OCR A Level Chemistry A - Acid-base & Redox

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Questions and Answers

What does concentration measure in a solution?

  • The total volume of the solvent used
  • The amount of solute dissolved in a given volume of solvent (correct)
  • The temperature of the solution
  • The pressure exerted by the solute

What is the purpose of placing a white tile under the conical flask during titration?

  • To reduce the heating effect of the burette
  • To absorb excess liquid spillage
  • To prevent the flask from sliding
  • To enhance the visibility of the color change (correct)

What does a concentrated solution imply about its solute?

  • The solute is evenly distributed throughout the solvent
  • The solute is insoluble in the solvent
  • There is a very small amount of solute present
  • There is a high concentration of solute (correct)

What uncertainty is recorded when using a burette marked to a precision of 0.10 cm3?

<p>±0.05 cm3 (D)</p> Signup and view all the answers

What is the equivalence point in titration?

<p>When the two solutions have completely reacted (D)</p> Signup and view all the answers

What is the oxidation number of sulfur in S4O6^2-?

<p>+2.5 (C)</p> Signup and view all the answers

What is the purpose of using Roman numerals in the notation of transition metals?

<p>To show the oxidation state of the metal (C)</p> Signup and view all the answers

Which of the following represents a redox reaction?

<p>Zinc + hydrochloric acid → zinc chloride + hydrogen (A)</p> Signup and view all the answers

Why can't single atoms have a fractional oxidation number?

<p>Because electrons are indivisible and cannot be split (C)</p> Signup and view all the answers

What is produced when a metal reacts with an acid?

<p>Salt and hydrogen (D)</p> Signup and view all the answers

What is true about strong acids when dissolved in water?

<p>They fully dissociate into ions. (C)</p> Signup and view all the answers

Which statement describes weak acids?

<p>They only partially dissociate in solution. (C)</p> Signup and view all the answers

When ethanoic acid (CH3COOH) is dissolved in water, what is the primary product?

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

How do weak bases behave when dissolved in solution?

<p>They partially dissociate and establish an equilibrium. (D)</p> Signup and view all the answers

What occurs during the dissociation of strong bases in solution?

<p>They completely dissociate into ions. (A)</p> Signup and view all the answers

Which of the following statements is incorrect regarding weak acids?

<p>They do not produce any ions in solution. (B)</p> Signup and view all the answers

What is the general reaction of a strong acid in solution?

<p>HCl (aq) ⇌ H+ (aq) + Cl- (aq) (B)</p> Signup and view all the answers

Which of the following acids is an example of a weak acid?

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

What does the oxidation state of an atom represent?

<p>The charge that would exist on an individual atom if bonding were completely ionic (D)</p> Signup and view all the answers

In a neutral molecule, the sum of the oxidation numbers must equal what value?

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

Which of the following statements is true regarding oxidation numbers in a compound?

<p>The sum of the oxidation numbers equals the charge of the compound (B)</p> Signup and view all the answers

How can oxidation numbers help in balancing redox equations?

<p>They indicate the oxidation and reduction that has taken place (A)</p> Signup and view all the answers

Which of the following scenarios would result in an element having a negative oxidation number?

<p>When it is more electronegative than the other atoms it bonds with (B)</p> Signup and view all the answers

What is the first step to determine the oxidation number of an atom in a molecule?

<p>Use the oxidation rules to assess the charge distribution (A)</p> Signup and view all the answers

What must happen for the oxidation number of an atom to be positive in a compound?

<p>It must lose electrons compared to its neutral state (A)</p> Signup and view all the answers

When considering CO2, what can be inferred about the oxidation states of the atoms involved?

<p>One atom must have a positive oxidation number while the other is negative (D)</p> Signup and view all the answers

What trend does electronegativity display across a period in the periodic table?

<p>It increases. (D)</p> Signup and view all the answers

Which element in P2O5 has the most negative oxidation number?

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

How can the oxidation state of an atom in a compound typically be determined?

<p>From its position in the periodic table. (A)</p> Signup and view all the answers

In which compound does sulfur have an oxidation state that is not a whole number?

<p>S4O62- (B)</p> Signup and view all the answers

What happens to electronegativity when moving down a group in the periodic table?

