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Section 6.3—Acidity, pH

Section 6.3—Acidity, pH. How does concentration of acid affect the pH of a sports drink?. Objectives Calculate the pH from the concentration of hydronium ions and vice versa Explain the connection between strength and pH

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Section 6.3—Acidity, pH

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  1. Section 6.3—Acidity, pH How does concentration of acid affect the pH of a sports drink? • Objectives • Calculate the pH from the concentration of hydronium ions and vice versa • Explain the connection between strength and pH • Calculate the concentration of hydroxide ions from hydronium ions and vice versa

  2. H +1 +1 H water O O H H H H Review of Acids – Arrhenius Definition • Acids produce a hydronium ion (H3O+1) in water • A hydronium ion is water + a hydrogen cation By this definition, if an acid is to give a H+1 to water, then all acids will have hydrogen as the cation (first element written).

  3. Cl- H+ O H H How do Acids produce Hydronium? water hydrochloric acid Hydrogen cation (H+) with Cl- anion

  4. H Cl- O H H How do Acids produce Hydronium? +1

  5. H Cl- O H H How do Acids produce Hydronium? +1 Hydronium ion Chloride anion

  6. -1 O H Bases – Arrhenius Definition • Bases produce the hydroxide ion in water Hydroxide Ion

  7. Characteristics of Acids & Bases Acids Bases Produce H3O+1 (hydronium ion) in water Produce OH-1 (hydroxide ion) in water Tastes sour Tastes Bitter React with most metals to form hydrogen gas Feels slippery

  8. + + + - - - Strong versus Weak Acids How many hydronium ions can you find? How many anions can you find? 3 How many intact acid molecules can you find? 1 Strong acid MOST of the acid molecules have donated their H+ to water

  9. + - Strong versus Weak Acids How many hydronium ions can you find? How many anions can you find? 1 How many intact acid molecules can you find? 3 Weak acid Only a FEW of the acid molecules have donated their H+ to water

  10. Concentrated versus Dilute solvent solute Lower concentration Only a FEW solute particle dissolved Higher concentration A LOT of solute particles dissolved

  11. Combinations of Concentration & Strength Concentrated Dilute Strong A lot of acid added & most dissociates(forms ions) Not much acid added, but most of what’s there dissociates (forms ions) A lot of acid added, but most stays together Not much acid added and most of what is there stays together Weak

  12. Acids and Bases as Electrolytes Acids and bases dissociate in ions in water Free-floating ions in water conduct electricity Acids & Bases are electrolytes Strong acids and bases are strong electrolytes. Weak acids and bases are weak electrolytes.

  13. pH • S.P.L. Sorenson developed the pH scale in 1909. pH stands for “power of hydrogen”. • [H+] (the concentration of H+) is not expressed as a small number in scientific notation but as a logarithm.

  14. pH • pH is defined as the negative logarithm of the [H+] or - log [H+]. • Think of it this way: the pH of a solution is the negative of the exponent on 10 that gives us the [H+]. • For example, if a solution has [H+] = 10-3 M, its pH is -(the exponent on 10), which is -(-3) or 3.

  15. pH This means as the concentration of H+ increases, pH decreases!! So . . . • Acids have a pH from 0 to below 7. The lower the number, the stronger the acid. • Neutral (not acidic or basic) is at 7. • Bases have a pH from above 7 to 14. The higher the number, the stronger the base.

  16. Practice Problems Calculate the pH of solutions having the following ion concentrations. • [H+] = 1 x 10-6 M • [H+] = 1 x 10-7 M • [H+] = 1 x 10-11 M Are these solution acidic, basic, or neutral?

  17. More Practice • What is the [H+] if the pH is • 2 • 13 • 8

  18. 1000x more acidic Level of acidity increases 100x more acidic 10x more acidic What does a “log” scale really mean? Every change of 1 in pH shows a change of 10x in concentration of hydronium. pH 4 3 2 1

  19. Auto-ionization of Water • Water molecules will split into their ions: • 2 H2O  H3O+1 + OH-1 OR • H2O  H+ + OH- • Studies have shown that at 25°C the following is true: • [H3O+1] [OH-1] = 10-14 • This means that if you know the hydronium concentration at 25°C (which can be found from pH), then you can also find the hydroxide concentration

  20. Practice Problems • The [H+] of an aqueous solution is 1 x 10-5 M. What is the [OH-]? Is the solution acidic, basic or neutral? • If the [OH-] of an aqueous solution is 1 x 10-3 M, what is the [H+]? Is the solution acidic, basic, or neutral? • Find the [OH-] if the pH is 13.

  21. pOH • pOH is the negative logarithm of the [OH-] or -log [OH-]. • Therefore, a solution with [OH-] = 1 x 10-2 M has a pOH of 2. • A simple relationship between pH & pOH makes it simple to calculate one when the other is known: pH + pOH = 14

  22. Practice Problems • If the [OH-] = 1 x 10-3 M, what is the pOH and pH of the solution? Is the solution acidic, basic, or neutral? • A solution has a pH of 8. What are the [H+] & [OH-]? Is the solution acidic, basic, or neutral? • A solution has a pOH of 4. What are the [H+] & [OH-]? Is the solution acidic, basic, or neutral?

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