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MATSEC IM Chemistry study materials
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MATSEC IM Chemistry practice questions

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Question 1Chemistry (IM syllabus content)

An ion contains 12 protons, 12 neutrons and 10 electrons. Which description correctly identifies its element and charge?

  1. Magnesium with charge 2+ Correct answer.
  2. Neon with charge 2+
  3. Magnesium with charge 2-
  4. Carbon with no charge

Explanation

Twelve protons identify magnesium. Having two fewer electrons than protons gives a net charge of plus two; the neutron count affects the isotope but not this charge calculation.

Question 2Chemistry (IM syllabus content)

An atom of chlorine-37 contains how many neutrons in its nucleus?

  1. 37
  2. 18
  3. 17
  4. 20 Correct answer.

Explanation

Chlorine has atomic number 17, so every chlorine atom has 17 protons. The mass number 37 equals protons plus neutrons, therefore neutrons = 37 - 17 = 20.

Question 3Chemistry (IM syllabus content)

Which statement describes a covalent bond in a molecule such as methane?

  1. Positive and negative ions attract electrostatically.
  2. Metallic cations are surrounded by a sea of electrons.
  3. Electrons are transferred completely from carbon to hydrogen.
  4. A shared pair of electrons is attracted to both nuclei. Correct answer.

Explanation

A covalent bond forms when two atoms share one or more pairs of electrons, and the shared pair is attracted to both nuclei simultaneously. This sharing holds the atoms together in a discrete molecule.

Question 4Chemistry (IM syllabus content)

Which formula represents the empirical formula of ethanoic acid, CH3COOH?

  1. C2H4O
  2. CH2O Correct answer.
  3. CHO
  4. C2H4O2

Explanation

The molecular formula is C2H4O2. Dividing each subscript by the highest common factor, 2, gives CH2O, which is the empirical formula showing the simplest whole-number ratio.

Question 5Chemistry (IM syllabus content)

In the reaction Zn + CuSO4 -> ZnSO4 + Cu, which species is oxidised?

  1. Copper ion
  2. Zinc Correct answer.
  3. Sulfate ion
  4. Copper metal

Explanation

Zinc loses two electrons to become Zn2+, and loss of electrons is oxidation. Copper ions gain those electrons and are reduced, so zinc is the species oxidised.

Question 6Chemistry (IM syllabus content)

Which gas is produced when a reactive metal such as magnesium reacts with dilute hydrochloric acid?

  1. Chlorine
  2. Carbon dioxide
  3. Oxygen
  4. Hydrogen Correct answer.

Explanation

A metal above hydrogen in the reactivity series displaces hydrogen from a dilute acid, giving a metal chloride and hydrogen gas. Magnesium chloride forms in solution while hydrogen bubbles off.

Question 7Chemistry (IM syllabus content)

Which pair of ions is present in aqueous sodium chloride in the highest concentrations?

  1. Na- and Cl+
  2. Na2+ and Cl2-
  3. Na+ and ClO-
  4. Na+ and Cl- Correct answer.

Explanation

Sodium chloride dissolves to give sodium ions, Na+, and chloride ions, Cl-. Each ion carries a single charge because sodium loses one electron and chlorine gains one.

Question 8Chemistry (IM syllabus content)

Which factor increases the rate of a reaction between marble chips and dilute hydrochloric acid?

  1. Lowering the temperature
  2. Diluting the acid further
  3. Using powdered calcium carbonate Correct answer.
  4. Using larger marble chips

Explanation

Powdering the solid greatly increases its surface area, so more acid particles can collide with the carbonate per unit time. A higher collision frequency raises the reaction rate.

Question 9Chemistry (IM syllabus content)

Phosphorus has a higher first ionization energy than sulfur despite sulfur's greater nuclear charge. Which feature explains this exception to the period trend?

  1. Sulfur's outer electron is in a 4s orbital
  2. Sulfur has a paired 3p electron with additional electron-electron repulsion Correct answer.
  3. Sulfur has one fewer occupied shell than phosphorus
  4. Phosphorus has a fully occupied 3p subshell

Explanation

Phosphorus has three singly occupied 3p orbitals, whereas sulfur introduces a paired electron. Repulsion within that pair makes one sulfur electron easier to remove despite the general period trend.

Question 10Chemistry (IM syllabus content)

Why is the first ionisation energy of aluminium lower than that of magnesium?

