a compound. Oxidation Numbers Oxidation/Reduction (ReDox)-- Whenever a substance loses electrons and another substance gains electrons Oxidation Numbers--system used to keep track of electron transfers Sometimes refered to as ‘ELECTRON ACCOUNTING’ NOTE: Oxidation numbers are given a ‘+’ or ‘–’ THEN an number (e.g. +7). (do not say 7+, IB does not like this at all) Oxidation Numbers Rules Oxidation numbers always refer to single atoms The oxidation number of an uncombined element is always 0 O2, H2, Ne Zn The oxidation number of Hydrogen is usually +1 Hydrides are an exception They are -1 HCl, H2SO4 The oxidation number of Oxygen is usually -2 Peroxides are an exception They are –1 In OF2 oxygen is a +2 H2O, NO2, et Oxidation numbers of monatomic ions follow the charge of the ion O2-, Zn2+ The sum of oxidation numbers is zero for a neutral compound. It is the charge on a polyatomic ion LiMnO4 SO42- Rules for determining Oxidation Number 1. Atoms of solid metals are neutral (i.e. 0) (no charge has they have the same number of protons and neutrons) E.g. Fe (s) , Mg (s), Sn (s) 2. Natural molecules of gases are Neutral (i.e. 0) E.g. H2, O2 , Cl2 , l2 GENERALLY if an element is in its natural elemental state its Oxidation number is 0 3. Most elements will always produce the same oxidation state (+, -) This is true for e.g. flourine will always be -1 most element in period 1-7 But some can change……….. e.g. Hydrogen is +1 (H+1) except in hydride ion where H is -1 (H-1) Oxygen is -2 (O-2) but +1 in Hydrogen peroxide (O+1) 4. The halogen group are always -1 except when bonded to oxygen or a halogen higher in the group. 5. The total charge on a compound is 0. The number of positives cancel out the negatives. e.g. MgO (Magnesium is +2 and Oxygen is -2) Practice Assigning Oxidation Numbers NO2 N2O5 HClO3 HNO3 Ca(NO3)2 KMnO4 Practice Assigning Oxidation Numbers Fe(OH)3 K2Cr2O7 CO32CNK3Fe(CN)6 Practice Assigning Oxidation Numbers NO2 N= +4, O = -2 N2O5 N = +5, O = -2 HClO3 H=+1, Cl=+5, O = -2 HNO3 H=+1, N = +5, O = -2 Ca(NO3)2 Ca=+2, N =+5, O= -2 KMnO4 K=+1, Mn=+7, O= -2 Practice Assigning Oxidation Numbers Fe(OH)3 Fe =+3, O=-2, H=+1 K2Cr2O7 K=+1, Cr=+6, O=-2 CO32- C=+4, O =-2 CN- C=+4, N=-5 K3Fe(CN)6 K=+1, Fe=+3, C=+4, N=-5 Assessment Statement 9.1.3 State the names of compounds using oxidation numbers. Transition Elements Sc Ti V Cr Mn Fe Co Ni Cu Zn +1 +3 +2 +2 +2 +2 +2 +2 +2 +2 +3 +3 +3 +3 +3 +3 +3 +3 +4 +4 +4 +4 +4 +4 +4 +5 +5 +5 +5 +5 +6 +6 +6 +2 +7 These elements can have multiple oxidation states. So when we talk to someone about, say, ‘Iron Oxide., we also have to tell them is the iron’s oxidation number +2, +3 etc…………. Different Oxidation Numbers The oxidation state of many transition metals can be different e.g. Fe can be +2 or +3 FeCl2 (ferrous Chloride, iron (II) Chloride) FeCl3 (ferric Chloride, iron (III) Chloride) These compounds have very different chemical and physical properties so it is important that we highlight which one we are talking about. Activity Give the oxidation number of the stated element in each of the following species: Iron in Fe 2+ Silicon in Si02 Sulphur in SCl4 Nitrogen in N2O Carbon in CH4OH Vandium in VO 2+ Bromine in BrO Iodine in IF6- Cromium in Cr2O7 2- Assessment Statements 9.1.4 Deduce whether an element undergoes oxidation or reduction in reactions using oxidation numbers, Using Oxidation Numbers An increase in the oxidation number indicates that an atom has lost electrons and therefore oxidized. A decrease in the oxidation number indicates that an atom has gained electrons and therefore reduced Example Zn + CuSO4 ZnSO4 + Cu 0 +2+6-2 +2+6-2 0 Zn: 0 + 2 Oxidized Cu: +2 0 Reduced Exercise For each of the following reactions find the element oxidized and the element reduced Cl2 0 + KBr +1-1 KCl + +1-1 Br increases from –1 to 0 -- oxidized Cl decreases from 0 to –1 -- Reduced K remains unchanged at +1 Br2 0 Exercise For each of the following reactions find the element oxidized and the element reduced Cu + HNO3 Cu(NO3)2 + NO2 + H2O 0 +1+5-2 +2 +5-2 +4 –2 +1-2 Cu increases from 0 to +2. It is oxidized Only part of the N in nitric acid changes from +5 to +4. It is reduced The nitrogen that ends up in copper nitrate remains unchanged Exercise For each of the following reactions find the element oxidized and the element reduced HNO3 + I2 HIO3 + NO2 1 +5 -2 0 +1+5-2 +4-2 N is reduced from +5 to +4. It is reduced I is increased from 0 to +5 It is oxidized The hydrogen and oxygen remain unchanged. Exercise For each of the following reactions find the element oxidized and the element reduced Cl2 + KBr Cu + HNO3 HNO3 + I2 KCl + Br2 Cu(NO3)2 + NO2 + H2O HIO3 + NO2 Activity Write the half equations for each. Work out which is being oxidised or reduced. 