<p>It decreases. (D)</p> Signup and view all the answers

Is it possible for the oxidation states of atoms to vary in a compound?

<p>Yes, depending on the other elements present. (D)</p> Signup and view all the answers

What oxidation state does chlorine typically have in ClO2-?

<p>+3 (A)</p> Signup and view all the answers

Which of the following statements about oxidation numbers is correct?

<p>Oxidation numbers are always whole numbers. (C)</p> Signup and view all the answers

What is the first step involved in calculating the concentration of hydrochloric acid in the worked example?

<p>Write the balanced symbol equation for the reaction. (C)</p> Signup and view all the answers

In the given example, how many moles of sodium carbonate reacted?

<p>0.00125 mol (B)</p> Signup and view all the answers

What is the molar ratio of sodium carbonate to hydrochloric acid in the reaction?

<p>1:2 (A)</p> Signup and view all the answers

How many moles of hydrochloric acid reacted in the example?

<p>0.00250 mol (B)</p> Signup and view all the answers

Which of the following equations is used to calculate the concentration of a solution?

<p>Concentration = Moles / Volume (C)</p> Signup and view all the answers

What is the concentration of hydrochloric acid in mol dm-3, as calculated in the worked example?

<p>0.125 mol dm-3 (A)</p> Signup and view all the answers

What is the purpose of converting cm3 to dm3 when calculating concentration?

<p>To ensure the units of volume and concentration are compatible. (B)</p> Signup and view all the answers

What is the molar mass of sodium carbonate (Na2CO3)?

<p>106 g mol-1 (B)</p> Signup and view all the answers

Flashcards

Concentration of a solution

The amount of solute dissolved in 1 dm³ of solvent.

Solute

The substance that dissolves in a solvent.

Solvent

The substance in which the solute is dissolved, often water.

Burette

The equipment used in titration to measure volumes precisely.

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End point of titration

The point at which reactions of solutions are complete, indicated by a color change.

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Strong acids

Acids that fully dissociate in solution, releasing all H+ ions.

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Weak acids

Acids that partially dissociate in solution, establishing an equilibrium.

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Strong bases

Bases that fully dissociate in solution, releasing all OH- ions.

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Weak bases

Bases that partially dissociate in solution, forming equilibrium.

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Dissociation

The process of a compound splitting into ions in solution.

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Equilibrium in solutions

A state where reactants and products are present at stable concentrations.

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Hydrogen chloride (HCl)

A strong acid that fully dissociates into H+ and Cl- ions in water.

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Ethanoic acid (CH3COOH)

A weak acid that partially dissociates, establishing an equilibrium with H+ and CH3COO- ions.

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Oxidation number of sulfur

In S4O62-, the oxidation number of sulfur is +2.5, but individual atoms have integer oxidation numbers.

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Integer oxidation numbers

Only single atoms can have oxidation numbers in whole numbers, no fractions allowed.

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Roman numerals in oxidation states

Used to indicate oxidation states of transition metals with multiple states, like iron (+2 and +3).

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Redox reaction

A reaction involving changes in oxidation states, such as a metal reacting with an acid to form salt and hydrogen.

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Example of a redox reaction

Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen; both oxidation and reduction occur.

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Oxidation Number

The charge on an atom if bonding were ionic; indicates 'status'.

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Determining Oxidation State

Process of finding the oxidation number using specific rules.

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Oxidation vs Reduction

Used to identify if oxidation or reduction occurs in a reaction.

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Half Equations

Equations representing the oxidation or reduction part of a redox reaction.

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Oxidation Numbers in Molecules

In compounds, oxidation numbers sum to zero for neutrality.

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Simple Ions Oxidation Numbers

For simple ions, the oxidation number equals the ion's charge.

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Sum of Oxidation Numbers

In a neutral molecule, oxidation numbers must add up to zero.

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Identifying Positive Oxidation

Look at the oxidation numbers to determine which atom is positive in a compound.

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Convert cm3 to dm3

To convert cubic centimeters to cubic decimeters, divide by 1000.

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Moles calculation formula

Number of moles = concentration (mol dm-3) x volume (dm3).

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Mass of solute formula

Mass of solute (g) = number of moles (mol) x molar mass (g mol-1).

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Balanced equation example

Na2CO3 + 2HCl → 2NaCl + H2O + CO2 shows the reaction balance.

