The last type of reaction in this unit are oxidation-reduction reactions, commonly called redox. These reactions deal with the transferring➡️ of electrons, which causes molecules to change oxidation states.
When a molecule loses an electron, it’s oxidized, and its oxidation number increases. When a molecule gains an electron, it’s reduced, and its oxidation number decreases. Electrons travel from the oxidized species to the reduced species.
Writing redox reactions reveals which species is which by illustrating the transfer of electrons between molecules.
💡 Mnemonic Device Time! There are two different ones, use the one that seems the most catchy to you:
OIL RIG = "oxidation is loss" and "reduction is gain"
LEO says GER "loss of electrons = oxidation" and "gain of electrons = reduced"
2Mg (s) + O2 (g) --> 2MgO (s)
Identify the charges of each species. Mg and O2 are both neutral molecules. MgO is made up of an ionic bond between Mg2+ and O2-, and we know their charges have to sum to 0 since MgO is a neutral compound.
With this information, we can write half reactions for each reactant. A half reaction just shows the transfer electrons for each molecule.
Start writing the half reaction equations by illustrating the change in charge. Neutral 2 moles of Mg becomes 2 moles of Mg2+, and 1 mole of neutral O2 becomes 1 mole of O2-:
2Mg --> 2Mg2+
O2 --> 2O2-
However, just like any other equation, we need to balance the products and reactants. We’re dealing with the change in charge, so not only do we need to ensure a conservation of mass, but we need to keep a conservation of charge❗❗
We can conserve charge by adding electrons to the appropriate side. In the first equation, the reactants have a charge of 0, but the products have a charge of +4 (2 moles of 2+ ion). We need to add a charge of -4 (or 4 electrons) to the products to make it so that both sides have 0:
2Mg --> 2Mg2+ + 4e-
In the second equation, the reactants also have a charge of 0, but the products have a charge of -4. Thus, we need a way to add a charge of +4 to balance the products out. However, we can only add electrons. The way around this is to add a charge of -4 (or 4 electrons) to the neutral side. Now, both sides have an equal charge of -4.
O2+ 4e- --> 2O2-
Finally, we’ll add the two reactions and simplify by cancelling out similar terms.
2Mg+ O2 --> 2Mg2+ + 2O2-
Congratulations! You made a redox reaction!
However, we’re not done yet😞. In ionic bonds, electrons are transferred completely from one element to another. MgO is an example of an ionic bond. Alternatively, in non-ionic bonds, electrons are shared between molecules. HCl and BrNO3 are some examples.
Scientists👨🔬👩🔬 decided to deal with this caveat by giving molecules with non-ionic bonds oxidation numbers instead of ionic charges. These oxidation numbers reflect the maximum number of electrons a molecule would give or accept if they were in an ionic bond.
💭Remember these rules for assigning oxidation numbers:
Free elements (ex. Br2, Na, P4) have an oxidation number of 0
Neutral molecules also have oxidation numbers of 0, so their elements’ numbers must sum to 0
Ex. In the compound IF6, let x = the oxidation number for iodine and y = the oxidation number for fluorine. This must be true: x + 6y = 0
Monatomic ions have an oxidation number equal to their charge (ex. Na+ = oxidation number of +1, Ba2+ = +2, Cl- = -1, Al3+ = +3)
Oxygen is -2, except in hydrogen peroxide (H2O2) and peroxide ion (O2-2), where it’s -1
Hydrogen is +1, except in metal hydrides (ex. LiH, BaH2), where it’s -1
Fluorine is -1. Other halogens are usually -1, but they vary (ex. Br2O3)
Oxidation numbers can be fractions, but it’s very rare (ex. superoxide, O2- = -½)
⬇️Here are the general rules for making redox equations:
If the molecule has an ionic bond, each element’s oxidation number is their ionic charge. If not, determine their oxidation numbers.
Start writing the half reaction for each molecule by writing the change in charge.
Make sure the elements are balanced
Finalize the half reaction. Balance them by adding electron(s) to the appropriate side.
Add the half reactions together and simplify the equation.
Above, we balanced a really easy equation in an acidic solution. You could also be asked to balance redox in a basic solution. Balancing in a basic solution follows the same steps, BUT there is an additional step at the end since OH- ions are present.
That extra step is to form H2O with the present H+ ions and oxygen atoms, and then add that mass onto the other side with OH-.
🎥 Watch: AP Chemistry - Redox Reactions
✍️ Free Response Questions
AP Chemistry Free Response Questions
⚛️ Unit 1: Atomic Structure and Properties
1.1Moles and Molar Mass
1.2Mass Spectroscopy of Elements
1.3Elemental Composition of Pure Substances
1.4Composition of Mixtures
1.5Atomic Structure and Electron Configurations
1.6Photoelectron Spectroscopy & Graph Interp.
🤓 Unit 2: Molecular and Ionic Compound Structures and Properties
2.0Unit 2 Overview: Molecular and Ionic Bonding
2.1Types of Chemical Bonds
2.2Intramolecular Force and Potential Energy
2.3Ionic Bonding and Ionic Solids
2.4Metallic Bonding and Alloys
2.5Lewis Dot Diagrams
2.6Resonance and Formal Charge
🌀 Unit 3: Intermolecular Forces and Properties
3.0Unit 3 Overview: Intermolecular Forces and Properties
3.2Properties of Solids
3.3Solids, Liquids, and Gases
3.4The Ideal Gas Law
3.5The Kinetic Molecular Theory of Gases
3.6Deviations from the Ideal Gas Law
3.7Mixtures and Solutions
3.8Representations of Solutions
3.9Separation of Solids/Mixtures
3.10Solubility and Solubility Rules
3.11Spectroscopy and the Electromagnetic Spectrum
3.12Quantum Mechanics and the Photoelectric Effect
🧪 Unit 4: Chemical Reactions
4.0Unit 4 Overview: Chemical Reactions
4.1Recognizing Chemical Reactions
4.2Net Ionic Equations
4.4Physical vs. Chemical Changes
4.5Stoichiometry & Calculations
4.6Titrations - Intro and Calculations
4.8Intro to Acid-Base Neutralization Reactions
👟 Unit 5: Kinetics
5.0Unit 5 Overview: Kinetics
5.1Defining Rate of Reaction
5.2Introduction to Rate Laws
5.3Rate and Concentration Change
5.4Writing Rate Laws
5.5Collision Model of Kinetics
5.6Reaction Energy and Graphs w/ Energy
5.7Reaction Mechanisms and Elementary Steps
5.8Writing Rate Laws Using Mechanisms
🔥 Unit 6: Thermodynamics
6.0 Unit 6 Overview: Thermochemistry and Reaction Thermodynamics
6.1Endothermic Processes vs. Exothermic Processes
6.2Energy Diagrams of Reactions
6.3Kinetic Energy, Heat Transfer, and Thermal Equilibrium
6.4Heat Capacity and Coffee-Cup Calorimetry
6.5Phase Changes and Energy
6.6Introduction to Enthalpy of Reaction
6.7Bond Enthalpy and Bond Dissociation Energy
6.8Enthalpies of Formation
⚖️ Unit 7: Equilibrium
🍊 Unit 8: Acids and Bases
8.0Unit 8 Overview: Acids and Bases
8.1Introduction to Acids and Bases
Unit 9: Applications of Thermodynamics
🤺 AP Chemistry Essentials
🧐 Multiple Choice Questions
*ap® and advanced placement® are registered trademarks of the college board, which was not involved in the production of, and does not endorse, this product.
© fiveable 2021 | all rights reserved.
2550 north lake drive
milwaukee, wi 53211