Chemistry - Oxidation‐Reduction Reactions - Quick Guides | Chemistry

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Study GuideChemistry–Oxidation‐Reduction Reactions1.Introduction to Oxidation–Reduction (Redox) ReactionsOxidation–reduction reactions, often calledredox reactions, are among the most important types ofchemical reactions you will study. They play a key role in many real-life processes—from producingenergy in factories to releasing energy inside your own body during respiration.1.1What Happens in a Redox Reaction?At the heart of every redox reaction is thetransfer of electrons. One substance gives up electrons,and another substance receives them.•The substance thatloses electronsis said to beoxidized.•The substance thatgains electronsis said to bereduced.Even though these terms come from older chemistry ideas, they are still widely used today. Animportant point to remember is this:oxidation and reduction always happen together. You cannothave one without the other because electrons lost by one substance must be gained by another.1.2Understanding Oxidation NumbersTo keep track of which elements are oxidized and which are reduced, chemists useoxidationnumbers. An oxidation number is a value assigned to an element in a compound or reaction thatshows how many electrons it has effectively lost or gained.Here’s how oxidation numbers help us:•If an element’s oxidation numberincreases(becomes more positive), that element isoxidized.•If an element’s oxidation numberdecreases, that element isreduced.By comparing oxidation numbers before and after a reaction, we can clearly identify which substancesare oxidized and which are reduced.

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Study Guide1.3Why Oxidation Numbers MatterOxidation numbers are not just for identification—they are also essential forbalancing redoxequations. When balancing these reactions, the goal is to make sure that:•Thetotal number of electrons lostduring oxidation•is equal to•Thetotal number of electrons gainedduring reduction.This balance reflects the conservation of charge and ensures the reaction is written correctly.2. Quiz: Introduction to Oxidation-Reduction Reactions1. QuestionSpontaneous redox reactions are known as __________ reactions in both industry and the humanbody.Answer Choices•energy consuming•energy producing•energy constantCorrect Answerenergy producingWhy This Is CorrectSpontaneous redox reactions release energy as electrons move from one substance to another.In industry, this energy may be captured as electrical energy (like in batteries), and in the humanbody, it is used to produce ATP during cellular respiration.

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Study Guide2. QuestionOxidation and reduction occurAnswer Choices•simultaneously•in separate reactions•on the product side of the reactionCorrect AnswersimultaneouslyWhy This Is CorrectOxidation and reduction always happen together because electrons lost by one species must begained by another.There is no oxidation without reduction.3. QuestionThe key characteristic of redox reactions is theAnswer Choices•fixed electron configurations of each species•noble gas electron configurations of each species•passing of one or more electrons from one species to anotherCorrect Answerpassing of one or more electrons from one species to anotherWhy This Is CorrectRedox reactions involve electron transfer.Oxidation is the loss of electrons, and reduction is the gain of electrons, making electron movementthe defining feature.

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Study Guide4. Question__________ are assigned to each element as a formalism to keep track of the movement of electronsbetween species.Answer Choices•Physical phases•Oxidation numbers•Molar massesCorrect AnswerOxidation numbersWhy This Is CorrectOxidation numbers are bookkeeping tools that help track electron loss and gain during chemicalreactions.5. QuestionSimply stated, the gain of electrons isAnswer Choices•reduction•oxidation•sublimationCorrect AnswerreductionWhy This Is CorrectReduction is defined as the gain of electrons.A helpful memory trick is“LEO the lion says GER”(Loss of Electrons is Oxidation, Gain of Electronsis Reduction).

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Study Guide6. QuestionDuring oxidation, the oxidation number of a species becomesAnswer Choices•more negative•more positive•more neutralCorrect Answermore positiveWhy This Is CorrectOxidation involves the loss of electrons.When electrons are lost, the oxidation number increases, becoming more positive.7. QuestionThe species that is being reduced will haveAnswer Choices•a more negative oxidation state after the reduction has occurred•a more positive oxidation state after the reduction has occurred•no change in its oxidation state after the reduction has occurredCorrect Answera more negative oxidation state after the reduction has occurredWhy This Is CorrectReduction involves gaining electrons.Gaining electrons makes the oxidation number decrease, becoming more negative.