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Stoichiometry of Na2CO3 and HCl

1 mol of Na2CO3 reacts with 2 mol of HCl.

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Calculate moles of Na2CO3

Moles = 0.025 dm3 x 0.050 mol dm-3 = 0.00125 mol of Na2CO3.

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Concentration of HCl

Concentration (HCl) = 0.125 mol dm-3 determined from reactions.

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Neutralisation reaction

Reactions where an acid and base form salt and water.

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Electronegativity

A measure of an atom's ability to attract electrons in a bond.

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Trend of Electronegativity

Increases across a period and decreases down a group in the periodic table.

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Determining Oxidation Number

Found from the element's position in the periodic table and other atoms present.

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Oxidation Number Rules

Specific guidelines that help deduce the oxidation state of atoms in compounds.

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Whole Numbers in Oxidation

Oxidation numbers are typically whole numbers, but can vary in certain cases.

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Oxygen Electronegativity

Oxygen is more electronegative than carbon, hence it has a negative oxidation state.

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Examples of Oxidation Numbers

Common compounds like P2O5, SO42-, and NH3 can help practice deducing oxidation states.

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Study Notes

OCR A Level Chemistry A - Acid-base & Redox Reactions

  • This topic covers acids, acid-base titrations, and redox reactions.
  • Strong acids fully dissociate in solution (e.g., HCl → H⁺ + Cl⁻).
  • Strong bases fully dissociate in solution (e.g., NaOH → Na⁺ + OH⁻).
  • Weak acids partially dissociate in solution, forming an equilibrium (e.g., CH₃COOH ⇌ CH₃COO⁻ + H⁺).
  • Weak bases partially dissociate in solution, forming an equilibrium (e.g., CH₃CH₂NH₂ + H₂O ⇌ CH₃CH₂NH₃⁺ + OH⁻).
  • Neutralisation is a reaction between an acid and a base/alkali, producing a salt and water (e.g., HCl + NaOH → NaCl + H₂O).
  • The proton of the acid reacts with the hydroxide of the base to form water (H⁺ + OH⁻ → H₂O).
  • Spectator ions are not involved in the formation of water and form the salt.
  • Examples of strong acids include HCl, H₂SO₄, HNO₃.
  • Examples of weak acids include HCOOH, CH₃COOH.
  • Examples of strong bases include NaOH, KOH, Ba(OH)₂.
  • Examples of weak bases include NH₃, CH₃NH₂, CH₃CH₂NH₂.
  • Metals react with acids to produce a salt and hydrogen gas (e.g., Zn + 2HCl → ZnCl₂ + H₂).
  • Metals react with metal oxides to produce a salt and water (e.g., 2HCl + CaO → CaCl₂ + H₂O).
  • Metals react with hydroxides to produce a salt and water (e.g., H₂SO₄ + Mg(OH)₂ → MgSO₄ + 2H₂O).
  • Metals react with carbonates to produce a salt, water, and carbon dioxide (e.g., 2HNO₃ + CuCO₃ → Cu(NO₃)₂ + H₂O + CO₂).
  • Volumetric analysis uses the volume and concentration of one reactant to determine the concentration of another unknown solution.

Acid-base Titrations

  • Titration is a technique to determine the concentration of an unknown solution using a standard solution.
  • Key apparatus include beakers, burettes, volumetric pipettes, and conical flasks.
  • Before titration, standard solutions must be precisely prepared, using precise measurements.
  • Burettes are used for precise measurement of volumes, often marked to 0.10 cm³.
  • Uncertainty is usually ±0.05 cm³ for analogue instruments.
  • End points (equivalence points) are recorded when the indicator color changes.
  • Concordant results are within 0.1 cm³ of each other, the non-concordant results should be discarded.

Redox

  • Oxidation numbers help determine whether a reaction is oxidation or reduction.
  • Rules exist for assigning oxidation numbers to determine oxidation state changes.
  • Oxidation involves an increase in oxidation number.
  • Reduction involves a decrease in oxidation number.
  • Redox reactions involve simultaneous oxidation and reduction.
  • Oxidizing agents cause oxidation and reduce their own oxidation state.
  • Reducing agents cause reduction and oxidize their own oxidation state.

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