2B5 Acids and Bases Flashcards
Explain how acids and bases are identified by their definitions and strength, and how buffers and indicators are used in acid-base chemistry.
What is an acid?
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.
What is a base?
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.
What is an Arrhenius acid?
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.
Fill in the blank:
An Arrhenius base increases the concentration of ______ ions in water.
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.
What does the Brønsted-Lowry theory state about acids?
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⁻.
What is a Brønsted-Lowry base?
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⁻.
True or False:
According to Brønsted-Lowry, acids must always be in aqueous solutions.
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.
True or False:
Dissociation involves the breaking of molecules into atoms.
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.
What does partial dissociation mean?
Only some of the molecules break into ions.
Weak acids like acetic acid (CH₃COOH) partially dissociate, with most molecules remaining intact in solution.
What makes an acid strong?
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.
Fill in the blank:
A weak acid ________ dissociates in water.
partially
For example, Acetic acid (CH₃COOH) dissociates only partially, releasing fewer H⁺ ions.
True or False:
Strong acids have a higher pH than weak acids.
False
Strong acids have lower pH values because they release more H⁺ ions, while weak acids only partially dissociate.
What is a strong base?
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.
True or False:
Weak bases are ineffective at raising pH.
False
Weak bases can raise pH but only partially dissociate, which limits their effectiveness compared to strong bases.
What is the range of the pH scale?
0 to 14
Values below 7 are acidic, values above 7 are basic, and 7 is neutral.
What does a pH of 7 indicate about a solution?
The solution is neutral.
Pure water is neutral because [H⁺] = [OH⁻].
Fill in the blank:
A pH of 3 indicates a/an _______ solution.
acidic
A pH lower than 7 indicates an acidic solution with a higher concentration of H⁺ ions.
What is the pH of a solution with equal concentrations of H⁺ and OH⁻?
7
Pure water and neutral solutions have equal concentrations of H⁺ and OH⁻ ions.
What is the formula used to calculate pH?
pH = -log[H₃O⁺]
This formula measures the acidity of a solution based on the concentration of hydrogen ions.
True or False:
A solution with a [H⁺] concentration of 1 × 10⁻⁴ M will have a pH of 4.
True
pH = -log(1 × 10⁻⁴) = 4.
What is the pH of a solution with a hydrogen ion concentration of 1 × 10⁻⁶ M?
6
pH = -log(1 × 10⁻⁶) = 6.
True or False:
A pH of 12 indicates a basic solution.
True
A pH above 7 is basic, indicating a lower concentration of H⁺ and higher concentration of OH⁻ ions.
What is the relationship between pH and pOH in an aqueous solution?
pH + pOH = 14
This relationship is true for all aqueous solutions at 25°C.
What is the hydronium ion?
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.
True or False:
In pure water, the concentration of H₃O⁺ is equal to the concentration of OH⁻.
True
In pure water, [H₃O⁺] = [OH⁻] = 1 × 10⁻⁷ M at 25°C.
What happens to the concentration of OH⁻ as the pH of a solution increases?
It increases.
As pH rises, the solution becomes more basic, meaning more OH⁻ ions are present.
Define:
Buffer solution
A solution that resists changes in pH when small amounts of acid or base are added.
Buffers typically consist of a weak acid and its conjugate base, or a weak base and its conjugate acid.
True or False:
Buffers can completely neutralize large amounts of acid or base.
False
Buffers are effective at neutralizing small amounts of acid or base, but their capacity is limited.
What are the two main components of most buffer solutions?
A weak acid and its conjugate base, or a weak base and its conjugate acid.
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.
Fill in the blank:
Buffers work by _________ added acid and base, preventing drastic pH changes.
neutralizing
The buffer neutralizes small amounts of H⁺ or OH⁻ ions, keeping the pH stable.
What happens if a buffer’s capacity is exceeded?
The pH will change significantly.
If too much acid or base is added, the buffer can no longer neutralize the excess ions, causing the pH to shift.
Fill in the blank:
Blood uses the ________ buffer system to maintain a stable pH.
bicarbonate
The bicarbonate buffer system in blood maintains pH by balancing the concentrations of carbonic acid (H₂CO₃) and bicarbonate ions (HCO₃⁻).
True or False:
The pH of a buffer solution changes when acid is added.
False
Buffers minimize pH changes even when acids or bases are added to the solution.
What is an acid-base indicator?
A substance that changes color depending on the pH of the solution.
Indicators are typically weak acids or bases that undergo a color change when they gain or lose protons.
What causes an acid-base indicator to change color?
The change in the concentration of H⁺ ions, which alters the indicator’s molecular structure.
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.
True or False:
Acid-base indicators work by directly neutralizing acids or bases.
False
Indicators do not neutralize acids or bases; they simply change color to signal the pH of a solution.
What color does phenolphthalein turn in a basic solution?
Pink
Phenolphthalein is colorless in acidic solutions and turns pink in basic solutions, usually above pH 8.3.
What is the pH range at which phenolphthalein changes color?
8.3 to 10.
Phenolphthalein is commonly used in titrations to determine when a solution has turned basic.
True or False:
Litmus paper turns red in acidic solutions and blue in basic solutions.
True
Litmus paper is a common indicator that changes color in response to the pH of the solution, red for acidic and blue for basic.
Fill in the blank:
pH paper is a type of indicator that provides a wide _______ chart to determine pH.
color
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.
What color does litmus paper turn in a neutral solution (pH 7)?
Purple
Litmus paper shows purple in neutral solutions because the pH is neither acidic nor basic.
Why are indicators used in titration?
To visually signal the endpoint of the reaction.
Indicators help determine when enough titrant has been added to react completely with the analyte.
Fill in the blank:
pH paper can measure a _______ range of pH values compared to litmus paper.
broader
pH paper provides more accurate and wide-ranging measurements of pH compared to litmus paper, which is more limited.
