Oct/Nov 2020

Which of the following statements is correct?

Atoms, molecules and stoichiometry

In aqueous solution, sulfuric acid dissociates as follows. $\text{H}_2\text{SO}_4 \rightarrow \text{HSO}_4^- + \text{H}^+$ (this reaction goes to completion) $\text{HSO}_4^- \rightleftharpoons \text{SO}_4^{2-} + \text{H}^+$ (this reaction reaches equilibrium with constant $K_c$) An analysis of a $2.00\,\text{mol dm}^{-3}$ solution of $\text{H}_2\text{SO}_4$ gave a $\text{HSO}_4^-$ concentration of $1.988\,\text{mol dm}^{-3}$. What is $K_c$?

Equilibria

An autocatalytic reaction is one where one of the products acts as a catalyst for the reaction. Which curve would be produced if the rate of an autocatalytic reaction were graphed against time?

Reaction kinetics

X and Y are two elements from Period 3 in the Periodic Table, and together they produce compound Z. X gives a soluble acidic oxide. In this oxide, the oxidation number of X is $+4$. Y gives an amphoteric oxide. What is the formula of compound Z?

The Periodic Table: chemical periodicity

This question concerns two elements in Group $2$, Q and R. Three of the statements given are true for metal Q. The remaining statement is true for metal R. Which statement applies to R?

Group 2

The electron arrangement of atoms from four elements is shown. Which element acts as the most powerful oxidising agent?

The Periodic Table: chemical periodicity

A student brings each pair of chemicals together in separate test-tubes. • excess calcium $(s)$ + water $(l)$ • barium chloride $(aq)$ + strontium hydroxide $(aq)$ • calcium carbonate $(s)$ + excess hydrochloric acid $(aq)$ • magnesium sulfate $(aq)$ + barium nitrate $(aq)$ For how many of these mixtures is a white solid formed?

Group 2

Which compound reacts with concentrated sulfuric acid in its roles as both a strong acid and an oxidising agent?

Electrochemistry

Ammonia can take part in an acid-base reaction with hydrogen chloride, producing ammonium chloride. Which statement is correct?

Equilibria

What patterns are observed in the stated properties as Group $2$ is descended from magnesium to barium?

Group 2

Sulfur dioxide, $\text{SO}_2$, reacts with calcium hydroxide in aqueous solution. What is the principal product formed first?

Nitrogen and sulfur

Strontium metal may be obtained from strontium oxide, $\text{SrO}$, by reducing it with aluminium. One possible equation is shown. $\text{6SrO} + \text{2Al} \rightarrow \text{3Sr} + \text{Sr}_3\text{Al}_2\text{O}_6$ What is the greatest mass of strontium metal that could be formed when $100\,\text{g}$ of strontium oxide is reduced by this reaction?

Atoms, molecules and stoichiometry

When answering this question, take both structural isomerism and stereoisomerism into account. The compounds X, Y and Z are shown. How many additional isomers of $\text{C}_3\text{H}_7\text{ClO}$ exist that are alcohols?

Hydroxy compounds

Two students each make a claim about 2-methylbut-1-ene. Student 1 says that 2-methylbut-1-ene has geometrical isomers. Student 2 says that 2-methylbut-1-ene undergoes an addition reaction with HBr to form 1-bromo-2-methylbutane as the major product. Which students are correct?

Hydrocarbons

Which statement is correct when PVC undergoes complete combustion?

Halogen compounds

Iodoethane, $\text{CH}_3\text{CH}_2\text{I}$, is heated with aqueous silver nitrate at $50^\circ\text{C}$. During this process, a precipitate is produced. Which row in the table gives the correct statement about this reaction?

Halogen compounds

A student changes 1-iodopropane, $\text{C}_3\text{H}_7\text{I}$, into butanoic acid, $\text{C}_3\text{H}_7\text{CO}_2\text{H}$, through a two-step chemical synthesis. Which reagent is used with 1-iodopropane in the first of the two steps?

Organic synthesis

1-chloro-1-methylcyclohexane is hydrolysed by heating with NaOH$(aq)$. The route of reaction is shown. In 1-chloro-1-methylcyclohexane, one carbon atom is attached to three other carbon atoms. What is the charge on this carbon atom at point Z?

