In this presentation you will: explore the principles of redox chemistry

Redox 1 • In this presentation you will: • explore the principles of redox chemistry Next >

Introduction Redox chemistry is reactions in which elements undergo simultaneous changes in oxidation state. Here you will examine the concepts of oxidation and reduction, in terms of electron transfer and oxidation states, and explore the displacement reactions that demonstrate the mechanisms of redox chemistry. Next >

Chemical Reactions When a chemical reaction occurs, the chemical composition of the original substances is changed. This process often involves the breaking of chemical bonds, the formation of new ones, and subsequently, an energy change resulting from the release or capture of energy. The new substances that are formed are known as chemical products. Next >

Chemical Reactions Chemists use special terminology to describe the type of chemical reactions that occur. For many years, chemical changes to elements were described in terms of reduction and oxidation. According to these old definitions, oxidation reactions involved the gain of oxygen, whereas reductions entailed the loss of oxygen from a compound. The formation and decomposition of metal oxides exemplifies this. Next >

Chemical Reactions Nowadays, the most common definition of oxidation is the loss of electrons (often an electron pair) from one substance to another. Electron loss = OXIDATION Na Reduction is the reverse process; that is, the gain of electrons (often a pair) from another substance. Electron gain = REDUCTION Cl This can be remembered with the mnemonic OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). Next >

Question 1 "Oxidation can be described in terms of electron transfer as the loss of electrons." Is this statement true or false? Answer True or False. Next >

Question 1 "Oxidation can be described in terms of electron transfer as the loss of electrons." Is this statement true or false? Answer True or False. True Next >

Question 2 "Reduction can be described as the loss of oxygen from, or the gain of electrons to, a compound." Is this statement true or false? Answer True or False. Next >

Question 2 "Reduction can be described as the loss of oxygen from, or the gain of electrons to, a compound." Is this statement true or false? Answer True or False. True Next >

Zn + 2HCI → ZnCI2 + H2 (Zn oxidized) (H+ reduced) Redox Reactions Because processes of electron loss and gain always go hand in hand in chemical reactions (one cannot occur without the other), reactions in which reduction and oxidation occur are known as redox reactions. In the reaction between hydrochloric acid and zinc, for example, zinc is oxidized by the hydrogen, while the hydrogen has been reduced by the zinc. A redox reaction has therefore taken place. Next >

Redox Reactions In a redox reaction, such as that between zinc and hydrochloric acid: • the element that reduces another element is called the reducing agent, and • the element that oxidizes another element is called the oxidizing agent. It is important to recognize thatthe reducing agent is itselfoxidized in the process of redox, and the oxidizing agent is reduced. Next >

Question 3 Is the reaction between hydrochloric acid and zinc a redox reaction? Answer Yes or No. Next >

Question 3 Is the reaction between hydrochloric acid and zinc a redox reaction? Answer Yes or No. Yes Next >

Question 4 In the reaction between hydrochloric acid and zinc, what is the reducing agent? A) The zinc B) The chloride ions C) The hydrogen ions D) The water in which the acid is dissolved Next >

Zn Reducing Agents A reducing agent is any chemical that reduces another element (and is itself oxidized) in a chemical reaction. The term can refer either to the whole compound, or the specific atom or ion within it, that causes the process of reduction. They essentially work as electron donors. Next >

Reducing Agents Strong reducing agents, because they are so reactive, can be dangerous and should be stored and handled with care. Next >

Sodium H2 Sodium borohydride Reducing Agents Examples of important industrial and laboratory reducing agents are: • group 1 and group 2 metals • Hydrides (containing H- ions) • Hydrogen • Sodium borohydride • Dissolved metals Next >

Oxidizing Agents Oxidizing agents are chemicals that oxidize other elements (and are themselves reduced) in chemical reactions. They are essentially electron acceptors. Like reducing agents, strong oxidizing agents are highly reactive. Handle with care! Peroxides Examples include: • Peroxides • Ozone (O3) • Thiosulfates • Halogens Metal cations Halogens • Metal cations Next >

Question 5 Group 1 metals and hydrides are examples of: A) Oxidizing agents B) Reducing agents C) Electron acceptors Next >

Question 6 Which of the following groups of substances do not generally act as oxidizing agents? A) Peroxides and thiosulfates B) Halogens C) Dissolved metals D) Metal cations Next >

FeCI2 + Mg → MgCI2 + Fe 2NaBr + CI2 → 2NaCI + Br2 Consider the formula AX + B → BX + A in which, usually, Single Displacement Reactions Displacement reactions occur when a more reactive element replaces a less reactive element in a compound. • A and B are different metals (or hydrogen),and X is an anion (negative ion), or • A and B are halogens, and X is a cation (positive ion). Next >

Mg + BaCl2→ Ba + MgCl2 Mg + 2Cl- + Ba2+→ Mg2+ + 2Cl- + Ba Mg is oxidized to Mg2+ ion Ba2+ ion is reduced to element Cl- is unaffected (spectator ion) Single Displacement Reactions Displacement reactions usually occur in aqueous solution, so the ions are dissociated. When A and B are metals, A is always oxidized, and B is always reduced. When A and B are halogens, A is reduced and B is oxidized. As the ions are dissociated,X is not affected by the reaction and is neither oxidized nor reduced. It is thus known as a spectator ion. Next >

