Acid and bases and Volumetric analysis

What does the Arrhenius model of acids state?

Acids will ionise into H+ ions when dissolved in water.

What does the Arrhenius model of bases state?

Bases release hydroxide ions (OH-) into solution.

Give examples of Arrhenius acids.

Hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4).

Give examples of Arrhenius bases.

Sodium hydroxide (NaOH), calcium hydroxide (Ca(OH)2), and iron (III) hydroxide (Fe(OH)3).

What is a monoprotic acid?

An acid that releases only one hydrogen ion into solution.

Give examples of monoprotic acids.

Hydrochloric acid (HCl), nitric acid (HNO3), and acetic acid (CH3COOH).

What is a polyprotic acid?

An acid that releases more than one hydrogen ion into solution.

Give examples of polyprotic acids.

Sulfuric acid (H2SO4), phosphoric acid (H3PO4), and ethanedioic acid ((COOH)2).

What does the Bronsted-Lowry theory of acids state?

Acids are substances that donate protons to other substances.

What does the Bronsted-Lowry theory of bases state?

Bases are substances that accept protons from other substances.

Give examples of Bronsted-Lowry acids.

Hydrochloric acid (HCl), nitric acid (HNO3), water (H2O).

Give examples of Bronsted-Lowry bases.

Sodium hydroxide (NaOH), ammonia (NH3), sodium carbonate (Na2CO3), water (H2O).

According to Arrhenius, why is HCl an acid in the reaction HCl(aq) + NaOH(aq) à NaCl(aq) + H2O(l)?

Because it releases H+ in solution.

According to Arrhenius, why is NaOH a base in the reaction HCl(aq) + NaOH(aq) à NaCl(aq) + H2O(l)?

Because it releases OH- in solution.

According to Bronsted-Lowry, why is HCl an acid in the reaction HCl(aq) + NaOH(aq) à NaCl(aq) + H2O(l)?

Because it donates a proton to NaOH to produce water.

According to Bronsted-Lowry, why is NaOH a base in the reaction HCl(aq) + NaOH(aq) à NaCl(aq) + H2O(l)?

Because it accepts a proton from the HCl to produce water.

What does concentration refer to?

The quantity of solute dissolved in a set volume of solvent.A concentrated solution contains a lot of solute, and a dilute solution contains a little.

What does strength refer to in the context of solutions?

How well a solute dissociates in solution.

What is a strong electrolyte?

A solution where a majority of the solute has dissociated.

What is a weak electrolyte?

A solution where only a small amount of the solute has dissociated.

What is a non-electrolyte?

A solution with no solute dissolved in it.

Give examples of strong acids.

Hydrochloric acid, nitric acid, sulfuric acid.

Give examples of weak acids.

Hydrofluoric acid, acetic acid, oxalic acid.

Give examples of strong bases.

Sodium hydroxide, calcium hydroxide, potassium hydroxide.

Give examples of weak bases.

Ammonia, aluminum hydroxide, methylamine.

How can the acidity constant (Ka) be used to describe a strong acid?

A high Ka value indicates that the equilibrium position resides to the right, demonstrating the high dissociation of strong acids into their ions.

High Ka = strong acid = more dissociation = more products = equilibrium to the right

How can the acidity constant (Ka) be used to describe a weak acid?

A low Ka value indicates that the equilibrium position sits to the left, showing that the acid prefers to remain in its molecular form.

Describe the pH of a strong acid.

Strong acids liberate maximum hydrogen ions into solution, resulting in very low pH values.

Describe the pH of a weak acid.

Weak acids liberate few hydrogen ions in solution, resulting in a pH value below 7 but closer to 7.

What is a conjugate acid?

The product of a base in an acid-base reaction.

What is a conjugate base?

The product of an acid in an acid-base reaction.

What does a conjugate base originate from?

An acid.

What is a pH indicator?

A compound that undergoes a color change in solution over a specific range of pH values, revealing whether a solution is acidic, neutral, or alkaline.

What determines the color of a pH indicator?

The increase or decrease in [H3O+] (hydronium ion concentration).

What happens to the indicator in an acid?

The [H3O+] increases, the reverse reaction is favored, resulting in a reduction in [H3O+], which increases the production of HIn, and shows the acid color.

What happens to an indicator in a base?

The [H3O+] decreases, the forward reaction is favored, resulting in an increase in [H3O+], which increases the production of In-, and shows the base color.

What is the pH range and color transition of phenolphthalein?

pH Range: Approximately 8 to 10. Color Transition: Colorless to pink.

What is the pH range and color transition of methyl red?

pH Range: 4.4 and 6.2. Color Transition: Red below pH 4.4, yellow above pH 6.2, orange within the range.

What is the pH range and color transition of bromothymol blue?

pH Range: 6 and 7.5. Color Transition: Yellow to blue.

