Introduction to Acid-Base Reactions
Identify the actual proton donor and acceptor and follow charge through conjugate pairs.
AP Chemistry Topic 4.8 · CED printed p. 86 / PDF p. 93. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Moles and molarity
- Ionic charges and molecular formulas
- Intermolecular versus intramolecular interactions
- Ideal gas equation with kelvin temperature
A Brønsted–Lowry acid donates a proton; a base accepts it. The same species can play either role in different reactions, so identify the actual transfer. In NH3 + H2O ⇌ NH4+ + OH−, ammonia is the base and water is the acid. A conjugate pair differs by exactly one H+, with a charge difference of one; losing a proton makes the charge one unit more negative.
In aqueous acid–base chemistry, water is a participant, not always a background spectator. It accepts a proton from HA to form H3O+, and it donates a proton to a base B to form OH−. H+(aq) is useful shorthand for solvated protons, not isolated bare protons freely floating in water. Water’s ability to donate or accept explains its role in both equations.
HA + H2O ⇌ A− + H3O+ pairs HA with A− and H2O with H3O+. A stronger acid gives a weaker conjugate base: if HA readily gives up a proton, A− has less tendency to take it back. Reaction favors the side with the weaker acid and weaker base. For a strong aqueous acid, its conjugate base has negligible basic behavior relative to water. Comparing strengths is about tendency to transfer, not how concentrated the bottle is. Lewis acid–base classification is outside AP assessment here.
Let's work through it
For HCO3− + H2O ⇌ CO32− + H3O+, identify the roles and pairs.
- HCO3− loses H+, so it is the acid and CO32− its conjugate base.
- Water accepts that proton, so it is the base and H3O+ its conjugate acid.
- Each pair differs by one H and one positive charge. In a different reaction, bicarbonate can accept a proton to form H2CO3; its role is reaction-dependent.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
What is the conjugate acid of CO32−?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
What is the conjugate acid of CO32−?
Worked answer and reason
HCO3−. Add one H+; charge increases by one.
Conjugacy changes one proton and one unit of charge.
Retrieval 2
If acid HA is stronger than HB, how do their conjugate bases compare?
Worked answer and reason
A− is weaker than B− in proton acceptance under the same solvent conditions.
The tendency to donate and take back the same proton are inversely related.
Retrieval 3
How does water act differently in HA +H₂O ⇌A⁻ +H₃O⁺ and B +H₂O ⇌BH⁺ +OH⁻?
Worked answer and reason
Water accepts a proton in the first and is a base; it donates a proton in the second and is an acid.
The actual transfer determines water’s role.
Catch the tempting mistake
- A base must have OH in its formula.
- A base needs to accept a proton; NH₃ does this using its lone pair.
- Conjugate pairs are whichever species lie on the same side.
- Match formulas differing by exactly H⁺ across the reaction.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 4 problem 15 · ap application · symbolic
- Unit 4 problem 16 · ap synthesis · particle
Scientific calculator workspace
Use this workspace for arithmetic, logarithms and elementary functions. Keep intermediate precision and report final values with appropriate units and significant figures. Graphing and general equation solving need a suitable approved tool.
Scientific calculator
This calculator requires JavaScript. Use the physics formula sheets and show your working manually.
+ − * / ^ ! · pi · e · sqrt · sin, cos, tan · asin, acos, atan · atan2(y,x) · ln, log · abs · exp · hypot(x,y). Use * between factors. Keep units in your written working.
For the teacher
- Ask students to draw an arrow showing which proton moves.
- Separate strong/weak chemical tendency from high/low concentration.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 4.8.A
- Identify species as Brønsted-Lowry acids, bases, and/or conjugate acid-base pairs, based on proton-transfer involving those species.
- 4.8.A.1
- By definition, a Brønsted-Lowry acid is a proton donor and a Brønsted-Lowry base is a proton acceptor.
- 4.8.A.2
- Only in aqueous solutions, water plays an important role in many acid-base reactions, as its molecular structure allows it to accept protons from and donate protons to dissolved species.
- 4.8.A.3
- When an acid or base ionizes in water, the conjugate acid-base pairs can be identified and their relative strengths compared.
- Lewis acid-base concepts will not be assessed on the AP Exam. The emphasis in AP Chemistry is on reactions in aqueous solution.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Classifying Chemical Reactions — Scientific check: precipitation, oxidation-number conventions, proton transfer; AP exclusions applied separately.
Glossary and deeper reading · 8 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
-
Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
-
Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
-
Molecular formula
A molecular formula gives the actual number of each kind of atom in a molecule. It can be a whole-number multiple of the empirical formula. For example, glucose has molecular formula C₆H₁₂O₆ and empirical formula CH₂O.
-
Ideal gas law
The ideal gas law relates pressure, volume, amount, and absolute temperature in an idealized gas model. It treats particles as having negligible volume and no intermolecular attractions. For example, at fixed amount and temperature, doubling the container volume halves the ideal gas pressure.
-
Conjugate acid–base pair
A conjugate acid–base pair consists of species differing by one proton. Losing a proton forms an acid's conjugate base, while gaining one forms a base's conjugate acid. For example, ammonium and ammonia form a conjugate acid–base pair.
-
Aqueous solution
An aqueous solution is a solution in which water is the solvent. The dissolved substance may originate as a solid, liquid, or gas, so aqueous does not mean that a solid has melted. For example, sodium chloride dissolved in water is aqueous even though the pure salt is solid.
-
Solvation
Solvation is the interaction of solvent particles with dispersed solute particles. When the solvent is water, the process is called hydration. For example, oriented water molecules form hydration shells around dissolved ions.
-
Lewis acid and base
A Lewis acid accepts an electron pair, and a Lewis base donates an electron pair to form a bonding interaction. This definition includes some acid–base interactions without proton transfer. For example, a nitrogen lone pair can be donated when ammonia binds to an electron-pair acceptor.