AX + BY BX + AY Double Displacement Reactions When two compounds in aqueous solution swap ions, they effectively displace each other to form two new compounds. This is known as a double displacement reaction. Next >

AX + BY → BX + AY Double Displacement Reactions The general formula for double displacement reactions is: Unlike single displacement reactions, which are always redox, double displacement reactions never feature a redox process. Next >

Question 7 Which of the following types of reactions involves the process of redox? A) Single displacement reactions B) Double displacement reactions C) Acid-base neutralization reactions D) All of the above Next >

Chlorine solution Iodine solution Bromine solution Displacement Reactions: Halogens The halogens are a group of closely related elements found in group 17 of the periodic table. Chlorine (Cl), bromine (Br), and iodine (I) are the most common halogens in laboratory use. The halogens all form diatomic molecules of the type X2 and also readily form the X- ion by gaining an electron. Next >

Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons Displacement Reactions: Halogens When reagents like halide ions are oxidized, electrons are taken from them. It is easier to oxidize (remove electrons from) the larger halide ions for the following reasons: • The number of energy levels is higher, shielding the electrons from the nucleus. I Next >

Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons Displacement Reactions: Halogens When reagents like halide ions are oxidized, electrons are taken from them. It is easier to oxidize (remove electrons from) the larger halide ions for the following reasons: • The atomic radius of the ion is larger, so the attraction for the outer electrons is lower. I The larger ions (like I-) are therefore stronger reducing agents. Next >

Question 8 Which of the following statements concerning the halogens and their ions is TRUE? A) It is more difficult to oxidize the larger halide ions because their atomic radius and number of energy levels increases B) It is easier to oxidize the larger halide ions because their atomic radius and number of energy levels increases Next >

CI CI Halogens: Oxidizing Ability While halide ions (e.g. Br-) are always reducing agents and are oxidized to Br2, halogen molecules (such as Cl2) are oxidizing agents and are reduced in the reverse process to the halide ion. Next >

Iodine displaced by chloride ions Halogens: Oxidizing Ability The relative oxidizing power of the halogens can be determined by displacement reactions with solutions containing halide ions. The displacement reactions occur when the strongest oxidizing molecules displace the weaker ions as molecules. Next >

F F Halogens: Oxidizing Ability For these reasons, the halogen that acts as the strongest oxidizing agent of all is fluorine (F). However, chlorine is the strongest halogen oxidizing agent found in most laboratories, as it is much less dangerous to handle. Next >

I I Halogens: Oxidizing Ability Iodine is the weakest due to its large size and weaker attraction of its electrons to the nucleus. This means that it accepts electrons less readily than the other halogens to form I- and thereby oxidize other elements. Next >

Question 9 "Halide ions are reducing agents because they gain electrons, whereas halogens are oxidizing agents because they lose electrons." Is this statement true or false? Answer True or False. Next >

Question 9 "Halide ions are reducing agents because they gain electrons, whereas halogens are oxidizing agents because they lose electrons." Is this statement true or false? Answer True or False. False Next >

Halide Ions: Reducing Ability When a halide ion behaves as a reducing agent, it loses an electron to the reagent it is reducing. As the radius of the halide ion increases, the energy required to remove the additional outer shell electron decreases and the diatomic molecule (e.g. Cl2) more readily reforms. Ion radius in pm 133 181 196 220 Ion F- Cl- Br- I- Thus the halide ions become more powerful reducing agents in the order F- < Cl- < Br- < I-. Next >

Question 10 Which of the following halogens is the strongest oxidizing agent? A) Chlorine B) Bromine C) Iodine D) Fluorine Next >

The reactions of concentrated sulfuric acid with solid sodium halides Observations Products Type of reaction NaCI Steamy fumes HCI Displacement NaBr Steamy fumes HBr Displacement Colorless gas SO2 H2SO4 reduction product Brown fumes Br2 Br- oxidation product NaI Steamy fumes Displacement HI Colorless gas H2SO4 reduction product SO2 Yellow solid H2SO4 reduction product S Smell of bad eggs H2SO4 reduction product H2S I-oxidation product Purple fumes I2 Halide Ions: Reducing Ability The reactivity series of halide ions as reducing agents can be shown by reacting solid halide salts with concentrated sulfuric acid (H2SO4). The strongest reducing agents reduce the sulfur in the sulfuric acid to a greater extent than the weaker ones, giving a greater range of chemical products. Many of them are highly toxic. Next >

The reactions of concentrated sulfuric acid with solid sodium halides Observations Products Type of reaction NaCI Steamy fumes HCI Displacement NaBr Steamy fumes HBr Displacement Colorless gas SO2 H2SO4 reduction product Brown fumes Br2 Br- oxidation product NaI Steamy fumes Displacement HI Colorless gas H2SO4 reduction product SO2 Yellow solid H2SO4 reduction product S Smell of bad eggs H2SO4 reduction product H2S I-oxidation product Purple fumes I2 Halide Ions: Reducing Ability I- can reduce sulfuric acid to hydrogen sulfide, sulfur, and sulfur dioxide, while Br- just reduces sulfuric acid to sulfur dioxide, and Cl- only undergoes a displacement reaction. Next >

Summary In this presentation you have seen: • definitions of reduction, oxidation, displacement reactions, and redox • important laboratory reducing and oxidizing agents • trends in the oxidizing ability of halogens and the reducing ability of halide ions (e.g. Cl-, Br-, I-) End >

In this presentation you will: explore the principles of redox chemistry