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Chemistry - Acids and Bases

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    Study Guide Chemistry - Acids and Bases 1. Quiz: The pH Scale 1. Question The neutral value on the pH scale is Answer Choices • less than 7 • greater than 7 • equal to 7 Correct Answer equal to 7 Why This Is Correct A neutral solution has equal concentrations of hydrogen ions (H ⁺ ) and hydroxide ions (OH ⁻ ). On the pH scale, this balance corresponds to a pH of 7. Pure water at 25°C is neutral and has a pH of 7. 2. Question Water dissociates into H ⁺ and OH ⁻ ions Answer Choices • to a large degree • to a small degree • not at all

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    Study Guide Correct Answer to a small degree Why This Is Correct Only a very small fraction of water molecules dissociate into ions at any given time. This limited dissociation is why pure water is a poor conductor of electricity. 3. Question The value of the ionization constant for water, Kw is Answer Choices • 1.0 × 10 ⁻ ⁷ • 1.0 × 10¹⁴ • 1.0 × 10 ⁻ ¹⁴ Correct Answer 1.0 × 10 ⁻ ¹⁴ Why This Is Correct Kw is defined as the product of the concentrations of H ⁺ and OH ⁻ ions in water. At 25°C, this constant value is always 1.0 × 10 ⁻ ¹⁴. 4. Question The pH scale is Answer Choices • a logarithmic scale • a linear scale • an exponential scale

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    Study Guide Correct Answer a logarithmic scale Why This Is Correct Each change of one pH unit represents a tenfold change in hydrogen ion concentration. Because it is based on powers of 10, the pH scale is logarithmic. 5. Question What is the [H ⁺ ] of a solution if the pH is 3.5? Answer Choices • 3.16 × 10⁴ • 3.16 × 10 ⁻ ⁴ • 3.5 × 10 ⁻ ⁴ Correct Answer 3.16 × 10 ⁻ ⁴ Why This Is Correct The hydrogen ion concentration is calculated using the formula: pH = −log[H ⁺ ] So, [H ⁺ ] = 10 ⁻ ³ ⋅ ⁵ = 3.16 × 10 ⁻ ⁴ M. 6. Question A solution has a [H ⁺ ] = 7.0 × 10 ⁻ ¹¹. What is its pH? Answer Choices • 10.15 • − 10.15 • 11.00

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    Study Guide Correct Answer 10.15 Why This Is Correct Using pH = −log[H ⁺ ]: pH = −log(7.0 × 10 ⁻ ¹¹) ≈ 10.15. A low hydrogen ion concentration corresponds to a high pH. 7. Question A solution has a [OH ⁻ ] = 1.3 × 10 ⁻ ³. This solution would be classified as Answer Choices • acidic • basic • neutral Correct Answer basic Why This Is Correct A high hydroxide ion concentration indicates a basic solution. Since [OH ⁻ ] is much greater than 1.0 × 10 ⁻ ⁷ , the pH will be above 7. 8. Question Bases have a pH Answer Choices • less than 7 • equal to 7 • greater than 7

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    Study Guide Correct Answer greater than 7 Why This Is Correct Basic solutions contain more hydroxide ions than hydrogen ions, resulting in a pH above 7 on the pH scale. 9. Question An acidic solution has Answer Choices • a greater concentration of hydroxide ions than hydrogen ions • a greater concentration of hydrogen ions than hydroxide ions • equal concentrations of hydrogen ions and hydroxide ions Correct Answer a greater concentration of hydrogen ions than hydroxide ions Why This Is Correct Acids release hydrogen ions in solution. A higher [H ⁺ ] compared to [OH ⁻ ] is the defining characteristic of an acidic solution. 10. Question A solution has a hydrogen ion concentration of 3.7 × 10 ⁻ ². What is its hydroxide ion concentration? Answer Choices • 2.7 × 10 ⁻ ¹³ • 3.7 × 10 ⁻ ¹⁶ • 3.7 × 10¹²

