Heat's Effect on Acids & Bases

The pH of a weak acid, like citric acid, varies depending on the temperature.

The strength, or pH, of acids and bases is dependent upon many factors. One of those factors is temperature, which changes the concentration of acids and bases.

Definitions

The Bronsted-Lowry definition of acids and bases states that an acid is anything that donates an H+, or positively charged hydrogen atom, and a base is anything that accepts an H+.

Equilibrium

Weak acids only give up some H+ ions, and weak bases only accept some H+ ions. When the ratio between the products and the reactants is stable, equilibrium has been reached.

Le Chatelier's Principle

Le Chatelier's principle says that if a system in equilibrium experiences a change, like a change in temperature, then the equilibrium of the system must change to compensate.

Heat and Reversible Reactions

In reversible reactions, the products can spontaneously turn back into the reactants, as acids and bases can, and will have heat as either a product or a reactant. If heat is added, then the reaction will reach a new equilibrium favoring products if heat was a reactant, or reactants if heat was a product.

Heat and Acids and Bases

Adding heat changes the equilibrium of an acid or base, altering the pH. Whether it will increase or decrease pH depends upon the individual reaction.

References

• Shodor Education Foundation: Acid-Base Chemistry
• Computer Assisted Chemistry Tutorial: Weak Acids and Bases
• Shodor Education Foundation: Chemical Equilibrium
• Newton Ask a Scientist: Temperature and pH Measurement