  1. Aluminium has fewer protons than magnesium.
  2. Aluminium has a larger nucleus than magnesium.
  3. Magnesium has a full outer shell.
  4. The outer electron of aluminium is in a 3p orbital, which is higher in energy and better shielded. Correct answer.

Explanation

Magnesium loses a 3s electron, whereas aluminium loses a 3p electron that lies slightly further from the nucleus and is shielded by the 3s pair. This makes the 3p electron easier to remove despite the larger nuclear charge.

Question 11Chemistry (IM syllabus content)

What is the shape of a molecule of sulfur dioxide, SO2?

  1. Tetrahedral
  2. Trigonal planar
  3. Linear
  4. Bent (V-shaped) Correct answer.

Explanation

The sulfur atom has two bonding pairs and one lone pair, giving three electron domains and a trigonal planar electron geometry. The lone pair compresses the O-S-O angle, so the molecular shape is bent.

Question 12Chemistry (IM syllabus content)

Which type of intermolecular force is chiefly responsible for the relatively high boiling point of water compared with hydrogen sulfide?

  1. London dispersion forces
  2. Dipole-induced dipole forces
  3. Ionic attraction
  4. Hydrogen bonding Correct answer.

Explanation

Water molecules form hydrogen bonds because hydrogen is attached to a highly electronegative oxygen atom with lone pairs. These strong intermolecular attractions require more energy to overcome, raising the boiling point.

Question 13Chemistry (IM syllabus content)

In an equilibrium mixture of nitrogen, hydrogen and ammonia, what happens when the total pressure is increased at constant temperature?

  1. The equilibrium shifts towards the products, ammonia. Correct answer.
  2. The equilibrium shifts towards the reactants.
  3. The value of the equilibrium constant changes.
  4. The equilibrium position is unaffected.

Explanation

Le Chatelier's principle states that raising pressure shifts the equilibrium towards the side with fewer gas molecules. The forward reaction converts four moles of gas into two, so the yield of ammonia increases.

Question 14Chemistry (IM syllabus content)

Which reagent and condition are used to oxidise ethanol to ethanoic acid?

  1. Acidified potassium dichromate(VI) with immediate distillation
  2. Sodium metal at room temperature
  3. Acidified potassium dichromate(VI) under reflux Correct answer.
  4. Concentrated sulfuric acid at room temperature

Explanation

Refluxing ethanol with acidified potassium dichromate(VI) keeps the aldehyde in contact with the oxidising agent so it is oxidised further to the carboxylic acid. The orange dichromate turns green as it is reduced.

Question 15Chemistry (IM syllabus content)

Which reagent gives a positive result with an aldehyde but not with a ketone?

  1. Bromine water
  2. Sodium carbonate solution
  3. Fehling's solution Correct answer.
  4. Litmus paper

Explanation

Aldehydes reduce the copper(II) complex in Fehling's solution to a brick-red precipitate of copper(I) oxide on warming. Ketones lack the oxidisable hydrogen on the carbonyl carbon and give no such change.

Question 16Chemistry (IM syllabus content)

What is the IUPAC name of CH3CH(CH3)CH2OH?

  1. Butan-2-ol
  2. 2-Methylpropan-2-ol
  3. Propan-1-ol
  4. 2-Methylpropan-1-ol Correct answer.

Explanation

The longest carbon chain containing the hydroxyl group has three carbons, numbered so the OH gets the lowest locant, giving propan-1-ol. A methyl group sits on carbon 2, so the name is 2-methylpropan-1-ol.

Question 17Chemistry (IM syllabus content)

Hydrogen peroxide decomposes more rapidly after manganese dioxide is added. If the manganese dioxide is recovered chemically unchanged overall, how should its role be classified?

  1. A substance that increases the reaction enthalpy
  2. A product that forces the equilibrium constant to increase
  3. A catalyst providing a lower-barrier pathway Correct answer.
  4. A reactant permanently incorporated into every product molecule

Explanation

The faster reaction and recovery of manganese dioxide unchanged overall are consistent with catalysis. A catalyst supplies an alternative reaction pathway and is regenerated rather than consumed stoichiometrically.

Question 18Chemistry (IM syllabus content)

Which cell is used industrially for the electrolysis of concentrated brine?

  1. The diaphragm or membrane cell Correct answer.
  2. The Daniell cell
  3. The Haber cell
  4. The standard hydrogen electrode

Explanation

Brine electrolysis in a membrane or diaphragm cell produces chlorine at the anode, hydrogen at the cathode and sodium hydroxide solution. The membrane separates the products to prevent them reacting together.