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Study Guide8. QuestionThe goal of a redox reaction is to keep the number of electrons lostAnswer Choices•less than the number of electrons gained•equal to the number of electrons gained•more than the number of electrons gainedCorrect Answerequal to the number of electrons gainedWhy This Is CorrectElectrons are conserved in chemical reactions.Every electron lost by one species must be gained by another.9. QuestionChanges in the __________ can also be used to balance redox equations.Answer Choices•physical states•number of species•oxidation numbersCorrect Answeroxidation numbersWhy This Is CorrectBalancing redox equations involves ensuring oxidation numbers change equally for oxidation andreduction, maintaining electron conservation.

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Study Guide3. Oxidation Numbers3.1What Are Oxidation Numbers?Atoms can gain or lose electrons.•If an atomgains electrons, it becomesnegatively charged.•If an atomloses electrons, it becomespositively charged.Atoms with charges are calledions. The idea of oxidation numbers comes from this electron transfer.Oxidation numbers arehypothetical chargeswe assign to atoms to help us keep track ofelectrons—especially incovalent bonds, where electrons are shared.Important: Oxidation numbers arenot real charges. They are just a helpful way to understandelectron movement.3.2How Do We Assign Oxidation Numbers?The basic rule is simple:Shared electrons are assigned to the more electronegative atom(the atom that pulls electronsmore strongly).Example: Water (H₂O)Let’s look at a water molecule using a Lewis structure.Figure1.Lewis structure for H2O.Oxygen is more electronegative than hydrogen. So, when assigning oxidation numbers:•All shared electrons in the O–H bonds are assigned tooxygen.•Each hydrogen is assumed to lose one electron.This leads to:•Oxygen: −2•Each Hydrogen: +1

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Study GuideFigure2.Assignment of oxidation numbers.Even though these look like charges, remember:the atoms in water are not ions.3.3Assigning Oxidation NumbersRule 1: Pure Elements Have an Oxidation Number of 0Any element by itself has an oxidation number ofzero.Examples:•Carbon in graphite or diamond → 0•Iron metal (Fe) → 0•Hydrogen in H₂→ 0Rule 2: Single-Atom IonsFor ions made of one atom, the oxidation number equals theactual charge.Examples:•Na⁺→ +1•Fe²⁺→ +2•Fe³⁺→ +3

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Study GuideRule 3: Total Oxidation Number Must Match the Overall Charge•In aneutral molecule, oxidation numbers add up to0.•In acharged ion, oxidation numbers add up to the ion’s charge.Example: Ammonia (NH₃)The molecule is neutral, so the total must be 0.•Nitrogen: −3•Each hydrogen: +1•(-3 + 3 × +1 = 0)Example: Carbonate ion (CO₃²⁻)The total must be −2.•Carbon: +4•Each oxygen: −2•(+4 + 3 × −2 = −2)Rule 4: Use Electronegativity PatternsSome elements almost always have the same oxidation numbers:•Oxygen:usually −2•Hydrogen:usually +1

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Study Guide•Halogens (F, Cl, Br, I):usually −1•Alkali metals (Group 1):always +1These patterns make assigning oxidation numbers much easier.Practice QuestionTry applying what you’ve learned:What is the oxidation number ofnitrogenin:1.Magnesium nitride (Mg₃N₂)2.Nitric acid (HNO₃)(Hint: Use the charge rules and common oxidation numbers.)4. Quiz: Oxidation Numbers1. QuestionWhen a neutral atom loses an electron, the ion which is formed hasAnswer Choices•a positive charge•a negative charge•no chargeCorrect Answera positive chargeWhy This Is CorrectWhen a neutral atom loses an electron, it loses a negative charge.Because protons remain unchanged, the atom now has more positive charges than negative ones,forming apositively charged ion (cation).

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