CHEMISTRY 40S UNIT 1: AQUEOUS REACTIONS IN SOLUTION LESSON 5

LEARNING OUTCOMES C : Identify reactions as redox or non-redox. Include: oxidizing agent, reducing agent, oxidized substance, and reduced substance.

DEFINING OXIDIZING AND REDUCING AGENTS Based on the below reaction: We can see that: The copper atom is oxidized because its oxidation number increases (becomes more positive), indicating a loss of electrons Each silver atom is reduced---the gain of electrons is indicated by a decrease in the oxidation number

OXIDIZING AND REDUCING AGENTS The two processes of reduction and oxidation cannot occur without one another Since silver is reduced, it causes copper to be oxidized Any substance that causes the oxidation of another substance is called an oxidizing agent In this example, AgNO3 is the oxidizing agent Since copper is oxidized, it causes silver to be reduced. Any substance that causes the reduction of another substance is called a reducing agent The oxidizing agent becomes reduced and the reducing agent becomes oxidized Analogy: travel agent A travel agent books tickets and holidays for you to travel. They don't do the traveling, but they are responsible for your travels. They are the agents of travel.

IDENTIFYING OXIDIZING AND REDUCING AGENTS Ex 1) Nitric acid reacts with hydrogen sulfide according to the balanced equation below. Identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent for the burning of propane. 2 HNO 3 (aq) + 3 H 2 S(g) → 2NO(g) + 3 S(s) + 4 H 2 O(l) Step 1: Assign oxidation numbers

IDENTIFYING OXIDIZING AND REDUCING AGENTS Step 2: Determine substances oxidized and reduced S is oxidized, loses electrons. N is reduced, gains electrons. H 2 S is the reducing agent. HNO 3 is the oxidizing agent. NOTE: oxidized and reduced substances are always atoms. The oxidizing and reducing agents are the compounds that undergo the change.

ELECTRONS TRANSFERRED A balanced chemical equation allows us to determine how many electrons are transferred in a redox reaction To determine the number of electrons transferred in a redox reaction: 1.Assign oxidation numbers 2.Determine atoms gaining or losing electrons 3.Multiply the number of electrons lost or gained by the number of atoms gaining or losing the electrons

EXAMPLE: ELECTRONS TRANSFERRED Example: for the redox reaction Cu + 2 AgNO 3 → Cu(NO 3 ) Ag How many electrons are transferred? Step 1 & 2: Step 3: Each copper atom loses 2 electrons. There is one copper atom = total of 2 electrons lost The silver atoms gain 1 electron each. There are 2 silver atoms = 2 electrons gained 2 electrons lost + 2 electrons gained = total of 2 electrons transferred

EXAMPLE: ELECTRONS TRANSFERRED Example 2) How many electrons are transferred in the reaction below: 2 HNO 3 (aq) + 3 H 2 S(g) → 2NO(g) + 3 S(s) + 4 H 2 O(l) Step 1 & 2:

EXAMPLE: ELECTRONS TRANSFERRED Step 3:

COMMON OXIDIZING AGENTS Strong oxidizers are those that readily accept electrons, that is, they will take or steal electrons from many substances. Strong oxidizing agents are usually atoms with high positive oxidation numbers, like +5, +6 and +7. Usually very reactive because they will take electrons from almost any substance, in order to lower their oxidation number. Common Oxidizing Agents Reaction Products O2O2 O 2 -, H 2 0, CO 2 F 2, Cl 2, Br 2, I 2 F -, Cl -, Br -, I - MnO 4 - Mn 2+ Cr 2 O 7 2- Cr 3+ HNO 3 NO, NO 2 H2O2H2O2 O 2, H 2 O

COMMON OXIDIZING AGENTS Example : Chlorine Often used in pools, drinking water and sewage to kill bacteria. Chlorine oxidizes the cell membrane of bacteria causing the membranes to be disrupted and the bacteria die. Hydrogen peroxide, H 2 O 2, works in a similar manner when used as a disinfectant on cuts and scrapes.

COMMON REDUCING AGENTS Good reducing agents are good sources of electrons. That is, reducing agents give up electrons easily. Examples of good reducing agents include metals that are easily oxidized, such as Na, K, Mg, Al, Zn and Fe. Other reducing agents are those substances that burn easily, like hydrogen gas and hydrocarbons.