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Chemistry - Oxidation‐Reduction Reactions

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    Study Guide Chemistry – Oxidation ‐ Reduction Reactions 1. Introduction to Oxidation – Reduction (Redox) Reactions Oxidation – reduction reactions, often called redox reactions , are among the most important types of chemical reactions you will study. They play a key role in many real - life processes — from producing energy in factories to releasing energy inside your own body during respiration. 1.1 What Happens in a Redox Reaction? At the heart of every redox reaction is the transfer of electrons . One substance gives up electrons, and another substance receives them. • The substance that loses electrons is said to be oxidized . • The substance that gains electrons is said to be reduced . Even though these terms come from older chemistry ideas, they are still widely used today. An important point to remember is this: oxidation and reduction always happen together . You cannot have one without the other because electrons lost by one substance must be gained by another. 1.2 Understanding Oxidation Numbers To keep track of which elements are oxidized and which are reduced, chemists use oxidation numbers . An oxidation number is a value assigned to an element in a compound or reaction that shows how many electrons it has effectively lost or gained. Here’s how oxidation numbers help us: • If an element’s oxidation number increases (becomes more positive), that element is oxidized . • If an element’s oxidation number decreases , that element is reduced . By comparing oxidation numbers before and after a reaction, we can clearly identify which substances are oxidized and which are reduced.

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    Study Guide 1.3 Why Oxidation Numbers Matter Oxidation numbers are not just for identification — they are also essential for balancing redox equations . When balancing these reactions, the goal is to make sure that: • The total number of electrons lost during oxidation • is equal to • The total number of electrons gained during reduction. This balance reflects the conservation of charge and ensures the reaction is written correctly. 2. Quiz: Introduction to Oxidation - Reduction Reactions 1. Question Spontaneous redox reactions are known as __________ reactions in both industry and the human body. Answer Choices • energy consuming • energy producing • energy constant Correct Answer energy producing Why This Is Correct Spontaneous 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 human body, it is used to produce ATP during cellular respiration.

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    Study Guide 2. Question Oxidation and reduction occur Answer Choices • simultaneously • in separate reactions • on the product side of the reaction Correct Answer simultaneously Why This Is Correct Oxidation and reduction always happen together because electrons lost by one species must be gained by another. There is no oxidation without reduction. 3. Question The key characteristic of redox reactions is the Answer Choices • fixed electron configurations of each species • noble gas electron configurations of each species • passing of one or more electrons from one species to another Correct Answer passing of one or more electrons from one species to another Why This Is Correct Redox reactions involve electron transfer. Oxidation is the loss of electrons, and reduction is the gain of electrons, making electron movement the defining feature.

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    Study Guide 4. Question __________ are assigned to each element as a formalism to keep track of the movement of electrons between species. Answer Choices • Physical phases • Oxidation numbers • Molar masses Correct Answer Oxidation numbers Why This Is Correct Oxidation numbers are bookkeeping tools that help track electron loss and gain during chemical reactions. 5. Question Simply stated, the gain of electrons is Answer Choices • reduction • oxidation • sublimation Correct Answer reduction Why This Is Correct Reduction is defined as the gain of electrons. A helpful memory trick is “LEO the lion says GER” (Loss of Electrons is Oxidation, Gain of Electrons is Reduction).

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    Study Guide 6 . Question During oxidation, the oxidation number of a species becomes Answer Choices • more negative • more positive • more neutral Correct Answer more positive Why This Is Correct Oxidation involves the loss of electrons. When electrons are lost, the oxidation number increases, becoming more positive. 7 . Question The species that is being reduced will have Answer 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 occurred Correct Answer a more negative oxidation state after the reduction has occurred Why This Is Correct Reduction involves gaining electrons. Gaining electrons makes the oxidation number decrease, becoming more negative.

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    Study Guide 8 . Question The goal of a redox reaction is to keep the number of electrons lost Answer Choices • less than the number of electrons gained • equal to the number of electrons gained • more than the number of electrons gained Correct Answer equal to the number of electrons gained Why This Is Correct Electrons are conserved in chemical reactions. Every electron lost by one species must be gained by another. 9 . Question Changes in the __________ can also be used to balance redox equations. Answer Choices • physical states • number of species • oxidation numbers Correct Answer oxidation numbers Why This Is Correct Balancing redox equations involves ensuring oxidation numbers change equally for oxidation and reduction, maintaining electron conservation.

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    Study Guide 3. Oxidation Numbers 3.1 What Are Oxidation Numbers? Atoms can gain or lose electrons. • If an atom gains electrons , it becomes negatively charged . • If an atom loses electrons , it becomes positively charged . Atoms with charges are called ions . The idea of oxidation numbers comes from this electron transfer. Oxidation numbers are hypothetical charges we assign to atoms to help us keep track of electrons — especially in covalent bonds , where electrons are shared. Important: Oxidation numbers are not real charges . They are just a helpful way to understand electron movement. 3.2 How Do We Assign Oxidation Numbers? The basic rule is simple: Shared electrons are assigned to the more electronegative atom (the atom that pulls electrons more strongly). Example: Water (H ₂ O) Let’s look at a water molecule using a Lewis structure. Figure 1. Lewis structure for H 2 O. Oxygen is more electronegative than hydrogen. So, when assigning oxidation numbers: • All shared electrons in the O – H bonds are assigned to oxygen . • Each hydrogen is assumed to lose one electron. This leads to: • Oxygen: −2 • Each Hydrogen: +1

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

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    Study Guide Rule 3: Total Oxidation Number Must Match the Overall Charge • In a neutral molecule , oxidation numbers add up to 0 . • In a charged ion , oxidation numbers add up to the ion’s charge. Example: Ammonia (NH ₃ ) The molecule is neutral, so the total must be 0. • Nitrogen: −3 • Each hydrogen: +1 • ( - 3 + 3 × +1 = 0) Example: Carbonate ion (CO ₃ ² ⁻ ) The total must be −2. • Carbon: +4 • Each oxygen: −2 • (+4 + 3 × −2 = −2) Rule 4: Use Electronegativity Patterns Some 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 +1 These patterns make assigning oxidation numbers much easier. Practice Question Try applying what you’ve learned: What is the oxidation number of nitrogen in: 1. Magnesium nitride (Mg ₃ N ₂ ) 2. Nitric acid (HNO ₃ ) (Hint: Use the charge rules and common oxidation numbers.) 4. Quiz: Oxidation Numbers 1. Question When a neutral atom loses an electron, the ion which is formed has Answer Choices • a positive charge • a negative charge • no charge Correct Answer a positive charge Why This Is Correct When 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 a positively charged ion (cation) .

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