Chemistry HL Paper 1A, specimen 2025

Two pigments extracted from a leaf are separated by thin-layer chromatography on a silica plate (a polar stationary phase) using a non-polar solvent as the mobile phase. Pigment X travels further up the plate than pigment Y. Which statement explains the…

Heavy water, $\ce{^{2}H2O}$, boils at 101.4 °C, whereas ordinary water, $\ce{^{1}H2O}$, boils at 100.0 °C at the same pressure. Which statement is correct? A. The boiling points differ because $\ce{^{2}H2O}$ molecules have a greater mass, while the chemical…

An unknown element E has the six successive ionization energies given below. Ionization number 1st 2nd 3rd 4th 5th 6th Ionization energy / $\mathrm{kJ\,mol^{-1}}$ 562.3 1091.8 4265.4 5637.9 7052.1 8893.6 Which is the formula of the chloride of E? A. $\ce{ECl}$…

A cylinder contains 80 g of argon and 12.0 g of helium. What is the ratio of the number of argon atoms to the number of helium atoms in the cylinder? A. 0.15 B. 0.67 C. 1.5 D. 6.7

A rigid cylinder of fixed volume contains an ideal gas at 250 K and a pressure of 75 kPa. One third of the gas molecules are released, and the temperature of the remaining gas is raised to 500 K. What pressure, in kPa, does the gas now have? A. 50 B. 100 C.…

Ammonium ions and phosphate ions combine to form an ionic compound. What is its formula? A. $\ce{NH4PO4}$ B. $\ce{(NH4)2PO4}$ C. $\ce{(NH4)3PO4}$ D. $\ce{NH4(PO4)3}$

In which set do all the species contain delocalized electrons? A. $\ce{NH4+}$, $\ce{C6H12}$, $\ce{Si(s)}$ B. $\ce{CH3OH}$, $\ce{SO2}$, $\ce{C}$ (diamond) C. $\ce{C2H4}$, $\ce{NO3-}$, $\ce{SiO2}$ D. $\ce{CO3^2-}$, $\ce{C6H6}$, $\ce{Cu(s)}$

What is the molecular geometry of $\ce{PCl3F2}$? A. trigonal bipyramidal B. trigonal planar C. square pyramidal D. pentagonal

Formal charges can be used to choose between possible Lewis structures for the cyanate ion, $\ce{NCO-}$. Which structure is preferred? Non-bonding pairs are not shown; each atom has a complete octet. A. $\ce{[N=C=O]^-}$ B. $\ce{[N#C-O]^-}$ C. $\ce{[N-C#O]^-}$…

Which pair of statements about the electrons in this molecule is correct? A. 3 non-bonding pairs of electrons and 8 electrons in $\pi$ bonds B. 5 non-bonding pairs of electrons and 8 electrons in $\pi$ bonds C. 5 non-bonding pairs of electrons and 10 electrons…

Copper can be hammered into thin sheets without shattering. Which feature of metallic bonding explains this? A. The delocalized electrons are free to move through the lattice, carrying charge. B. The metallic bonds are weak, so the cations are easily displaced…

The repeating unit of an addition polymer is $\ce{[CH2-CH(CN)]_n}$. Which monomer forms this polymer? A. $\ce{H2C=CHCN}$ B. $\ce{CH3CH2CN}$ C. $\ce{NCCH=CHCN}$ D. $\ce{H2C=CHCH2CN}$

Aqueous solutions of nickel(II) sulfate, which contain $\ce{[Ni(H2O)6]^2+}$ ions, are green. Use section 15 of the data booklet. Which statement explains the colour? A. Green light is absorbed when an electron is promoted between the split 3d orbitals. B.…

Which molecule contains an ester functional group? A. $\ce{CH3CH2COOH}$ B. $\ce{CH3CH2OCH2CH3}$ C. $\ce{CH3COOCH2CH3}$ D. $\ce{CH3CH2CHO}$

Which characteristic determines the block of the periodic table in which an element is placed? A. the number of valence electrons the element has B. the number of occupied main energy levels C. whether the element is a metal, a metalloid or a non-metal D. the…

Which element forms a stable compound in which its oxidation state is +6? A. Ti B. V C. Cr D. Zn

Methane, carbon dioxide and water vapour are all greenhouse gases. Which property do these gases share that makes them greenhouse gases? A. They absorb the ultraviolet part of solar radiation before it reaches the Earth's surface. B. They are all produced only…

The $\ce{^{1}H}$ NMR spectra of four compounds are described below. Which is the spectrum of propanone, $\ce{CH3COCH3}$? Use section 21 of the data booklet. A. one singlet at 2.2 ppm B. a quartet at 2.4 ppm and a triplet at 1.0 ppm, with relative areas 2 : 3…

The mass spectra of four compounds are shown below. Which is the spectrum of propanal, $\ce{CH3CH2CHO}$? Use section 22 of the data booklet. A. spectrum A B. spectrum B C. spectrum C D. spectrum D

A student mixes a solution of citric acid with solid sodium hydrogencarbonate in an insulated polystyrene cup fitted with a thermometer. The energy profile of the reaction is shown. Which combination is correct? A. the temperature of the solution rises;…

Methanol can be synthesized from carbon monoxide and hydrogen: $\ce{CO(g) + 2H2(g) -> CH3OH(l)}$ Reaction $\Delta H$ / $\mathrm{kJ\,mol^{-1}}$ $\ce{CH3OH(g) + 3/2 O2(g) -> CO2(g) + 2H2O(l)}$ $-764$ $\ce{CO(g) + 1/2 O2(g) -> CO2(g)}$ $-283$…

