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CK-12 Chemistry Concepts - Intermediate
Answer Key
Chapter 21: Acids & Bases
21.1 Properties of Acids
Practice
Questions
- Which of the four solutions are acids?
- Stop the video at 0:30 minutes and predict conductivities.
- What color did the acids turn when universal indicator was added?
Answers
- HCl and CH 3 COOH (acetic acid).
- The acids and NaCl are ionized and will conduct electricity.
- Red.
Review
Questions
- Are all acids electrolytes in water?
- What color does blue litmus turn in the presence of an acid?
- What does the reaction of an acid and a base produce?
Answers
- Yes.
- Red.
- Water and a salt.
21.2 Properties of Bases
Practice
Questions
- What does the reaction between an acid and a base produce?
- What is this reaction called?
- What does the reaction of a base with an ammonium salt form?
Answers
- A salt and water.
- Neutralization.
- A metal salt, ammonia, and water.
Review
Questions
- Are bases electrolytes?
- What color does a base turn phenolphthalein?
- Do bases react with metals the same way that acids do?
Answers
- Yes, some are strong electrolytes and some are weak.
- Red.
- No.
21.3 Arrhenius Acids
Practice
- What was Arrhenius’ first scientific idea?
- What did his Ph.D committee think about it?
- What did he win for this idea?
- Write the generic equation for dissociation of an Arrhenius acid.
Answers
- Electrolytes dissociated in solution.
- They did not like it and gave him a D for his research.
21.5 Bronsted-Lowry Acids and Bases
Practice
Questions
- Why is water considered a base in the reaction between water and HCl?
- Why is H 3 O+^ considered an acid?
- Why is the chloride anion considered a base?
Answers
- Because it can accept a proton from the HCl to form the hydronium ion.
- Because it can donate a proton.
- Because it can accept a proton to form HCl.
Review
Questions
- What is a Brønsted-Lowry acid?
- What is a Brønsted-Lowry base?
- How does ammonia function as a Brønsted-Lowry base?
Answers
- A molecule or ion that donates a proton.
- A molecule or ion that accepts a proton.
- The lone pair electrons on the N atom can accept a proton.
21.6 Bronsted-Lowry Acid-Base Reactions
Practice
Questions
Questions
- In the reaction between ammonia and water, how does the ammonia serve as a base on the left-hand side of the equation?
- How does the ammonium ion serve as an acid on the right-hand side of the equation?
- Write the conjugate acid-base pairs for the reaction between two water molecules illustrated above.
Answers
- It accepts a proton.
- It donates a proton.
- H 2 O/H 3 O+^ and OH -^ /H 2 O.
21.7 Lewis Acids and Bases
Practice
Questions
- What is the difference between a Lewis acid or base and a Brønsted Lowry acid or base?
- What is required to happen in Lewis acid-base reactions?
- Do all Lewis acid-base reactions involve protons?
Answers
- What is a buffer?
- How much more acidic is vinegar than grapefruit?
- How much more basic is soapy water than milk of magnesia?
Answers
- A solution that resists pH changes when acid or base or added.
- Ten times more acidic.
- One hundred times more basic.
Review
Questions
- What is one value of using pH instead of molar concentrations?
- Is coffee an acidic or a basic substance?
- If a material has a pH of 9.3, is it acidic or basic?
Answers
- A simple number can be used instead of an exponential expression.
- Acidic.
- Basic.
21.10 Calculating pH of Acids and Bases
Practice
Questions
- What is the pH of a 4.5 x 10-3^ M HI solution?
- What is the pH of a 3.67 x 10 -5^ M NaBr solution?
- If we have a weak base with a low ionization constant, can we assume that the [OH -^ ] in the solution is equal to the concentration of the base?
Answers
- pH = 2.4.
- pH = 9.56.
- No, the hydroxide ion concentration will not be equal to the molar concentration of the base because all the OH -^ will not be in the dissociated form.
21.11 The pOH Concept
Practice
Questions
- What is the formula for calculating pOH?
- What pOH value is indicative of an acidic solution?
- A pOH value of 3 would indicate what type of solution?
Answers
- pOH = -log[OH-]
- A value of less than 7.
- A basic solution.
21.12 Strong and Weak Acids and Acid Ionization Constant (Ka)
Practice
Questions
- The further the equilibrium is to the left, the weaker the base.