Question 19Chemistry (IM syllabus content)

In a galvanic cell, which statement about the direction of electron flow in the external circuit is correct?

  1. Electrons do not flow; only ions move.
  2. Electrons flow from the cathode to the anode.
  3. Electrons flow through the electrolyte.
  4. Electrons flow from the more negative electrode to the more positive electrode. Correct answer.

Explanation

The electrode with the more negative standard electrode potential releases electrons, so it acts as the anode. Electrons travel through the external wire from this negative electrode to the positive cathode.

Question 20Chemistry (IM syllabus content)

Which pair of half-equations correctly combines to give the overall reaction in a hydrogen-oxygen fuel cell in alkaline conditions?

  1. H2 + 2OH- -> 2H2O + 2e- and O2 + 2H2O + 4e- -> 4OH- Correct answer.
  2. 2H2O + 2e- -> H2 + 2OH- and 4OH- -> O2 + 2H2O + 4e-
  3. H2 + 2e- -> 2H+ and O2 -> 2O2-
  4. H2 -> 2H+ + 2e- and O2 + 4H+ + 4e- -> 2H2O

Explanation

In alkaline solution the anode reaction is oxidation of hydrogen by hydroxide ions to water, releasing electrons. At the cathode oxygen gains electrons and reacts with water to regenerate hydroxide ions, giving an overall formation of water.

Question 21Chemistry (IM syllabus content)

Which reagent distinguishes ethane from ethene in a simple laboratory test?

  1. Dilute hydrochloric acid
  2. Aqueous sodium chloride
  3. Bromine water Correct answer.
  4. Sodium carbonate solution

Explanation

Ethene contains a carbon-carbon double bond that adds bromine rapidly, decolourising the orange bromine water. Ethane is saturated and does not react, so the colour remains.

Question 22Chemistry (IM syllabus content)

Which polymer is formed by the addition polymerisation of chloroethene?

  1. Poly(chloroethene), PVC Correct answer.
  2. Poly(ethene)
  3. Nylon
  4. Polyester

Explanation

Addition polymerisation opens the C=C bond of chloroethene so that units join head to tail, leaving the chlorine substituent on the chain. The product is poly(chloroethene), commonly called PVC.

Question 23Chemistry (IM syllabus content)

A reaction has an enthalpy change of -890 kJ mol-1 and a large positive entropy change. Which statement about its feasibility is correct?

  1. It is feasible only at high temperature.
  2. It is feasible at all temperatures because both terms favour the reaction. Correct answer.
  3. It is never feasible.
  4. It is feasible only at low temperature.

Explanation

Feasibility depends on the sign of the Gibbs free energy change, given by delta-G = delta-H - T delta-S. With delta-H negative and delta-S positive, delta-G is negative at every temperature, so the reaction is always feasible.

Question 24Chemistry (IM syllabus content)

Which observation confirms that a reaction is exothermic?

  1. A gas is evolved rapidly.
  2. The temperature of the surroundings falls.
  3. A precipitate forms immediately.
  4. The temperature of the surroundings rises. Correct answer.

Explanation

An exothermic reaction releases energy to the surroundings, so the surroundings warm up and their temperature rises. The enthalpy change of the system is negative.

Question 25Chemistry (IM syllabus content)

Which statement explains why the second electron affinity of oxygen is endothermic while the first is exothermic?

  1. Electron affinity always decreases with atomic number.
  2. The first electron affinity involves bond breaking.
  3. The second electron is added to a smaller atom.
  4. The first electron is attracted to a neutral atom; the second is repelled by the already negative ion. Correct answer.

Explanation

The first electron is attracted into the neutral oxygen atom, releasing energy. The second electron must be forced onto an already negatively charged ion, and the electrostatic repulsion makes this step endothermic.

Question 26Chemistry (IM syllabus content)

How many moles of oxygen atoms are present in 0.50 mol of aluminium oxide, Al2O3?

  1. 1.0 mol
  2. 1.5 mol Correct answer.
  3. 3.0 mol
  4. 0.50 mol

Explanation

Each formula unit of Al2O3 contains three oxygen atoms, so one mole of Al2O3 contains three moles of oxygen atoms. Therefore 0.50 mol contains 0.50 × 3 = 1.5 mol of oxygen atoms.

Question 27Chemistry (IM syllabus content)

Which statement describes the structure of a solid ionic compound such as sodium chloride?