2 H2 (g) + O2 (g) 2 H2O (l) Ca (S) + H2O(l) CaO (S) + H2 (g) Mg (S) + CuO (S) → Mg0 (S) + Cu (S) Zn (S) + Cu S04 (aq) → ZnS04 (aq) + Cu (S) Assignment Oxidation Numbers/Naming Review packet Balancing Redox Reactions Many Redox rxns are complex and difficult to balance . A systematic approach to balancing these reaction is required. Balancing Redox Equations 1 1. 2. Divide the equation into 2 half reactions—one for oxidation, one for reduction. Balance each half reaction 1. Balance elements other than H and O 2. Balance O by adding H2O as needed 3. Balance H by adding H+ as needed.(acidic solution) 4. Balance charge by adding e- as needed. 3. Multiply half reactions by integers so that the # of e- lost in one reaction = # of e- gained in the other reaction. 4. Add the two half reactions. Simplify by canceling species that appear on both sides of the arrow. MnO41- (aq) + C2O42- (aq) Mn2+(aq) + CO2 (aq) MnO4 + 5e- Mn+2 C2O4 2CO2 + 2e- (Mn7+ Mn2+ ) (C+3 2C+4 ) 8H+ + MnO4 + 5e- Mn+2 + 4H2O C2O4 2CO2 + 2e- 16H+ + 2MnO4 + 10e- 2Mn+2 + 8H2O 5C2O4 10CO2 + 10e16H+ + 2MnO4 + 5C2O4 2Mn+2 + 8H2O + 10CO2 5. Check your work. Make sure that both the atoms and charges balance Balancing Redox Equations 1 1. 2. Divide the equation into 2 half reactions—one for oxidation, one for reduction. Cr2O72- (aq) + Cl1- (aq) Cr 3+ (aq) + Cl2 (g) Balance each half reaction 1. Balance elements other than H and O 2. Balance O by adding H2O as needed 3. Balance H by adding H+ as needed.(acidic solution) 4. Balance charge by adding e- as needed. 3. Multiply half reactions by integers so that the # of e- lost in one reaction = # of e- gained in the other reaction. 4. Add the two half reactions. Simplify by canceling species that appear on both sides of the arrow. 5. Check your work. Make sure that both the atoms and charges balance Balancing Redox Equations 1 1. 2. Divide the equation into 2 half reactions—one for oxidation, one for reduction. Cu (s) + NO31- (aq) Cu 2+ (aq) + NO2 (aq) Balance each half reaction 1. Balance elements other than H and O 2. Balance O by adding H2O as needed 3. Balance H by adding H+ as needed.(acidic solution) 4. Balance charge by adding e- as needed. 3. Multiply half reactions by integers so that the # of e- lost in one reaction = # of e- gained in the other reaction. 4. Add the two half reactions. Simplify by canceling species that appear on both sides of the arrow. 5. Check your work. Make sure that both the atoms and charges balance Balancing Redox Equations 1 1. 2. Divide the equation into 2 half reactions—one for Mn 2+ (aq) + NaBiO3 (s) oxidation, one for Bi 3+ (aq) + MnO4 1- (aq) + Na reduction. 1+ (aq) Balance each half reaction 1. Balance elements other than H and O 2. Balance O by adding H2O as needed 3. Balance H by adding H+ as needed.(acidic solution) 4. Balance charge by adding e- as needed. 3. Multiply half reactions by integers so that the # of e- lost in one reaction = # of e- gained in the other reaction. 4. Add the two half reactions. Simplify by canceling species that appear on both sides of the arrow. 5. Check your work. Make sure that both the atoms and charges balance Plenary: TOK Are Oxidation Numbers ‘Real’? Lets use the analogy of money. Different parts of the world have a different belief of money. Capitalists the value of money is a measure of work. Communist everyone is equal and money is distributed equally. Monarchist it is a measure of nations worth and means little to the population. BUT is money REAL or just a value depending on a collective belief?????? TOK CHARLES SANDERS PIERCE (philosopher) came up with the concept of ‘PRAGMATIC TRUTH’ in which he states that it is more convenient for a society to believe (truth is more an attribute of a society rather than an attribute of the physical world). TOK: Are Oxidation Numbers Useful? They able to to give us some answers of a chemical change. But is it just a matter of ‘electron accounting’ that helps us balance equations. It tells us what is more energetically favourable. They are useful………but so many exceptions. BUT is it actually what is happening………..