# R3: What are the mechanisms of chemical change?

> IB Chemistry HL · IB 2025+
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u6-overview/
> Weight: n/a

This unit explores how and why chemical reactions proceed, covering core topics in organic chemistry, redox chemistry, electrochemistry, and reaction mechanism fundamentals for both standard and higher level IB Chemistry.

**Prerequisites:** Foundational understanding of atomic structure, chemical bonding, and basic thermodynamics

## Learning objectives

- Identify organic functional groups and apply IUPAC nomenclature rules to name and draw organic compounds
- Classify common organic reaction types and interpret step-by-step reaction mechanisms
- Apply redox principles to explain the operation of both voltaic and electrolytic cells
- Explain how catalysts alter reaction mechanisms to influence the rate of chemical change

## Unit at a Glance

This unit builds sequentially from foundational skills to advanced content, starting with core organic chemistry basics, then moving into reaction classification, mechanism fundamentals, and core electrochemistry. After mastering core topics, you will progress to higher level (AHL) extensions including stereoisomerism, advanced mechanisms, electrolytic cells, catalysis, and spectroscopic identification of organic compounds.

This unit includes the following sub-topics, ordered from core to AHL content:
- [Introduction to organic chemistry: functional groups](https://www.owlsprep.com/study/ib-chemistry-hl-u6-introduction-to-organic-chemistry-functional/) — Classify organic compounds by their functional groups and recognize key structural features.
- [Nomenclature of organic compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u6-nomenclature-of-organic-compounds/) — Apply IUPAC rules to name and draw straight-chain and branched organic compounds.
- [Types of organic reactions](https://www.owlsprep.com/study/ib-chemistry-hl-u6-types-of-organic-reactions/) — Classify common reaction types including substitution, addition, elimination, and condensation.
- [Redox reactions and electrochemistry basics](https://www.owlsprep.com/study/ib-chemistry-hl-u6-redox-reactions-and-electrochemistry-basics/) — Introduce core oxidation and reduction concepts and their role in electrochemical processes.
- [Oxidation numbers and voltaic cells](https://www.owlsprep.com/study/ib-chemistry-hl-u6-oxidation-numbers-and-voltaic-cells/) — Calculate oxidation numbers and explain the structure and operation of simple voltaic cells.
- [Reaction mechanism basics](https://www.owlsprep.com/study/ib-chemistry-hl-u6-reaction-mechanism-basics/) — Learn how mechanisms describe stepwise bond breaking and forming during chemical change.
- [AHL: Stereoisomerism](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-stereoisomerism/) — Explore stereoisomerism including enantiomers and apply Cahn-Ingold-Prelog naming rules.
- [AHL: Advanced organic reaction mechanisms](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-advanced-organic-reaction-mechanisms/) — Study detailed mechanisms for common organic reactions including SN1, SN2, and electrophilic addition.
- [AHL: Electrolytic cells and standard electrode potentials](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-electrolytic-cells-and-standard/) — Calculate standard cell potentials and explain the operation of electrolytic cells and electrolysis.
- [AHL: Catalysis and reaction mechanisms](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-catalysis-and-reaction-mechanisms/) — Explain how homogeneous and heterogeneous catalysts alter reaction mechanisms to speed up reactions.
- [AHL: Spectroscopic identification of organic compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-spectroscopic-identification-of-organic/) — Use IR, mass spectrometry, and NMR data to determine the structure of unknown organic compounds.

## Common pitfalls

- **Wrong:** Confusing anode/cathode charge and redox roles in voltaic vs electrolytic cells
  - Why it fails: The charge of electrodes swaps between cell types, leading to confusion about where oxidation/reduction occurs
  - Correct: Remember 'AN OX RED CAT' applies to all cells: oxidation at anode, reduction at cathode, regardless of charge
- **Wrong:** Numbering parent chains or naming organic compounds without checking functional group priority
  - Why it fails: Many students number chains or assign names based on the first substituent they see instead of the highest priority group
  - Correct: Always identify the highest priority functional group first to set parent chain numbering
- **Wrong:** Drawing curly arrows in mechanisms starting from the electrophile
  - Why it fails: Curly arrows represent electron movement, not atom movement
  - Correct: Always start curly arrows at a lone pair or bonding electron pair, moving to the electron-deficient site

## Cheatsheet

| Concept / Key Formula | Description |
| --- | --- |
| Organic oxidation | Loss of H / gain of O / loss of electrons |
| Organic reduction | Gain of H / loss of O / gain of electrons |
| AN OX RED CAT | Oxidation at anode, reduction at cathode (all electrochemical cell types) |
| $E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode}$ | Calculates standard cell potential; positive $E^\circ_{cell}$ = spontaneous reaction |
| Curly arrow notation | Represents movement of a pair of electrons from electron-rich to electron-poor site |
| Common functional group priority | Carboxylic acids > esters > aldehydes > ketones > alcohols > alkenes > alkanes |
| Catalysis definition | Catalysts lower activation energy by providing an alternate reaction mechanism, and are not consumed |

## What's next

Begin your study of this unit with the first sub-topic on organic functional groups, then progress through the sub-topics in the order listed to build core knowledge before moving to advanced AHL content. Core concepts build sequentially, so working in order will help you master reaction mechanism logic. After completing all topics in this unit, you will move on to the next unit covering chemical kinetics.

- [Introduction to organic chemistry: functional groups](https://www.owlsprep.com/study/ib-chemistry-hl-u6-introduction-to-organic-chemistry-functional/)
- [Nomenclature of organic compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u6-nomenclature-of-organic-compounds/)
- [Types of organic reactions](https://www.owlsprep.com/study/ib-chemistry-hl-u6-types-of-organic-reactions/)

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