  1. A giant covalent network of shared electron pairs
  2. Layers of atoms held together by metallic bonding
  3. Discrete molecules each containing one sodium and one chloride ion
  4. A lattice of alternating cations and anions held by electrostatic attraction Correct answer.

Explanation

Ionic solids form a regular three-dimensional lattice in which each cation is surrounded by anions and vice versa. The lattice is held together by strong electrostatic attraction between oppositely charged ions, which explains the high melting points.

Question 28Chemistry (IM syllabus content)

A sample of gas occupies 240 cm3 at 300 K. Assuming the pressure stays constant, what volume does it occupy at 600 K?

  1. 480 cm3 Correct answer.
  2. 240 cm3
  3. 360 cm3
  4. 120 cm3

Explanation

At constant pressure, volume is directly proportional to absolute temperature (Charles's law). Doubling the temperature from 300 K to 600 K therefore doubles the volume to 480 cm3.

Question 29Chemistry (IM syllabus content)

Which type of reaction is the conversion of ethene into poly(ethene)?

  1. Addition polymerisation Correct answer.
  2. Condensation polymerisation
  3. Esterification
  4. Nucleophilic substitution

Explanation

Ethene molecules contain a carbon-carbon double bond that opens up so that monomers join together without losing any small molecule. This is addition polymerisation, producing a saturated carbon backbone.

Question 30Chemistry (IM syllabus content)

Which reagent is used to test for a carbonate by observing effervescence?

  1. Bromine water
  2. Dilute hydrochloric acid Correct answer.
  3. Fehling's solution
  4. Acidified potassium dichromate(VI)

Explanation

Carbonates react with dilute acid to release carbon dioxide, which is seen as effervescence and turns limewater milky. This is a standard identification test for the carbonate ion.

Question 31Chemistry (IM syllabus content)

Which statement describes a strong acid in aqueous solution?

  1. It has a pH greater than 7.
  2. It is almost completely dissociated into ions. Correct answer.
  3. It is only slightly dissociated into ions.
  4. It accepts protons from water molecules.

Explanation

A strong acid donates its proton essentially completely to water, so almost all of its molecules are ionised in dilute solution. This gives a high concentration of hydrogen ions and a low pH.

Question 32Chemistry (IM syllabus content)

Which pair of elements shows the same group chemistry because of the same number of outer-shell electrons?

  1. Fluorine and neon
  2. Lithium and sodium Correct answer.
  3. Carbon and nitrogen
  4. Sodium and magnesium

Explanation

Lithium and sodium are both in Group 1 and each has one electron in its outer shell, giving similar chemical properties. Elements in the same group show related reactions because their outer-shell configurations match.

Question 33Chemistry (IM syllabus content)

Which statement correctly describes an isotope?

  1. Ions of the same element with different charges
  2. Atoms of the same element with different numbers of neutrons Correct answer.
  3. Atoms with the same number of neutrons but different numbers of protons
  4. Atoms of different elements with the same mass number

Explanation

Isotopes are atoms of the same element, so they have the same number of protons, but they differ in the number of neutrons in the nucleus. This changes the mass number but not the chemical identity.

Question 34Chemistry (IM syllabus content)

In the reaction Zn + Cu2+ → Zn2+ + Cu, which change in oxidation state occurs at the zinc?

  1. Zinc is reduced from +2 to 0.
  2. Zinc is reduced from 0 to +2.
  3. Zinc is oxidised from 0 to +2. Correct answer.
  4. Zinc is oxidised from +2 to 0.

Explanation

Zinc metal starts at oxidation state 0 and becomes Zn2+, so it loses two electrons and is oxidised. Oxidation is a loss of electrons and an increase in oxidation state.

Question 35Chemistry (IM syllabus content)

Which expression gives the units of the rate constant for a reaction that is second order overall?

  1. mol-1 dm3 s-1 Correct answer.
  2. mol dm-3 s-1
  3. s-1
  4. mol-2 dm6 s-1

Explanation

For a second-order overall reaction, rate = k[A][B], so k = rate/(concentration)2. The units are (mol dm-3 s-1)/(mol dm-3)2, which simplifies to mol-1 dm3 s-1.

Question 36Chemistry (IM syllabus content)

Which statement explains why a mixture of ice and water at 0 °C can absorb heat without its temperature rising?

  1. The ice has a higher specific heat capacity than the water.
  2. The energy supplied is used to increase the average kinetic energy of the molecules.
  3. The water molecules are chemically decomposing into hydrogen and oxygen.
  4. The energy supplied is used to overcome the forces between water molecules during melting. Correct answer.

