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The Arrhenius Theory of Acids and Bases

The Arrhenius Theory of Acids and Bases. Acids are substances which produce hydrogen ions H + , in solution. Bases are substances which produce hydroxide ions OH- in solution. Neutralization happens because hydrogen ions and hydroxide ions react to produce water. Modified Arrhenius Theory.

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The Arrhenius Theory of Acids and Bases

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  1. The Arrhenius Theory of Acids and Bases • Acids are substances which produce hydrogen ions H+, in solution. • Bases are substances which produce hydroxide ions OH- in solution. • Neutralization happens because hydrogen ions and hydroxide ions react to produce water.

  2. Modified Arrhenius Theory • The Arrhenius definition does not explain or predict the pH of many substances, for example: • CO2(g) - forms an acid in water • NH3(g) – forms a base in water • NaHCO3(s) – acts as both an acid and a base

  3. The modified definition can explain a wider range of species as acid or base. • Acid solutions are formed when substance react with water to form hydronium ions H30+(aq) • Basic solutions are formed when substances react with water to form hydroxide ions OH-(aq) • still doesn’t explain why some substances, like sodium bicarbonate, behave like acids in some reactions and bases in others.

  4. Bronsted-Lowry Acid-Base Concept • proton transfer theory • focuses on acids and bases in reactions • a competition for protons. • Acid donates proton H+, bases accepts H+. • Water does not have to be a reactant or even present.

  5. Polyprotic Acids and Bases • More than one proton can be transferred or accepted • The first proton is most easily removed (weakest bond) • Only one proton is lost at a time • All the strongest acid (or base) reacts before the next strongest can react

  6. Examples of Polyprotic and Polybasic Substances

  7. Some substances can act as an acid in one reaction and as a base in another. These are called amphiprotic(or amphoteric) substances.

  8. Bronsted-Lowry conjugate pairs • A pair of substances that differs by only one proton • A stronger acid will have a weaker conjugate base. • The acid-base table organizes substances in order from strongest to weakest acids (top down) on the left, and weakest to strongest bases on the right.

  9. Bronsted-Lowry Acid-Base Reactions • A Bronsted-Lowry reaction is a competition for protons – the strongest acid will react with the strongest base. • The stronger the acid, the more easily it gives up a proton. This means that the bond with the H+ is weaker if the acid is stronger. • A stronger base has a stronger affinity for a proton. It forms a stronger bond with H+ than the acid did.

  10. Predicting B-L Reactions • List all entities as they exist in a water • Strong acids are H3O+ and conjugate base pair • Weak acids are undissociated/molecules • Ionic compounds are dissociated into ions • Water is present in all aqueous solutions • Identify each entity as an acid, base or both. • Find the strongest acid (highest on left) and strongest base (lowest on right) and react to form conjugates

  11. Predict the equilibrium position for the reaction. • 100% if H3O+ or OH- is a reactant • >50% if acid is above the base on the table • < 50% if acid is below the base on the table

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