Which of these fuels comes from a renewable energy source? A. kerosene obtained from crude oil B. biodiesel produced from rapeseed oil C. lignite (brown coal) D. hydrogen produced from natural gas

Which processes in the Born–Haber cycle for magnesium oxide are endothermic? I. Enthalpy of formation of magnesium oxide, $\ce{Mg(s) + 1/2 O2(g) -> MgO(s)}$ II. Second electron affinity of oxygen, $\ce{O^-(g) + e^- -> O^2-(g)}$ III. Lattice enthalpy of…

A reaction mixture is moving towards equilibrium, and the forward reaction is proceeding faster than the reverse reaction. Which statement is correct? A. $Q > K$ and $\Delta G > 0$ B. $Q < K$ and $\Delta G < 0$ C. $Q$ decreases towards zero while $K$ remains…

$10.0\ \mathrm{cm^3}$ of a gaseous hydrocarbon, $\ce{C_xH_y}$, is burned completely in exactly the required volume of oxygen. The gaseous products occupy $70.0\ \mathrm{cm^3}$. This falls to $30.0\ \mathrm{cm^3}$ once the water vapour present condenses, with…

In the energy profile of a reaction shown below, P, Q and R are energies measured from the same baseline. Which expressions give the activation energy of the forward reaction, $E_a$, and the enthalpy change of the forward reaction, $\Delta H$? A. $E_a = Q$;…

Raising the temperature of a reaction mixture by 10 °C can double the rate of reaction, even though the collision frequency increases by only a few per cent. What is the main reason for the large increase in rate? A. The activation energy of the reaction is…

Which expression correctly gives the equilibrium constant for the following reaction? $\ce{2NO(g) + Cl2(g) <=> 2NOCl(g)}$ A. $K_c = \dfrac{[\ce{NOCl}]^2}{[\ce{NO}]^2[\ce{Cl2}]}$ B. $K_c = \dfrac{[\ce{NO}]^2[\ce{Cl2}]}{[\ce{NOCl}]^2}$ C.…

The energy profile of a reaction that occurs in two steps is shown. Which statement is correct? A. Q is a transition state, and the first step is rate-determining. B. Q is an intermediate, and the first step is rate-determining. C. Q is an intermediate, and…

The equation for a reaction between gases X and Y is: $\ce{2X(g) + Y(g) <=> 2Z(g)}$ At equilibrium at 700 K the concentrations of X, Y and Z are $0.50\ \mathrm{mol\,dm^{-3}}$, $0.40\ \mathrm{mol\,dm^{-3}}$ and $1.0\ \mathrm{mol\,dm^{-3}}$ respectively.…

Which reaction is expected to have the largest Arrhenius factor, $A$, at a given temperature? A. $\ce{Cl2(g) + Br2(g) -> 2BrCl(g)}$ B. $\ce{Cl(g) + CH4(g) -> HCl(g) + CH3(g)}$ C. $\ce{C2H4(g) + HBr(g) -> C2H5Br(g)}$ D. $\ce{Cl(g) + Br(g) -> BrCl(g)}$

Which reactions involve the transfer of a proton? I. $\ce{CH3COOH(aq) + NH3(aq) -> CH3COONH4(aq)}$ II. $\ce{2CH3COOH(aq) + Mg(s) -> (CH3COO)2Mg(aq) + H2(g)}$ III. $\ce{CH3COOH(aq) + NaHCO3(s) -> CH3COONa(aq) + H2O(l) + CO2(g)}$ A. I and II only B. I and III…

An indicator, HIn, is a weak acid with $\mathrm{p}K_a = 5.0$. HIn is yellow and its conjugate base, $\ce{In-}$, is blue. $\ce{HIn(aq) <=> H+(aq) + In^-(aq)}$ Which statements are correct? I. At pH 2.0 the indicator appears yellow. II. At pH 5.0 the…

The overall reaction during the discharge of a lead–acid battery is: $\ce{Pb(s) + PbO2(s) + 2H2SO4(aq) -> 2PbSO4(s) + 2H2O(l)}$ Which statement is correct? A. The oxidation state of lead in $\ce{Pb(s)}$ changes from +2 to 0. B. The oxidation state of sulfur…

In a redox titration, dichromate(VI) ions are reduced to chromium(III) ions and iron(II) ions are oxidized to iron(III) ions. $\ce{Cr2O7^2-(aq)}$ reduced to $\ce{Cr^3+(aq)}$ $\ce{Fe^2+(aq)}$ oxidized to $\ce{Fe^3+(aq)}$ What volume of…

Two observations are made about the halon $\ce{CF2ClBr}$ in the stratosphere. Observation 1 Observation 2 Ultraviolet light breaks $\ce{CF2ClBr}$ down to give bromine radicals rather than chlorine or fluorine radicals. A single bromine radical can destroy many…

Which term cannot describe the chloride ion, $\ce{Cl-}$? A. Lewis base B. ligand C. Brønsted–Lowry acid D. nucleophile

Which halogenoalkane undergoes nucleophilic substitution with aqueous hydroxide ions at the lowest rate? A. 1-bromopropane B. 1-chloropropane C. 1-fluoropropane D. 1-iodopropane

1-Bromobutane reacts with aqueous sodium hydroxide to form butan-1-ol. Which statement about this reaction is correct? A. A carbocation intermediate is formed. B. The hydroxide ion attacks the bromine atom. C. The $\ce{C-Br}$ bond breaks before the nucleophile…

Benzene reacts with the electrophile $\ce{Br+}$, generated from bromine and iron(III) bromide, by electrophilic substitution. Which structure correctly represents the intermediate formed when $\ce{Br+}$ bonds to the ring? A. structure A B. structure B C.…