Explanation

During a change of state the temperature stays constant because the heat supplied breaks the intermolecular forces holding the solid lattice together. The energy does not increase the average kinetic energy of the molecules, so the thermometer reading does not change.

Question 37Chemistry (IM syllabus content)

Which statement explains why the melting point of magnesium is higher than that of sodium?

  1. Magnesium has weaker metallic bonding because of its greater charge.
  2. Magnesium has more delocalised electrons per atom and a smaller cation. Correct answer.
  3. Sodium has a giant covalent lattice while magnesium is metallic.
  4. Magnesium has a simple molecular structure with stronger van der Waals forces.

Explanation

Both metals have metallic bonding, but a magnesium atom contributes two delocalised electrons per atom and its Mg2+ ion is smaller than Na+. The stronger attraction between the ions and the greater electron density raises the melting point.

Question 38Chemistry (IM syllabus content)

Which half-equation correctly shows the reduction of manganate(VII) ions in acidic solution?

  1. MnO4- → Mn2+ + 4H2O + 5e-
  2. MnO4- + 4H2O → Mn2+ + 8H+ + 5e-
  3. MnO4- + 8H+ + 5e- → Mn2+ + 4H2O Correct answer.
  4. MnO4- + 5e- → Mn2+ + 4H2O

Explanation

In acidic solution, manganate(VII) is reduced to manganese(II) while gaining five electrons. The half-equation must balance atoms and charge, so eight hydrogen ions and four water molecules are needed on the appropriate sides.

Question 39Chemistry (IM syllabus content)

Which statement describes the bonding in a molecule of ammonia, NH3?

  1. Three covalent bonds from overlap of nitrogen and hydrogen orbitals, plus a lone pair on nitrogen Correct answer.
  2. Three metallic bonds with delocalised electrons around nitrogen
  3. Two covalent bonds and one hydrogen bond within the molecule
  4. Three ionic bonds between N3- and H+ ions

Explanation

Each hydrogen atom shares one electron pair with nitrogen to form a single covalent bond, giving three bonds. Nitrogen keeps one non-bonding lone pair, which gives ammonia its pyramidal shape and its basic properties.

Question 40Chemistry (IM syllabus content)

Which statement explains why the rate of reaction between magnesium and dilute hydrochloric acid decreases as the reaction proceeds?

  1. The concentration of hydrogen ions in the acid falls as it is used up. Correct answer.
  2. The temperature of the mixture falls below the activation energy.
  3. The magnesium becomes a catalyst for the reverse reaction.
  4. The activation energy of the reaction increases as the acid is used up.

Explanation

As the acid is consumed, the concentration of hydrogen ions decreases, so fewer collisions occur per unit time between the acid and the metal surface. The rate therefore falls as the reaction proceeds.

Question 41Chemistry (IM syllabus content)

Which statement about the equilibrium constant Kc for an exothermic reaction is correct?

  1. Kc depends only on the total pressure.
  2. Kc decreases when the temperature is raised. Correct answer.
  3. Kc is unchanged by temperature changes.
  4. Kc increases when the temperature is raised.

Explanation

For an exothermic reaction, raising the temperature shifts the position of equilibrium towards the reactants. This reduces the value of Kc because the equilibrium expression then has a smaller numerator relative to the denominator.

Question 42Chemistry (IM syllabus content)

Which statement correctly describes the trend in atomic radius across Period 3 from sodium to chlorine?

  1. The radius stays constant because the number of outer electrons is unchanged.
  2. The radius increases because extra electron shells are added.
  3. The radius decreases because the nuclear charge increases while electrons enter the same shell. Correct answer.
  4. The radius increases because shielding by inner electrons increases sharply.

Explanation

Across Period 3, protons are added to the nucleus while the outer electrons remain in the same shell, so the effective nuclear charge increases. The greater attraction pulls the outer electrons closer, giving a smaller atomic radius.

Question 43Chemistry (IM syllabus content)

Which statement explains why the enthalpy of hydration of the chloride ion is less exothermic than that of the fluoride ion?

  1. Chloride carries a smaller charge than fluoride, so fewer water molecules can bind.
  2. Chloride has a higher charge density because it has more electrons.
  3. Chloride is less polarisable, so water molecules are attracted more strongly.
  4. Chloride is larger, so its charge density and the attraction to water molecules are lower. Correct answer.

