Acids and Bases in Chemistry

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Questions and Answers

What characterizes Lewis acids?

  • They are always weak acids.
  • They don’t accept lone pairs of electrons.
  • They can only be organic compounds.
  • They are electron-poor molecules or cations. (correct)

Why does an aqueous solution of FeCl3 turn out to be acidic?

  • Electron density flows towards the metal cation, weakening O-H bonds. (correct)
  • The hydrogen atoms in water become negatively charged.
  • It dissociates into FeCl2 and H2O.
  • FeCl3 is a strong base in water.

What conclusion can be drawn about weak acids and their conjugate bases?

  • Weak acids have weaker conjugate bases compared to strong acids.
  • The weaker the acid, the stronger its conjugate base. (correct)
  • The stronger the acid, the stronger its conjugate base.
  • Conjugate bases of weak acids are always strong acids.

How does the equilibrium lie for a strong acid like HCl in water?

<p>The equilibrium lies on the far right. (B)</p> Signup and view all the answers

What is the characteristic of a weak acid in terms of dissociation?

<p>It dissociates only partially in water. (B)</p> Signup and view all the answers

What is the role of water in the dissociation reaction 2 H2O(l) H3O+(aq) + OH-(aq)?

<p>Water can act as both an acid and a base. (D)</p> Signup and view all the answers

Which of the following concentrations is correct for hydronium ions in pure distilled water at 25 oC?

<p>1.0 × 10^-7 M (C)</p> Signup and view all the answers

What is the molarity of undissociated water in a liter of pure water?

<p>55.4 M (C)</p> Signup and view all the answers

What is the ion product constant for water (Kw) defined as?

<p>Kw = [H3O+][OH-] (C)</p> Signup and view all the answers

In the reaction HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq), what role does water play?

<p>Water acts as a Bronsted-Lowry acid. (D)</p> Signup and view all the answers

According to Brönsted-Lowry theory, which of the following correctly describes a Brönsted-Lowry acid?

<p>A substance that donates a proton (D)</p> Signup and view all the answers

What is the role of NH3 in the reaction where it interacts with H2O as described by Brönsted-Lowry theory?

<p>Bronsted-Lowry base (C)</p> Signup and view all the answers

Which pair represents a conjugate acid-base pair according to Brönsted-Lowry theory?

<p>H2O and OH- (A), NH3 and NH4+ (B)</p> Signup and view all the answers

What defines a Lewis acid?

<p>A lone pair acceptor (D)</p> Signup and view all the answers

In the context of Lewis acids and bases, what is the role of BF3 when reacting with NH3?

<p>Lewis acid (A)</p> Signup and view all the answers

Which of the following statements about the Brönsted-Lowry theory is correct?

<p>It cannot explain reactions involving substances like FeCl3. (D)</p> Signup and view all the answers

What is a characteristic of a strong acid compared to a weak acid?

<p>It completely dissociates in solution. (C)</p> Signup and view all the answers

Which of the following is true regarding the dissociation of distilled water?

<p>Distilled water can dissociate into H+ and OH- ions. (B)</p> Signup and view all the answers

What is the role of water in acid-base reactions according to the definitions provided?

<p>It can act as either a base or an acid. (D)</p> Signup and view all the answers

Which of the following statements about acids and bases is true?

<p>Lewis acids accept lone pairs of electrons. (A)</p> Signup and view all the answers

How does the dissociation of strong acids differ from weak acids?

<p>Strong acids dissociate completely, while weak acids dissociate only partially. (D)</p> Signup and view all the answers

What is the interpretation of Kw at 25°C?

<p>It represents the product of the concentrations of H+ and OH- ions in water. (C)</p> Signup and view all the answers

Which method can be used to measure pH?

<p>The use of a universal indicator or electronic pH meter. (C)</p> Signup and view all the answers

What is the molarity of a solution containing 1000 g of a substance with a molar mass of 18.02 g/mol in 1.00 L?

<p>55.4 M (C)</p> Signup and view all the answers

At a temperature of 25 °C, what is the value of the ion-product constant, Kw?

