2B5 Acids and Bases

Question 1

What is an acid?

Answer 1

A substance that donates hydrogen ions (H⁺).

Acids donate protons (H⁺) in both aqueous and non-aqueous solutions (Brønsted-Lowry theory). They typically taste sour and turn blue litmus paper red.

Question 2

What is a base?

Answer 2

A substance that accepts hydrogen ions (H⁺) or donates electron pairs.

Bases feel slippery and turn red litmus paper blue. For example, NaOH dissociates in water to produce OH⁻ ions.

Question 3

What is an Arrhenius acid?

Answer 3

A substance that increases the concentration of H⁺ ions in water.

Arrhenius acids dissociate in water to release H⁺ ions. For example, HCl dissociates in water to form H⁺ and Cl⁻ ions.

Question 4

Fill in the blank:

An Arrhenius base increases the concentration of ______ ions in water.

Answer 4

OH⁻

An Arrhenius base is a substance that, when dissolved in water, increases the concentration of hydroxide ions (OH⁻).

For example, NaOH dissociates completely in water to produce Na⁺ and OH⁻ ions, making the solution basic.

Question 5

What does the Brønsted-Lowry theory state about acids?

Answer 5

Acids are proton (H⁺) donors.

According to the Brønsted-Lowry theory, an acid donates a proton (H⁺) to another substance.

For example, HCl donates an H⁺ to water, forming H₃O⁺ and Cl⁻.

Question 6

What is a Brønsted-Lowry base?

Answer 6

A substance that accepts a proton (H⁺).

A Brønsted-Lowry base accepts a proton (H⁺) from an acid.

For example, NH₃ accepts an H⁺ from H₂O, forming NH₄⁺ and OH⁻.

Question 7

True or False:

According to Brønsted-Lowry, acids must always be in aqueous solutions.

Answer 7

False

Brønsted-Lowry acids can exist in non-aqueous solutions as long as they donate protons (H⁺), so the theory is not limited to aqueous solutions.

Question 8

True or False:

Dissociation involves the breaking of molecules into atoms.

Answer 8

False

Dissociation refers to the separation of molecules into ions, not atoms. For example, HCl dissociates into H⁺ and Cl⁻, not H and Cl atoms.

Question 9

What does partial dissociation mean?

Answer 9

Only some of the molecules break into ions.

Weak acids like acetic acid (CH₃COOH) partially dissociate, with most molecules remaining intact in solution.

Question 10

What makes an acid strong?

Answer 10

It completely dissociates in water.

Strong acids fully dissociate, meaning almost all the acid molecules break into H⁺ ions and their conjugate anion, with very few intact molecules remaining in solution.

Question 11

Fill in the blank:

A weak acid ________ dissociates in water.

Answer 11

partially

For example, Acetic acid (CH₃COOH) dissociates only partially, releasing fewer H⁺ ions.

Question 12

True or False:

Strong acids have a higher pH than weak acids.

Answer 12

False

Strong acids have lower pH values because they release more H⁺ ions, while weak acids only partially dissociate.

Question 13

What is a strong base?

Answer 13

A base that completely dissociates into OH⁻ ions in water.

For example, NaOH is a strong base because it dissociates fully in water to give Na⁺ and OH⁻ ions.

Question 14

True or False:

Weak bases are ineffective at raising pH.

Answer 14

False

Weak bases can raise pH but only partially dissociate, which limits their effectiveness compared to strong bases.

Question 15

What is the range of the pH scale?

Answer 15

0 to 14

Values below 7 are acidic, values above 7 are basic, and 7 is neutral.

Question 16

What does a pH of 7 indicate about a solution?

Answer 16

The solution is neutral.

Pure water is neutral because [H⁺] = [OH⁻].

Question 17

Fill in the blank:

A pH of 3 indicates a/an _______ solution.

Answer 17

acidic

A pH lower than 7 indicates an acidic solution with a higher concentration of H⁺ ions.

Question 18

What is the pH of a solution with equal concentrations of H⁺ and OH⁻?

Answer 18

7

Pure water and neutral solutions have equal concentrations of H⁺ and OH⁻ ions.

Question 19

What is the formula used to calculate pH?

Answer 19

pH = -log[H₃O⁺]

This formula measures the acidity of a solution based on the concentration of hydrogen ions.

Question 20

True or False:

A solution with a [H⁺] concentration of 1 × 10⁻⁴ M will have a pH of 4.

Answer 20

True

pH = -log(1 × 10⁻⁴) = 4.

Question 21

What is the pH of a solution with a hydrogen ion concentration of 1 × 10⁻⁶ M?

Answer 21

6

pH = -log(1 × 10⁻⁶) = 6.

Question 22

True or False:

A pH of 12 indicates a basic solution.

Answer 22

True

A pH above 7 is basic, indicating a lower concentration of H⁺ and higher concentration of OH⁻ ions.

Question 23

What is the relationship between pH and pOH in an aqueous solution?