Explanation

Both ions carry a single negative charge, but the chloride ion has a larger radius and therefore a lower charge density. The weaker attraction between the chloride ion and the water dipoles releases less energy on hydration.

Question 44Chemistry (IM syllabus content)

Which reagent and condition convert propan-2-ol into propanone?

  1. Bromine water at room temperature
  2. Sodium hydroxide in aqueous solution
  3. Concentrated sulfuric acid at 170 °C
  4. Acidified potassium dichromate(VI) with heating under reflux Correct answer.

Explanation

Propan-2-ol is a secondary alcohol, and oxidation with acidified potassium dichromate(VI) converts it into the ketone propanone. Reflux ensures the oxidation goes to completion.

Question 45Chemistry (IM syllabus content)

Which statement explains why the boiling point of propan-1-ol is higher than that of butane?

  1. Propan-1-ol has a lower relative molecular mass and so is more volatile.
  2. Propan-1-ol molecules form hydrogen bonds through the hydroxyl group. Correct answer.
  3. Butane contains a hydroxyl group that forms strong hydrogen bonds.
  4. Butane molecules are held together by ionic attractions.

Explanation

The O-H group in propan-1-ol allows hydrogen bonding between molecules, which requires more energy to overcome than the van der Waals forces in butane. This raises the boiling point of the alcohol despite its similar size.

Question 46Chemistry (IM syllabus content)

Which statement is true about the entropy change when a solid dissolves in water to give a solution of ions?

  1. The entropy change is zero because the number of particles is unchanged.
  2. The entropy of the system always decreases because ions become hydrated.
  3. The entropy of the system usually increases because the ordered lattice is broken up. Correct answer.
  4. The entropy change depends only on the enthalpy of solution.

Explanation

Breaking up an ordered ionic lattice and dispersing the ions in solution increases the number of ways the particles can be arranged, so the entropy of the system usually increases. Hydration orders some water molecules around the ions, but the overall effect is normally an increase.

Question 47Chemistry (IM syllabus content)

A solution contains 0.10 mol dm-3 of a weak acid, HA, with Ka = 1.0 × 10-5 mol dm-3. What is the approximate pH of the solution?

  1. 1.0
  2. 5.0
  3. 7.0
  4. 3.0 Correct answer.

Explanation

For a weak acid, [H+] ≈ √(Ka × [HA]) = √(1.0 × 10-5 × 0.10) = √(1.0 × 10-6) = 1.0 × 10-3 mol dm-3. The pH is therefore -log(1.0 × 10-3) = 3.0.

Question 48Chemistry (IM syllabus content)

Which statement explains why the lattice enthalpy of magnesium oxide is much more exothermic than that of sodium chloride?

  1. MgO contains covalent bonds, which are stronger than ionic bonds in NaCl.
  2. Both ions in MgO carry double charges and are small, so electrostatic attraction is much stronger. Correct answer.
  3. MgO has a higher coordination number, which weakens the electrostatic attraction.
  4. NaCl has a larger lattice because it contains more ions per formula unit.

Explanation

Lattice enthalpy depends on ionic charge and size according to electrostatic attraction. Mg2+ and O2- have double charges and smaller radii than Na+ and Cl-, so the attraction between them is far stronger and much more energy is released on formation.

Question 49Chemistry (IM syllabus content)

Which statement about a buffer made from ethanoic acid and sodium ethanoate is correct?

  1. The pH of the buffer changes greatly when small amounts of acid or base are added.
  2. Added acid is removed by the ethanoic acid and added base by the ethanoate ion.
  3. The buffer works because both components are strong electrolytes that fully dissociate.
  4. Added acid is removed by the ethanoate ion and added base by the ethanoic acid. Correct answer.

Explanation

In an ethanoate buffer, the large reserve of ethanoate ions neutralises added acid, while the reserve of ethanoic acid molecules neutralises added base. Both components are present in large amounts, so the pH changes only slightly.

Question 50Chemistry (IM syllabus content)

Which statement explains why the reaction between hydrogen and iodine is slow at room temperature even though it is thermodynamically feasible?

  1. The iodine is a solid and cannot react with a gas.
  2. The reaction is exothermic, which makes it kinetically slow.
  3. The reaction has a positive enthalpy change that prevents it occurring.
  4. The activation energy is high because a strong H-H bond must be broken. Correct answer.

Explanation

Reaction rate depends on the activation energy, and breaking the strong H-H bond requires a large input of energy. At room temperature few collisions have enough energy to overcome this barrier, so the reaction is slow despite being feasible.