Review
Questions
- Define a strong base.
- Define a weak base.
- Which is the stronger base: pyridine or urea?
Answers
- A base which ionizes completely in an aqueous solution.
- A base that ionizes only a small amount in an aqueous solution.
- Pyridine.
21.14 Calculating Ka and Kb
Practice
Questions
- What does an Arrhenius base dissociate to?
- What does a Brønsted-Lowry base form in water?
- How is percent ionization determined?
Answers
- The cation of the base plus OH -^.
- The base reacts with water. The products are the protonated base plus a hydroxide ion.
- % dissociation (ionization) = [OH -^ at equilibrium] ÷ [base initial] x 100.
Review
Questions
- What approach is used for calculation of ionization constants?
- What initial assumptions are made?
- What equilibrium assumptions are made?
Answers
- The ICE approach.
- The concentrations of H +^ and base anion are zero.
- The concentrations of H +^ and base anion are equal.
21.15 Calculating pH of Weak Acid and Base Solutions
Practice
Questions
- What does x stand for in the equation?
- What simplifying assumption is made?
- What would x stand for if we were calculating pOH?
Answers
- The hydrogen ion concentration.
- The extent of ionization of a weak acid is small and x in the denominator can be ignored.
- Concentration of the hydroxide ion.
21.16 Neutralization Reaction and Net Ionic Equations for Neutralization Reactions
Practice
Review
Questions
- What is the standard solution?
- How do you know you have reached the end-point?
- What is the reaction that occurs during a titration?
Answers
- A known concentration of sodium hydroxide.
- The phenolphthalein turns pink.
- A neutralization reaction: acid + base → salt + water.
21.18 Titration Calculations
Practice
Questions
- What assumption is made about the amounts of materials at the neutral point?
- What is different about the calculation using sulfuric acid?
- Why is the mole ratio important?
Answers
- The moles of acid equal the moles of base.
- There are two protons to neutralize in a molecule of sulfuric acid.
- We need to know how many moles of acid are involved in the reaction.
21.19 Titration Curves
Practice
Questions
- Why is the equivalence point less than pH 7 for the titration of ammonia with HCl?
- Why is it difficult to do a titration of a weak acid and a weak base?
- Why do we get two inflection points for the titration of ethanedioic acid?
Answers
- Some of the product formed is ammonium chloride, which can dissociate into ammonia and a proton.
- There is no steep break in the curve to indicate where the equivalence point is.
- There are two different acid dissociation constants.
Review
Questions
- What does a titration curve tell us?
- At what pH are the moles of acid and base equal?
- Is the equivalence point for a weak acid-strong base titration the same as for a strong-acid-strong base titration?
Answers
- The change in pH as base is added.
- pH 7.
- No, the pH is greater than 7.
21.20 Indicators
Practice
Questions
- What was the acid used.
- What was the base used?
- What color was methyl orange in acid? in base?
Answers
- It reacts somewhat with water to form HF and OH -^.
- It releases a proton when it forms ammonia.
- There are no acid-base equilibrium reactions occurring.
21.22 Calculating pH of Salt Solutions
Practice
Questions
- In the first example, how do we know that we can ignore x when determining [F-]?
- In example two, how do we know the ammonium ion concentration?
- Could we write the equilibrium in example two as NH 4 +
⇌ H+^ + NH 3?
Answers
- The K (^) b is very small.
- Ammonium chloride dissociated completely in water.
- Yes, this equation is functionally equivalent to the more complete one.
21.23 Buffers
Practice
Questions
- What is a weak acid?
- What does a conjugate base rarely do?
- Write the generic equation for neutralization of a base by a weak acid.
- Write the generic equation for neutralization of an acid by a weak base.
Answers
- One that rarely loses a proton to water.
- It rarely steals a proton from water.
- OH -^ + HA → A-^ + H 2 O.
- HCl + A -^ → HA + Cl-^.
Review
Questions
- What is a buffer?
- How would the acetic acid/ acetate buffer system neutralize added base?
- If acid is added to the carbonic acid/ carbonate buffer system, how is it neutralized?
Answers
- A solution of a weak acid or base and its salt.
- Some acetic acid would release protons to neutralize the base.
- The carbonate anion would react with the excess acid.
