Reactions Oxidation-reduction of Oxidation-reduction 2019.02.28 prof. V. Paulauskas

Oxidation-reduction is a chemical process wherein the oxidation number of an element is changed The abbreviated term used for oxidation-reduction is redox Redox reactions Redox process involves the complete transfer of electrons in the formation of ionic bonds, or partial transfer or shift of electrons in the formation of covalent bonds 2019.02.28 prof. V. Paulauskas

Ag+(aq) + Cl– (aq) → Ag+Cl–(s) AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq) IONIC REACTION Ag+(aq) + Cl– (aq) → Ag+Cl–(s) Element have the same oxidation number 2Na (s) + Cl2 (g) → NaCl (s) REDOX REACTION 2Na0(s) + Cl20(g) → 2Na+Cl–(s) Oxidation number of the element has changed 2019.02.28 prof. V. Paulauskas

One electron is transferred from a neutral sodium atom to a neutral chlorine atom to form Na+ and Cl- ions that have filled-shell configuration ▫▫ ▫▫ Na + Cl → Na+ Cl – ▫ ▫▫ ▫ ▫▫ ▫▫ ▫▫ ▫▫ Sodium (alkali metals) has only one electron in its valence shell Chlorine (halogens) is missing just one electron from having a complete shell configuration Valence shell – the outermost shell of an atom 2019.02.28 prof. V. Paulauskas

Oxidation numbers (ON) are positive and negative numbers assigned to the atoms of the elements in a compound – they result from the transfer of electrons from one atom to another in ionic cmpds or sharing between in covalent cmpds ON are not real charges - they are the results of an accounting method whereby we can keep track of electrons during a chemical reaction Thus ON are defined using a premise, that the bonds in compounds for which you wish to assign oxidation numbers are assumed to be 100% ionic in nature 2019.02.28 prof. V. Paulauskas

RULES for determination of ON: 1. Free element has always ON=0, e.g. ON of solid, metallic Cu is 0 2. Hydrogen in almost all compounds has ON=+1 3. Oxygen in compounds is always ON=-2, except with F or in peroxides 4. Alkali metals in compounds with non-metals have ON=+1 5. Alkaline earth metals in compounds have ON=+2 6. Halogens (except in cmpnds. with O or one another) have ON=-1 7. Charge of the ion or molecule equals sum of ON of all atoms 8. Algebraic sum of ON in a molecular compound must equal 0 2019.02.28 prof. V. Paulauskas

Oxidation Is Losing electrons Reduction Is Gaining electrons In oxidation, the ON of an element increases in a positive direction as a result of losing electrons Oxidation ───────────────────────► -7 -6 -5 -4 -3 -2 -1 0 +1 +2 +3 +4 +5 +6 +7 ◄─────────────────────── Reduction In reduction, the ON of an element decreases, becoming less positive and more negative, as a result of gaining electrons OIL RIG Oxidation Is Losing electrons  Reduction Is Gaining electrons 2019.02.28 prof. V. Paulauskas

the oxidation half and the reduction half The loss and gain of electrons is a characteristic feature of all redox reactions +4 -2 S + O2 → SO2 Redox reaction may be written as two half-reactions – the oxidation half and the reduction half 2019.02.28 prof. V. Paulauskas

O0 + 2e → O-2 S0 – 4e → S+4 reduction process oxidation process oxidising agent S0 – 4e → S+4 oxidation process reducing agent Substance that gains electrons (does the oxidising) – is called the oxidising agent (oxidator) Typical oxidators have high ON – N+5 Substance that loses electrons (does the reducing) – is called the reducing agent (reductor) Typical reductors have low ON – N-3 2019.02.28 prof. V. Paulauskas

_________________________ Oxidation-reduction – a joint process, so in redox reactions: the number of electrons lost by the reducing agent must be equal to the number of electrons gained by the oxidizing agent 3 │ S+6 + 8e → S-2 (oxidator) │ N+2 – 3e → N+5 (reductor) 8 _________________________ ±24e 3S+6 + 8N+2 → 3S-2 + 8N+5 2019.02.28 prof. V. Paulauskas

Natural redox processes – burning, respiration, photosynthesys, soil formation processes Industrial – metal extraction from ores, production of nitrogen fertilisers, fuel combustion, metal plating, chemical industries Daily life – diesel combustion, galvanic processes (galvanic elements, lead batteries), metal corrosion 2019.02.28 prof. V. Paulauskas

Types of Redox Reactions In combination reactions two elements are combined In a decomposition reaction, one element is broken down into its constitutive parts In displacement reactions, one or more atoms is swapped out In combustion reactions, a compound is combined with oxygen to produce heat In disproportion reactions, a molecule may be simultaneously reduced and oxidized 2019.02.28 prof. V. Paulauskas

Types of Redox Reactions 1. Intermolecular 2. Intramolecular 3. Dismutation 4. Comutation 2019.02.28 prof. V. Paulauskas

ELECTROLYSIS Process where electrical energy is used to bring about a chemical change Application: Manufacturing of sodium, chlorine Production of pure hydrogen and oxygen Purification of metals (silver) Electroplating of metals Charging of batteries 2019.02.28 prof. V. Paulauskas