What solutions will produce an acidic salt?

The reaction of a strong acid and a weak base.

What solutions will produce a basic salt?

The reaction of a weak acid and a strong base.

What is a buffer solution?

A buffer solution is one that can resist small changes in pH.

They will return to a nominal pH value when small volumes of acid or base are added to them.

What could be used to prepare a low pH buffer?

A weak acid and the salt of the weak acid (conjugate base)

Weak acid neutralises base

-If a base was added, the OH- will neutralise hydrogen ions that the weak acid ionizes to (H₃O+), causing the concentration to decrease. The equilibrium will shift to the right to increase H₃O+ concentration and prevent pH from rising

Conjugate base neutralizes acid

-If an acid is added, the H₃O+ concentration will increase, and so the equilibrium will shift to the left to decrease H₃O+ concentration

What could be used to prepare a high pH buffer?

A weak base and its salt (e.g., ammonia and ammonium chloride).

Outline the effect of adding acid to an acidic buffer.

The [H3O+] increases (and the pH decreases), the system counteracts the addition of H3O+ by increasing the rate of the reverse reaction relative to the forward reaction, which in turn, decreases [H3O+] and brings the pH back to where it began.

Outline the effect of adding base to an acid buffer.

The [H3O+] decreases (and the pH increases) as the OH- ions cause the production of water, the system counteracts the reduction of H3O+ by increasing the rate of the forward reaction relative to the reverse reaction, which in turn, increases [H3O+] and brings the pH back to where it began.

Outline the effect of adding acid to a basic buffer.

The [OH-] decreases (and the pH decreases) as H3O+ causes the formation of water, the system counteracts the reduction of OH- by increasing the rate of the forward reaction relative to the reverse reaction, which in turn, increases [OH-] and brings the pH back to where it began.

Outline the effect of adding base to a basic buffer.

The [OH-] increases (and the pH increases), the system counteracts the addition of OH- by increasing the rate of the reverse reaction relative to the forward reaction, which in turn, decreases [OH-] and brings the pH back to where it began.

What is buffer capacity?

The term ‘buffer capacity’ refers to how much acid or alkali can be added to a buffer solution before there is a drastic change in the pH value.

The more concentrated the reagents used for the buffer are, the higher the buffer capacity will be.

Buffer capacity measures a solution's ability to resist pH changes upon addition of acids or bases. It depends on the concentrations of the buffered components; higher concentrations result in greater buffer capacity.

What is the bicarbonate buffer solution

CO₂ + H₂O —> ← (catalyst) H₂CO₃ —> ← HCO₃^- _(aq) + H⁺_(aq)

When an acid is introduced, it causes an excess H+ present in the blood, and so the bicarbonate ion (HCO₃⁻) neutralizes it by forming carbonic acid (H₂CO₃) (shifts to the left)

When a base is introduced, it makes the blood too basic, carbonic acid (H₂CO₃) neutralizes it by releasing a hydrogen ion (H⁺) and forming bicarbonate ion (HCO₃⁻) (shifts to the right)

Why is the bicarbonate buffer system important?

It is a crucial acid-base homeostatic mechanism that helps maintain the pH balance in the blood and other tissues.

What is the role of the respiratory system in the bicarbonate buffer system?

Helps regulate the levels of CO₂ by adjusting the rate and depth of breathing.

What is the role of the kidneys in the bicarbonate buffer system?

Help regulate the concentration of bicarbonate ions by reabsorbing HCO₃⁻ and secreting H⁺ into the urine.

Why is the phosphate buffer system important?

It is another important mechanism for maintaining pH balance in the body, particularly in the intracellular fluid and the kidneys. In the kidneys, it helps to buffer the pH of urine.

What is the phosphate buffer system?

H₂PO_4(aq) —> ← H⁺_(aq) + HPO₄^-_(aq)

When an acid is introduced, the monohydrogen phosphate (HPO₄²⁻) neutralizes it by forming dihydrogen phosphate (H₂PO₄⁻).

When a base is introduced, dihydrogen phosphate (H₂PO₄⁻) neutralizes it by releasing a hydrogen ion (H⁺) and forming monohydrogen phosphate (HPO₄²⁻).

Give some applications of buffer systems.

Biological systems, chemical reactions, analytical chemistry, and industrial processes.

What is volumetric analysis?

A set of analytical techniques (called titrations) that use volumes of reagents (substance or compound that can facilitate a reaction) to determine concentration.

What is a primary standard?

A highly pure and stable chemical reagent (substance or compound that can facilitate a reaction) which is used to prepare standard solutions.

A primary standard solution is an accurately known concentration which can be used in titration to find teh concentration of another reagent

What are the key properties of an ideal primary standard solution?

•High Purity: The reagent should be exceptionally pure.

