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Polyprotic Acids & Acid Strength

Polyprotic Acids & Acid Strength. Polyprotic Acids. We have seen examples of acids that contain more than one hydrogen ion that can be lost. Sulfuric acid, H 2 SO 4 , for example, has two hydrogen ions that it can give up. The first hydrogen ion is released as:

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Polyprotic Acids & Acid Strength

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  1. Polyprotic Acids & Acid Strength

  2. Polyprotic Acids • We have seen examples of acids that contain more than one hydrogen ion that can be lost. • Sulfuric acid, H2SO4, for example, has two hydrogen ions that it can give up. The first hydrogen ion is released as: H2SO4 (aq) → H+(aq) + HSO4-(aq)

  3. Polyprotic Acids • The second hydrogen will be more difficult to remove because it must now be removed from a negative ion, HSO4-. • Remember that this is because it is difficult to pull something with a positive charge (like a proton) away from something with a negative charge. HSO4-(aq) → H+(aq) + SO42-(aq)

  4. Main Idea: For acids that can donate more than one proton, it will always be easier to donate the first H+ than the second H+.Acids that can donate More than one proton are called polyprotic.

  5. Practice Question: • How many H+ can citric acid, H3C6H5O7, release? • Complete the dissociation equation (without showing water) for each consecutive dissociation equation.

  6. Practice Question Answer: Citric acid can release three H+. The reactions would be: 1.    H3C6H5O7 (aq) → H+(aq) + H2C6H5O7-(aq) 2.    H2C6H5O7-(aq) → H+(aq) + HC6H5O72-(aq) 3.    HC6H5O72-(aq) → H+(aq) + C6H5O73-(aq)

  7. Acid Strength... What Makes an Acid Strong?

  8. Strong and Weak Acids & Bases • You have undoubtedly heard of the pH scale before and know that it has something to do with indicating how strong or weak an acid is. • In this part of the unit we will work towards defining acid and base strength in terms of pH, but there are several important steps along the way. • It will be important that you understand each step.

  9. Acids & Bases are Electrolytic Solutions Electrolytes Acids & Bases Strong acids produce many H+ ions(or H3O+ ions)weak acids produce few H+ ions The stronger the acid, the more H+ ions are produced • Strong electrolytes conduct electricity well because the compound produces many ions in solution • Weak electrolytes conduct electricity poorly because they produce few ions in solution

  10. The Same Goes for Bases • Strong bases produce many OH- ions weak bases produce few OH- ions • The stronger the base, the more OH- ions are produced. • Strong acids and bases are essentially one-way reactions - the acid or base breaks down completely to produce ions

  11. Strong Acids & Bases • At equilibrium there are very few reactants left (very low concentration); only products - the ions. It is impossible to tell just by looking at the formula whether an acid or base is strong or weak, you have to be told or given more information! Notice that the single arrow is used – sometimes, the single arrow is used to imply that the reaction heavily favours the products.

  12. Weak Acids & Bases • Weak acids and bases, however, do not ionize completely. • For weak electrolytes, equilibrium lies to the left side of the equation (the reactant side) and there will be few ions present.

  13. Weak Acids & Bases • The double arrow is commonly used to indicate the partial ionization of the solution. Some examples: Notice here that the double arrow is used, whereas in the previous reactions (strong acids and bases), only a single arrow was used.

  14. Strong & Weak vs. Dilute & Concentrated • It is important that you don't confuse acid and base strength with dilution. • Dilute and concentrated, you should remember, refer to the relative amounts of solute and solvent in a solution. By contrast, acid and base strength specially refer to the concentration of ions in the solution. • A strong acid such as HCl is still a strong acid (completely ionizes to produce many ions) even when it is dilute (lots of water and relatively small amounts of HCl). Acetic acid is still a weak acid even when it is concentrated.

  15. Conjugate Acid and Base Strength • Recall that conjugate acid-base pairs differ in their structure by only one proton. For example, the conjugate base of HNO3 acid is NO3-.

  16. Conjugate Acid and Base Strength • The conjugate base of a strong acid is always a weak base. • The conjugate acid of a strong base is always a weak acid. • The conjugate base of a weak acid is always a strong base. • The conjugate acid of a weak base is always a strong acid.

  17. Practice Question • H2SO4 is a strong acid. What is the formula for its conjugate base? Is it weak or strong? • When NaOH reacts with water, the products are strongly favoured. NaOH reacts with HCl (a strong acid) to form NaCl, and H2O. What are the conjugate acids and bases? Are they weak or strong? Hint: Write the balanced chemical equation for this reaction, and it will help to identify the conjugate acids and bases.

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