Metal chemical properties Neutral metals are good reducing agents – in the course of reaction, free Me atom loses electrons to form Me2+ ion: Meo – ne  Men+ reductor/oxidation Cuo – 2e  Cu2+ reductor/oxidation The more negative the standard reduction potential of the Me, the stronger are reductive (metalic) properties 2019.02.28 prof. V. Paulauskas

Metalic properties are determined by electrones in the outer shell that can be easily lost: Meo – ne  Men+ oxidation With an increasing atom diameter, the metallic properties become stronger, as element is is much more likely to lose electrons: Fr  Cs  Rb  K  Na  Li 2019.02.28 prof. V. Paulauskas

Activity Series of the Metals (Electromotive Series) Li-K-Ca-Mg-Al-Zn-Cr-Fe-Cd-Pb-H-Cu-Hg-Ag-Pt-Au -3 -2,4 -0,76 -0,4 0,0V +0,8 +1,7 ------- METAL ACTIVITY DECREASING ------- REDUCTIVE PROPERTIES DECREASING It is a listing of metals in decreasing order of their reactivity with hydrogen-ion sources such as water and acids Metal is oxidized to a metal ion, and the hydrogen ion is reduced to H2 2019.02.28 prof. V. Paulauskas

Activity of Metals Elements toward the bottom left corner of the periodic table are the metals that are the most active in the sense of being the most reactive Lithium, sodium, and potassium all react with water. The rate of this reaction increases as we go down this column, because these elements become more active as they become more metallic 2019.02.28 prof. V. Paulauskas

Activity Series of the Metals Li-K-Ca-Mg-Al-Zn-Cr-Fe-Cd-Pb-H-Cu-Hg-Ag-Pt-Au -3 -2,4 -0,76 -0,4 0,0V +0,8 +1,7 ↔ Very active Me Non-reactive Me Strong metallic properties Weak metallic properties It is related to the standard reduction potential of a Me cation The more positive the standard reduction potential of the cation, the more difficult it is to oxidize the metal to a metal cation, and the later that metal falls in the series 2019.02.28 prof. V. Paulauskas

Activity of the Metals Zn + 2HCl  ZnCl2 + H2 Cu + HCl  no reaction Li-K-Ca-Mg-Al-Zn-Cr-Fe-Cd-Pb-H-Cu-Hg-Ag-Pt-Au All Me with a negative reduction potential will react with acids releasing hydrogen: Zn + 2HCl  ZnCl2 + H2 Cu + HCl  no reaction Very active metals (alkaline) react even with water to release hydrogen: 2Na + 2H2O  2NaOH + H2 2019.02.28 prof. V. Paulauskas

Metal Displacement Reactions Li-K-Ca-Mg-Al-Zn-Cr-Fe-Cd-Pb-H-Cu-Hg-Ag-Pt-Au A metal will displace (take the place of) a less reactive metal in a metal salt solution: Zn + CuSO4  ZnSO4 + Cu Cu + ZnSO4  no reaction Mg + ZnSO4  MgSO4 + Zn The greater the difference between metal potentials – the greater is the reaction rate 2019.02.28 prof. V. Paulauskas

Metal Displacement Reactions Li-K-Ca-Mg-Al-Zn-Cr-Fe-Cd-Pb-H-Cu-Hg-Ag-Pt-Au 2AgNO3(aq) + Cu(s) → 2Ag(s) + Cu(NO3)2(aq) Copper displaces silver in solution when a copper wire is dipped in a silver nitrate solution – solid silver precipitates out 2019.02.28 prof. V. Paulauskas

Metal reaction with acids When metal reacts with non-oxidizing acid (notably HCl and diluted H2SO4), a salt and hydrogen are produced: metal + acid → salt + hydrogen Fe + H2SO4  FeSO4 + H2 Li-K-Ca-Mg-Al-Zn-Cr-Fe-Cd-Pb-H-Cu-Hg-Ag-Pt-Au Hydrogen is included in the table to separate those metals that will not react with acids (the noble metals) from those that will Metals higher than Hydrogen will always replace it in an acid 2019.02.28 prof. V. Paulauskas

Metal reaction with acids Li-K-Ca-Mg-Al-Zn-Cr-Fe-Cd-Pb-H-Cu-Hg-Ag-Pt-Au Oxidising acids (notably concentrated sulfuric and nitric acids), react with metal to give much more interesting products For example, concentrated nitric acid react with copper metal to give salt, water AND a particularly unfriendly gas, NO2 Cu + HNO3  Cu(NO3)2 + NO2 + H2O Metals lower than Hydrogen (the noble metals) will also react with oxidising acids Fe + H2SO4  Fe2(SO4)3 + H2S + H2O 2019.02.28 prof. V. Paulauskas

Metal reactions with acids Reduced products with oxidizing acids (HNO3, concentrated H2SO4) depends upon Me activity: HNO3(N+5)  NO2(N+4), NO (N+2), N2 (N0), NH3(N-3) -------------------------------------------- Cu …..… Pb……. Zn…….. Mg 2019.02.28 prof. V. Paulauskas