<p>1.0 × 10−14 (B)</p> Signup and view all the answers

How can pH be measured with high accuracy?

<p>By using a pH electronic meter (C)</p> Signup and view all the answers

What is the pH range that indicates a basic solution?

<p>pH &gt; 7 (A)</p> Signup and view all the answers

Which statement is true regarding the use of methyl orange as an indicator?

<p>It turns yellow at pH levels above approximately 4.2. (C)</p> Signup and view all the answers

What pH level is considered neutral?

<p>pH = 7 (D)</p> Signup and view all the answers

What will a solution with a yellow color when tested with bromocresol purple indicate?

<p>pH is between 4.2 and 5.6 (B)</p> Signup and view all the answers

What is the approximate pH of a solution that is yellow with both methyl orange and bromocresol purple?

<p>Between 4.2 and 5.6 (C)</p> Signup and view all the answers

What is the pH of a 0.050 M hydrochloric acid solution?

<p>1.30 (A)</p> Signup and view all the answers

Which formula is used to find the concentration of hydronium ions from hydroxide concentrations?

<p>[H3O+] = Kw / [OH-] (B)</p> Signup and view all the answers

What is the molarity of sodium hydroxide in a 100.0 mL solution made from dissolving 2.50 g of NaOH?

<p>0.625 M (A)</p> Signup and view all the answers

Why do alkaline-earth metal hydroxides have lower pH values compared to alkali metal hydroxides, despite being strong bases?

<p>They are not fully soluble. (B)</p> Signup and view all the answers

If 5.00 g of Ca(OH)2 is dissolved in 1250.0 mL of solution, what is the pH of the resulting solution?

<p>13.03 (C)</p> Signup and view all the answers

How does the degree of dissociation of strong diprotic acids compare to strong monoprotic acids?

<p>Strong diprotic acids can have both strong and weak dissociations. (D)</p> Signup and view all the answers

What is the final pH of a solution where [H3O+] = 1.60 × 10^-14?

<p>13.80 (B)</p> Signup and view all the answers

What will happen to the pH if a strong acid is mixed with a strong base in equal concentrations?

<p>The solution will have a neutral pH. (C)</p> Signup and view all the answers

Flashcards

Brønsted-Lowry Acid

A substance that can donate a proton (H+ ion) in a chemical reaction.

Brønsted-Lowry Base

A substance that can accept a proton (H+ ion) in a chemical reaction.

Conjugate Base

A substance formed when an acid loses a proton. It can accept a proton to reform the acid.

Conjugate Acid

A substance formed when a base gains a proton. It can donate a proton to reform the base.

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Lewis Acid

A substance that can accept a lone pair of electrons in a chemical reaction.

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Lewis Base

A substance that can donate a lone pair of electrons in a chemical reaction.

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Proton Transfer Reaction

A reaction where a proton is transferred between an acid and a base.

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Conjugate Acid-Base Pair

A pair of molecules that differ by only a proton. One is an acid, the other is its conjugate base.

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Nucleophiles

They are electron-rich molecules or anions that can donate a lone pair of electrons.

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Electrophiles

They are electron-poor molecules or cations that can accept a lone pair of electrons.

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Weak Acid

A weak acid dissociates only partially in water, leading to a small equilibrium constant. For example, acetic acid has an equilibrium constant of 1.8 x 10-5.

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What is Kw?

The equilibrium constant for the auto-ionization of water, where two water molecules react to form hydronium and hydroxide ions. It is a measure of the extent of water's dissociation at a given temperature.

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Amphoteric substance

A substance that can act as both an acid and a base.

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Water as a base

The reaction where a water molecule acts as a base, accepting a proton from an acid, forming hydronium and a conjugate base.

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Water as an acid

The reaction where a water molecule acts as an acid, donating a proton to a base, forming hydroxide and a conjugate acid.

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What is the concentration of H3O+ in distilled water at 25°C?

The concentration of hydronium ions in pure water at 25°C is 1.0 x 10^-7 M, making it very weakly acidic.

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Arrhenius Acid

A substance that increases the concentration of hydrogen ions (H+) when dissolved in water.

