Calculator Inputs
Equilibrium Graph
The chart shows how Keq changes when pKa difference changes. A log scale is used because Keq changes very fast.
Formula Used
For acid-base reactions
HA + B⁻ ⇌ A⁻ + HB
Keq = 10^[pKa(HB) - pKa(HA)]
A positive ΔpKa gives Keq greater than one. Products are favored.
For acid dissociation
HA ⇌ H⁺ + A⁻
Keq ≈ Ka = 10^(-pKa)
This mode treats the equilibrium constant as the acid dissociation constant.
Free energy relation
ΔG° = -RT ln(Keq)
R is 8.314462618 J mol⁻¹ K⁻¹. Temperature is entered in Kelvin.
How to Use This Calculator
- Select the calculation mode.
- Enter the reactant acid pKa value.
- Enter the product conjugate acid pKa value for reaction mode.
- Add optional starting concentrations for HA and B⁻.
- Enter temperature in Kelvin for ΔG°.
- Press the calculate button.
- Review Keq, ΔpKa, direction, and mixture estimates.
- Download the result as CSV or PDF.
Example Data Table
| Reaction example | pKa(HA) | pKa(HB) | ΔpKa | Keq | Direction |
|---|---|---|---|---|---|
| Acetic acid + ammonia | 4.76 | 9.25 | 4.49 | 3.09 × 10⁴ | Product favored |
| Ammonium + acetate | 9.25 | 4.76 | -4.49 | 3.24 × 10⁻⁵ | Reactant favored |
| Phenol + ethoxide | 10.00 | 16.00 | 6.00 | 1.00 × 10⁶ | Product favored |
| HCl + water | -7.00 | -1.74 | 5.26 | 1.82 × 10⁵ | Product favored |
| Water + acetate | 15.70 | 4.76 | -10.94 | 1.15 × 10⁻¹¹ | Reactant favored |
Values are illustrative. Use verified pKa data for formal laboratory or academic work.
Understanding Keq from pKa
Why pKa matters
pKa is a compact way to describe acid strength. A lower pKa means a stronger acid. A stronger acid gives up its proton more easily. A weaker acid holds its proton more strongly. These facts help predict acid-base equilibrium direction.
Reaction direction
Acid-base reactions usually move from the stronger acid side toward the weaker acid side. The calculator compares the pKa of the reactant acid with the pKa of the product acid. That difference is called ΔpKa. When ΔpKa is positive, the product acid is weaker. The reaction is then product favored.
What Keq means
Keq shows the ratio of products to reactants at equilibrium. A Keq above one means products are larger. A Keq below one means reactants are larger. A Keq near one means both sides are important. Because the pKa scale is logarithmic, one pKa unit changes Keq by a factor of ten.
Advanced interpretation
This tool also calculates log10(Keq), Ka values, and standard free energy. These values help compare reactions that look similar. The concentration estimate uses a simple one-step equilibrium model. It assumes a 1:1 reaction. It also assumes no side reactions.
Practical use
Students can use the calculator to check homework. Teachers can create fast examples. Chemists can estimate whether a proton transfer is reasonable before planning a reaction. The export buttons help save results for reports. The graph helps show trends clearly. Always remember that solvents, temperature, ionic strength, and activity effects can change real measurements.
Important limits
Keq from pKa is a thermodynamic estimate. It does not prove that a reaction is fast. Some reactions are favorable but slow. Some are affected by solubility or competing equilibria. For careful work, use pKa values measured in the same solvent and temperature. That gives the most reliable comparison.
FAQs
1. What does this calculator find?
It finds Keq from pKa values. It can estimate acid-base reaction direction, Ka, ΔpKa, log10(Keq), and ΔG°.
2. Which pKa values should I enter?
For reaction mode, enter the pKa of HA and HB. HA is the reactant acid. HB is the conjugate acid formed from the base.
3. What formula is used for reaction mode?
The calculator uses Keq = 10^[pKa(HB) - pKa(HA)]. This predicts whether proton transfer favors products or reactants.
4. What does positive ΔpKa mean?
A positive ΔpKa means the product acid is weaker than the reactant acid. The equilibrium usually favors products.
5. What does negative ΔpKa mean?
A negative ΔpKa means the product acid is stronger. The equilibrium usually favors reactants instead of products.
6. Is Keq the same as Ka?
For acid dissociation mode, Keq is treated as Ka. For acid-base reaction mode, Keq compares products and reactants.
7. Why use a logarithmic graph?
Keq changes by powers of ten. A logarithmic graph makes very small and very large values easier to compare.
8. Can solvent change the answer?
Yes. pKa depends on solvent and conditions. Use pKa values measured in the same solvent for the best estimate.