Answer 23

pH + pOH = 14

This relationship is true for all aqueous solutions at 25°C.

Question 24

What is the hydronium ion?

Answer 24

A water molecule combined to a proton (H⁺).

The hydronium ion, H₃O⁺, forms when a proton (H⁺) bonds with a water molecule, representing the concentration of hydrogen ions in a solution.

Question 25

# True or False: In pure water, the concentration of H₃O⁺ is **equal** to the concentration of OH⁻.

Answer 25

True ## Footnote In pure water, [H₃O⁺] = [OH⁻] = 1 × 10⁻⁷ M at 25°C.

Question 26

What **happens** to the concentration of OH⁻ as the pH of a solution increases?

Answer 26

It increases. ## Footnote As pH rises, the solution becomes more **basic**, meaning more OH⁻ ions are present.

Question 27

# Define: Buffer solution

Answer 27

A solution that **resists** changes in pH when small amounts of acid or base are added. ## Footnote Buffers typically consist of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Question 28

# True or False: Buffers can **completely** neutralize large amounts of acid or base.

Answer 28

False ## Footnote Buffers are effective at neutralizing **small amounts** of acid or base, but their capacity is limited.

Question 29

What are the **two** main components of most buffer solutions?

Answer 29

A weak acid and its conjugate base, or a weak base and its conjugate acid. ## Footnote Buffers maintain pH stability by neutralizing small amounts of added acid or base. For example, acetic acid (CH₃COOH) and acetate (CH₃COO⁻) form a common *buffer pair*.

Question 30

# Fill in the blank: Buffers **work** by \_\_\_\_\_\_\_\_\_ added acid and base, preventing drastic pH changes.

Answer 30

neutralizing ## Footnote The buffer neutralizes small amounts of H⁺ or OH⁻ ions, keeping the pH stable.

Question 31

What happens if a buffer’s capacity is **exceeded**?

Answer 31

The pH will **change** significantly. ## Footnote If too much acid or base is added, the buffer can no longer neutralize the excess ions, causing the pH to shift.

Question 32

# Fill in the blank: Blood uses the \_\_\_\_\_\_\_\_ buffer system to **maintain** a stable pH.

Answer 32

bicarbonate ## Footnote The bicarbonate buffer system in blood maintains pH by balancing the concentrations of carbonic acid (H₂CO₃) and bicarbonate ions (HCO₃⁻).

Question 33

# True or False: The pH of a buffer solution **changes** when acid is added.

Answer 33

False ## Footnote Buffers minimize pH changes even when acids or bases are added to the solution.

Question 34

What is an **acid-base** indicator?

Answer 34

A substance that changes **color** depending on the pH of the solution. ## Footnote Indicators are typically weak acids or bases that undergo a color change when they *gain or lose* protons.

Question 35

What **causes** an acid-base indicator to change color?

Answer 35

The change in the **concentration** of H⁺ ions, which alters the indicator's molecular structure. ## Footnote The molecular structure of the indicator changes when it gains or loses protons (H⁺), which causes a visible color change. This change is due to the difference in the form of the indicator at different pH levels.

Question 36

# True or False: Acid-base indicators work by **directly** neutralizing acids or bases.

Answer 36

False ## Footnote Indicators do not neutralize acids or bases; they simply change color to signal the pH of a solution.

Question 37

What color does **phenolphthalein** turn in a basic solution?

Answer 37

Pink ## Footnote Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions, usually above pH 8.3.

Question 38

What is the **pH range** at which phenolphthalein changes color?

Answer 38

8.3 to 10. ## Footnote Phenolphthalein is commonly used in **titrations** to determine when a solution has turned basic.

Question 39

# True or False: Litmus paper turns **red** in acidic solutions and **blue** in basic solutions.

Answer 39

True ## Footnote **Litmus paper** is a common indicator that changes color in response to the pH of the solution, red for acidic and blue for basic.

Question 40

# Fill in the blank: **pH paper** is a type of indicator that provides a wide \_\_\_\_\_\_\_ chart to determine pH.

Answer 40

color ## Footnote pH paper provides a color chart that corresponds to **specific** pH values, allowing a more precise determination of the solution’s pH compared to litmus paper, which only identifies whether a solution is acidic or basic.

Question 41

What **color** does litmus paper turn in a neutral solution (pH 7)?

Answer 41

Purple ## Footnote Litmus paper shows purple in neutral solutions because the pH is neither acidic nor basic.

Question 42

Why are indicators used in **titration**?

Answer 42

To visually **signal** the endpoint of the reaction. ## Footnote Indicators help *determine* when enough titrant has been added to react completely with the analyte.

Question 43

# Fill in the blank: pH paper can **measure** a \_\_\_\_\_\_\_ range of pH values compared to litmus paper.

Answer 43

broader ## Footnote pH paper provides more **accurate** and wide-ranging measurements of pH compared to litmus paper, which is more limited.