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Arrhenius Base

A substance that increases the concentration of hydroxide ions (OH-) when dissolved in water.

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Ion-Product Constant (Kw)

The product of the hydronium ion ([H3O+]) concentration and the hydroxide ion ([OH-]) concentration, which is constant at 25°C for pure water.

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pH Scale

A measure of the acidity or alkalinity of a solution, defined as the negative logarithm (base 10) of the hydronium ion concentration ([H3O+]).

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Acidic, Basic, and Neutral Solutions

A solution with a pH less than 7 is acidic, while a solution with a pH greater than 7 is basic (alkaline). A neutral solution has a pH of 7.

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pH Meter

An electronic device used to measure pH by detecting the difference in electrical potential between a reference electrode and a pH-sensitive electrode.

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Acid-Base Indicators

Organic dyes that change color depending on the pH of the solution. They can be used to estimate the approximate pH of a solution.

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Universal Indicator

A combined indicator that changes color over a wide range of pH values, allowing for a rough estimation of the pH.

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Buffer Solution

A solution that resists changes in pH when small amounts of acid or base are added.

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Calibration of pH Meter

Calibration is the process of adjusting a measuring device, such as a pH meter, to make it give accurate readings. This is usually done by comparing the device's readings to known standard solutions.

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Strong Acid

A strong acid completely dissociates in water, meaning all of its molecules donate a proton (H+) to water, resulting in a high concentration of hydronium ions (H3O+).

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pH of Strong Acids

The pH of a strong acid depends solely on the initial concentration of the acid because the acid fully dissociates. For example, a 0.1 M solution of a strong monoprotic acid will have a pH of 1.

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Strong Base

A strong base completely dissociates in solution, releasing all of its hydroxide ions (OH-) into the solution. This results in a high concentration of OH- and a correspondingly low concentration of H3O+.

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pH of Strong Bases

The pH of a strong base solution is determined by the concentration of hydroxide ions (OH-) produced by the complete dissociation of the base. A higher concentration of OH- leads to a higher pH.

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Solubility of Strong Bases

Some alkaline earth metal hydroxides, like Ca(OH)2, are strong bases but have limited solubility in water, meaning they don't fully dissociate, leading to a lower pH than expected from their concentration.

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Metal Oxides as Bases

Metal oxides, when reacted with water, can form hydroxides, contributing to the alkalinity of the solution.

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Strong Bases and Solubility

While strong bases are defined as completely dissociating in solution, some, like alkaline earth metal hydroxides, have limited solubility in water, meaning they don't fully dissociate.

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Calculating pH of Strong Acids and Bases

The pH of a solution can be calculated using the concentration of H3O+ ions, which can be obtained from the initial concentration of the strong acid or by calculating the concentration of OH- ions and then using the Kw equation.

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Study Notes

Acids and Bases 1

  • Two definitions of acids and bases are examined: Brønsted-Lowry and Lewis.
  • Brønsted-Lowry theory defines an acid as a proton (H⁺) donor and a base as a proton acceptor.
  • An example of a Brønsted-Lowry acid-base reaction is shown with water and ammonia.
  • In this reaction, water acts as an acid and ammonia acts as a base.
  • Another example of a Brønsted-Lowry reaction is given with sulphuric acid and water.
  • The Brønsted-Lowry theory has a disadvantage: it doesn't explain highly acidic solutions resulting from substances that aren't proton donors, such as FeCl₃.
  • Lewis theory defines a Lewis acid as a lone pair acceptor and a Lewis base as a lone pair donor.
  • An example illustrates a Lewis acid-base reaction where ammonia donates a lone pair to boron trifluoride.
  • Another example shows the reaction between hydroxide and hydrogen chloride, where hydroxide acts as a Lewis base and hydrogen chloride acts as a Lewis acid.
  • Lewis definitions are more comprehensive than previous definitions of acids and bases.
  • These definitions are widely used in organic chemistry.
  • Aqueous solutions of FeCl₃ are acidic because the hydrogen atoms in water are attracted to the metal cation, weakening their bonds with oxygen, which can then be abstracted by water molecules to form more H₃O⁺ ions.