•High Stability/Low Reactivity: It must remain stable both in air and solution.

•High Equivalent Weight: This minimizes mass measurement errors.

•Non-Hygroscopic: To prevent mass changes due to water absorption.

•Non-Toxic and Inexpensive: Ideally, it should be readily available.

What is the equivalence point?

•The equivalence point of a titration is the theroetical point where neither acid nor base remain, and the reaction is complete.

•At the equivalence point there is no reactant in excess (exist in the flask) because a sufficent amount of acid and base has been added to neutralise each other

What is the end point?

•The end point of titration occurs when one drop of titrant changes the colour of the pH indicator.

•The end point and the equivalence point should be very similar but not necessarily equal.

Why is sodium hydroxide not suitable as a primary standard?

It absorbs water and carbon dioxide from the air, affecting mass measurements.

What is a secondary standard?

A secondary standard (in burette) is still found by titration which can then be used finding concentration of other titrates

Once standardised against a primary standard, a solution can be used as a secondary standard.

•Standardisation is when you titrate a prepared  solution against a primary standard solution to obtain an accurate value.

•Although slightly less accurate due to process (technique) errors, a secondary standard still provides reliable concentration values.

What could compromise a secondary standard's accuracy?

Process (technique) errors.

Outline the process of preparing a standard solution.

1. Selectan appropriate primary standard

2.Weigh the primary standard

3.Dissolve in solvent

4.Pour dissolved solution to volumetric flask

5.Dilute to volume

6.Standardise the solution to get an accurate value for the secondary standard (perform titration with secondary standard, whichs concentration is unkown)

7.Label and store the solution

What mass of sodium carbonate crystals is required to prepare 500 mL of a 0.05 molL^-1 solution?

n(Na₂CO₃) = c x V = 0.05 x 0.5 = 0.025 mol

M(Na₂CO₃) = (22.99 x 2) + 12.01 + (16.00 x 3) = 105.99 gmol^-1

m(Na₂CO₃) = n x M = 0.025 x 105.99 = 2.65 g

What is titration?

An analystical procedure involving the reaction between an acid solution and base solution

The purpose of a titration is to determine the concentration of the acid or base in the solution of unkown concentration

What is the steps in titration?

1.An aliquot (known volume but unkown conc) of unknown solution is accurately pipetted into a conical flask or beaker.

-The pipette should be cleaned, with the final rinse being with the reagent (titre) to ensure no dilution occurs

2.A few drops of a selected indicator is added to the unknown solution/aliquot (which is now transferred to a conical flask)

3.A solution of known concentration (titre) is poured into the burette. The volume is recorded as the initial volume.

4.The known solution (titre) is then carefully added/released to the unknown solution (with mixing) until one drop changes the colour of the indicator. The volume is then recorded as the final volume.

5.The titre is calculated by subtracting the initial volume from the final volume.

6.The procedure is repeated until the results are within 0.20 mL of each other.

7. Stiochmeistry used to calculate concentration of unknown

How can cleaning techniques create errors in titrations?

Inadequate cleaning can lead to dilution or contamination, affecting reagent concentrations.

How should analytical glassware be rinsed?

A pipette, burette, or storage bottle must first be washed with distilled water and then rinsed with a small amount of the solution that goes into it.

What errors can be caused by draining a pipette?

Applying pressure can cause a systematic error where the volume released is slightly greater than what it is supposed to be.

What can affect the determination of an endpoint?

A consistent approach is required to ensure the endpoint is determined the same way between titrations.

How does the strength of an acid or base affect the equivalence point?

The strength of an acid or base affects equivalence point in the following ways: strong acid + strong base = equivalence point at pH 7. strong acid + weak base = equivalence point below pH 7. weak acid + strong base = equivalence point above pH 7. weak acid + weak base = equivalence point close to pH 7.

What is accuracy?

How close a value is to a true or measured value.

What is precision?

How close values are to each other.

What is systematic error?

A type of error where the values are either consistently higher or lower than the actual value (bias).

Example: Choosing wrong indicaor

What is random error?

A type of error which causes inconsistent values (scatter)

Example: Inconsistent rinising techniques or careless attention to colour change

What happens when a strong acid reacts with a strong base?

•If a strong acid is reacted with a strong base, the salt solution will be neutral, and the equivalence point will be close to, if not, pH 7.

What happens when a strong acid reacts with a weak base?

•If a strong acid is reacted with a weak base, the salt solution will be acidic, and the equivalence point will be below pH 7.

What happens when a weak acid reacts with a strong base?

•If a weak acid is reacted with a strong base, the salt solution will be basic, and the equivalence point will be above pH 7.

What happens when a weak acid reacts with a weak base?

•If a weak acid is reacted with a weak base, the salt solution will be neutral, and the equivalence point will be close to, if not, pH 7.