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    Study Guide Correct Answer 2.7 × 10 ⁻ ¹³ Why This Is Correct Using the equation Kw = [H ⁺ ][OH ⁻ ]: [OH ⁻ ] = (1.0 × 10 ⁻ ¹⁴) ÷ (3.7 × 10 ⁻ ²) ≈ 2.7 × 10 ⁻ ¹³ M. 2. Strong and Weak Acids 2.1 What Happens When Substances Dissolve in Water? When a substance is placed in water, it can behave in different ways: • Some substances break completely into ions . • Some do not break at all . • Others break only partially . Understanding this behavior helps us classify substances as strong electrolytes or weak electrolytes . 2. 2 Strong Electrolytes: Complete Dissociation Substances that completely separate into ions in water are called strong electrolytes . Because they produce many ions, their solutions can conduct electricity very well . • Most ionic compounds behave this way. • A common example is sodium chloride (NaCl) . 2. 3 Compounds with Both Ionic and Covalent Bonds Some compounds contain both ionic and covalent bonds . A good example is sodium carbonate (Na ₂ CO ₃ ) .

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    Study Guide Why is Na ₂ CO ₃ a strong electrolyte? • In water, each formula unit of Na ₂ CO ₃ completely dissociates into three ions: o Two sodium ions (Na ⁺ ) o One carbonate ion (CO ₃ ² ⁻ ) Dissociation equation: • The sodium ions are free in solution. • The carbonate ion stays intact internally because its atoms are held together by covalent bonds . Figure 1. Ionic and covalent bonding in Na 2 CO 3 . 2.4 Weak Electrolytes: Partial Dissociation Some substances contain polar covalent bonds and only partially dissociate in water. These are called weak electrolytes . An example is sulfurous acid (H ₂ SO ₃ ) . Equilibrium reaction: • Most of the solution remains as undissociated H ₂ SO ₃ molecules . • Only a small number of ions are formed.

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    Study Guide 2. 5 Strong vs Weak Electrolytes: Key Difference • Strong electrolytes → completely dissociate into ions. • Weak electrolytes → only partially dissociate. This difference becomes very important when studying acids and bases . 2. 6 Strong Acids and Weak Acids Acids are classified as strong or weak based on how much they ionize in water. The general dissociation of an acid is written as: • HA = acid molecule • H ⁺ = hydrogen ion • A ⁻ = conjugate base (anion) 2.7 Acid Ionization Constant (Ka) The extent to which an acid ionizes is measured by the acid ionization constant , called Ka . Ka expression: • Large Ka → strong acid (more ionization) • Small Ka → weak acid (less ionization) 2.8 Common Acids and Their Ka Values Different acids have very different Ka values.

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    Study Guide What the table shows: • Strong acids like HCl, HNO ₃ , and H ₂ SO ₄ have very large Ka values . • Weak acids like acetic acid and carbonic acid have small Ka values . • Every acid produces H ₃ O ⁺ (hydronium ions) in solution. 2. 9 Calculating Ion Concentrations: Example with Carbonic Acid Let’s look at a 1 M solution of carbonic acid (H ₂ CO ₃ ) . Equilibrium reaction: Let x = amount of H ₂ CO ₃ that dissociates. Ka expression: x² / (1 − x) = 4.3 × 10 ⁻ ⁷ Because carbonic acid is weak, x is very small , so:

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    Study Guide x² ≈ 4.3 × 10 ⁻ ⁷ x = 6.56 × 10 ⁻ ⁴ M • This is the hydronium ion concentration • pH = 3.18 2. 10 Organic Acids: Carboxylic Acids Carboxylic acids contain a – COOH functional group . • Only one hydrogen in this group can dissociate. • As a result, these acids are weak acids . Examples: • Acetic acid • Propionic acid Summary • Strong acids dissociate almost completely. o Example: 0.5 M HCl → [H ₃ O ⁺ ] ≈ 0.5 M • Weak acids dissociate only slightly. o Their ion concentrations must be calculated using Ka . • The smaller the Ka , the weaker the acid .

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