Chemistry Acids and Bases Definitions

Brønsted-Lowry Definition of Acids and Bases

• Acids are proton donors.
• Bases are proton acceptors.
• This definition is more general than the Arrhenius definition, encompassing a wider range of substances.
• Examples:
  • HCl (hydrochloric acid) donates a proton to form Cl− (chloride ion), acting as an acid.
  • NH3 (ammonia) accepts a proton to form NH4+ (ammonium ion), acting as a base.

Arrhenius Definition of Acids and Bases

• Acids produce H+ ions in aqueous solutions.
• Bases produce OH− ions in aqueous solutions.
• This definition is limited to aqueous solutions.
• Examples:
  • Hydrochloric acid (HCl) dissociates into H+ and Cl− ions in water.
  • Sodium hydroxide (NaOH) dissociates into Na+ and OH− ions in water.

Conjugate Acid-Base Pairs

• When an acid loses a proton, it forms its conjugate base.
• When a base gains a proton, it forms its conjugate acid.
• These pairs are related by the transfer of a single proton.
• Example:
  • HCl (acid) loses a proton to form Cl− (conjugate base)
  • H2O (base) gains a proton to form H3O+ (conjugate acid)

Strengths of Acids and Bases

• Strong acids and bases completely dissociate in water.
• Weak acids and bases partially dissociate in water.
• The strength of an acid or base is determined by its tendency to donate or accept protons.
• Strong acids: HCl, H2SO4, HNO3
• Strong bases: NaOH, KOH, Ca(OH)2
• Weak acids: acetic acid (CH3COOH), carbonic acid (H2CO3)
• Weak bases: ammonia (NH3), amines

pH Scale

• A measure of the concentration of H+ ions in a solution.
• Ranges from 0 to 14.
• pH 7 is neutral (water).
• pH < 7 is acidic.
• pH > 7 is basic.
• Logarithmic scale (a change of one pH unit corresponds to a tenfold change in H+ concentration).

Neutralization Reactions

• A reaction between an acid and a base.
• The products are a salt and water.
• The H+ ions from the acid react with the OH− ions from the base to form water.
• Example:
  • HCl + NaOH → NaCl + H2O

Salts

• Ionic compounds formed from the reaction of an acid and a base.
• Consist of a cation (positive ion) from the base and an anion (negative ion) from the acid.
• Can be acidic, basic, or neutral depending on the strength of their constituent ions.
• Example:
  • NaCl (sodium chloride) is a neutral salt.
  • NH4Cl (ammonium chloride) is an acidic salt.
  • Na2CO3 (sodium carbonate) is a basic salt.

Acid-Base Titrations

• A method used to determine the concentration of an unknown acid or base solution.
• A solution of known concentration (standard solution) is added to the unknown solution until the reaction is complete.
• The equivalence point is reached when the moles of acid equal the moles of base.
• Indicators are used to visually detect the equivalence point.

Indicators

• Substances that change color depending on the pH of the solution.
• Used to signal the end point of a titration.
• Common indicators include phenolphthalein (colorless in acid, pink in base) and methyl orange (red in acid, yellow in base).

Acid Rain

• Acidic deposition from the atmosphere.
• Formed when sulfur dioxide (SO2) and nitrogen oxides (NOx) react with water, oxygen, and other chemicals to form various acidic compounds.
• Causes damage to ecosystems, including water bodies, forests, and structures.

Buffers

• Solutions that resist changes in pH.
• Consist of a weak acid and its conjugate base, or a weak base and its conjugate acid.
• Maintain a relatively constant pH when small amounts of acid or base are added.
• Important in biological systems to maintain homeostasis.

Explore the Brønsted-Lowry and Arrhenius definitions of acids and bases in this quiz. Understand the concept of conjugate acid-base pairs and their importance in chemical reactions. Test your knowledge with examples and definitions that illustrate these foundational ideas.