Brønsted-Lowry Theory - Revision Slide

  • In the Brønsted-Lowry theory example, H₂O donates a proton (H⁺ ion) to NH₃, making water a Brønsted-Lowry acid.
  • In turn, NH₃ accepts a proton, making it a Brønsted-Lowry base.
  • The reverse reaction shows a conjugate acid-base pair. NH₄⁺ donates a proton (H⁺ ion) to hydroxide (OH⁻), making NH₄⁺ a Brønsted-Lowry acid and OH⁻ a Brønsted-Lowry base.
  • In summary, conjugate acid-base pairs are formed by an acid and a base in an equilibrium reaction.

Strong and Weak Acids

  • A strong acid dissociates completely in water.
  • The equilibrium lies to the right for strong acids.
  • Strong acids have a very large equilibrium constant (HCl, for example).
  • The conjugate base of a strong acid is very weak.

Weak Acids

  • Weak acids dissociate only partially in water.
  • The equilibrium for weak acids lies towards the left.
  • Weak acids have a very small equilibrium constant, for example acetic acid.
  • The conjugate base of weak acids is relatively strong.

Strengths of Acids and Their Conjugate Bases

  • A table displays the relative strengths of various acids and their conjugate bases.
  • The table is ordered from strong acids to weak acids.
  • The stronger the acid, the weaker its conjugate base.
  • The weaker the acid, the stronger its conjugate base.

Dissociation of Water

  • Water is amphoteric, acting as both an acid and a base.
  • Water as a base accepts protons (H⁺ ions), for example, when reacting with hydrochloric acid.
  • Water as an acid donates protons (H⁺ ions), for example, when reacting with ammonia.
  • A reaction between two water molecules illustrates how one water molecule acts as an acid and the other acts as a base, forming hydronium (H₃O⁺) and hydroxide (OH⁻) ions.

Water Dissociation

  • Distilled water is a weak electrolyte.
  • The concentration of hydronium ions ([H₃O⁺]) and hydroxide ions ([OH⁻]) in distilled water at 25 °C is 1.0 × 10⁻⁷ M.
  • Two molecules in a billion dissociate.

Ion-Product Constant

  • The ion-product constant (Kw) of water is a measure of the equilibrium between water molecules, hydronium (H₃O⁺) and hydroxide (OH⁻).
  • Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25ºC
  • The concentrations of [H₃O⁺] and [OH⁻] in pure water are equal, making it neutral.
  • Solutions with greater [H₃O⁺] than [OH⁻] are acidic, and basic solutions have a higher [OH⁻] concentration than [H₃O⁺].

The pH Scale

  • pH is a measure of the acidity or basicity of a solution.
  • pH = -log[H₃O⁺]
  • Acidic solutions have pH values less than 7.
  • Neutral solutions have a pH of 7.
  • Basic solutions have pH values greater than 7.
  • Values for pH can be negative or greater than 14, which depends on temperature.

pH of Some Common Substances

  • A chart shows the pH values of common substances.

Measuring pH

  • pH can be measured using an electronic meter, which is highly accurate.
  • The electronic meter requires calibration with solutions of known pH like distilled water or buffer solutions.
  • Acid-base indicators can also approximate pH.
  • Universal indicators (paper strips or liquid) are another way to measure pH, offering relatively less precise measurements compared to the electronic meter.

pH for Strong Acids

  • Strong monoprotic acids dissociate completely in water.
  • Use the initial concentration of the strong acid to calculate [H₃O⁺], which is used to determine pH.

pH for Strong Bases

  • Alkali metal hydroxides (MOH) dissociate almost completely in water, producing metal cations (M⁺) and hydroxide anions (OH⁻) in a 1:1 molar ratio.
  • The concentration of hydroxide ions ([OH⁻]) can be used to calculate [H₃O⁺] and then pH.

Strong Bases (continued)

  • Some alkaline-earth metal hydroxides, while being strong bases, do not completely dissociate; this results in fewer hydroxide ions, impacting their pH to become lower.
  • Metal oxides like CaO react with water to form alkaline solutions.

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