Chemical bonding

An alcohol with the molecular formula $\text{C}_5\text{H}_{12}\text{O}$ decolourises warm acidified potassium manganate$(VII)$. This alcohol also produces a yellow precipitate with alkaline aqueous iodine. What is the alcohol likely to be?

Hydroxy compounds

Which pair of test results would show that substance X is a ketone?

Carbonyl compounds

Carvone occurs in spearmint oil. Which product is produced when carvone is reacted with NaBH$_4$?

Carbonyl compounds

Which compound is chiral and, on treatment with Na$_2$CO$_3$, gives off CO$_2$?

Carboxylic acids and derivatives

A lone $^ {32}\text{P}$ nucleus may be formed if one $^ {32}\text{S}$ nucleus combines with particle $X$. During this reaction, a proton is released. Which particle is $X$?

Atomic structure

The skeletal formula for compound X is displayed. What molecular formula does compound X have?

Atoms, molecules and stoichiometry

Nitrogen can form several oxides. The enthalpies of formation are listed below. $\Delta H_f^\circ(\text{NO}(g)) = +90\,\text{kJ mol}^{-1}$ $\Delta H_f^\circ(\text{N}_2\text{O}(g)) = +82\,\text{kJ mol}^{-1}$ $\Delta H_f^\circ(\text{NO}_2(g)) = +33\,\text{kJ mol}^{-1}$ Which of the following statements are true?

Chemical energetics

Which of the following statements are correct?

Atoms, molecules and stoichiometry

For an equilibrium reaction, the units of $K_c$ are $\text{mol}^{-1}\,\text{dm}^3$. Which equation might describe the equilibrium?

Equilibria

Methanol, CH$_3$OH, may be made on an industrial scale by reacting CO with H$_2$. $\text{CO}(g) + 2\text{H}_2(g) \rightleftharpoons \text{CH}_3\text{OH}(g) \quad \Delta H = -91\,\text{kJ mol}^{-1}$ This synthesis can be run at $4 \times 10^3\,\text{kPa}$ and 1150 K. Which of the statements about this reaction are correct?

Equilibria

Which rows accurately display the relative electrical conductivities of the three Period 3 element sets?

Chemical bonding

Three test-tubes, X, Y and Z, each contain water. A small quantity of NaCl is added to test-tube X. A small quantity of SiCl$_4$ is added to test-tube Y. A small quantity of AlCl$_3$ is added to test-tube Z. After a brief interval, two drops of universal indicator solution are put into each test-tube. Which of the following statements could be true?

The Periodic Table: chemical periodicity

The structure of compound R is shown. Which of the following statements about compound R are correct?

An introduction to AS Level organic chemistry

When methane is brominated, the free radical CH$_3\cdot$ is formed. One possible termination step in this reaction is the creation of C$_2$H$_6$ when two free radicals join together. What could be formed in a termination step in the bromination of propane?

Reaction kinetics

Three reactions of primary alcohols are given. Which of these reactions produces water as one of the products?

Hydroxy compounds

For which of the following substances, when they are in the liquid state, do the particles experience only intermolecular forces due to temporary dipoles?

Chemical bonding

The diagram illustrates a section of the structure of polymer X. Which reagents react with polymer X?

Chemical bonding

Copper melts at a high temperature. What causes copper to have a high melting point?

Chemical bonding

Which two standard enthalpy changes have the same numerical value?

Chemical energetics

An energy cycle is shown for this reaction. $\text{Br}_2(\text{l}) + 3\text{F}_2(\text{g}) \rightarrow 2\text{BrF}_3(\text{l})$ The standard enthalpy of formation of $\text{BrF}_3(\text{l})$ is $-301\,\text{kJ mol}^{-1}$. The enthalpy change from $\text{BrF}_3(\text{l})$ to $\text{BrF}_3(\text{g})$ is $+44\,\text{kJ mol}^{-1}$. What is the average bond energy of the Br-F bond in $\text{BrF}_3$?

Chemical energetics

In which reaction is the change in sulfur's oxidation number the greatest?

Nitrogen and sulfur

During the initial step of the chloride process used to produce titanium, the reaction below takes place. $2\text{TiO}_2 + 4\text{Cl}_2 + 3\text{C} \rightarrow 2\text{TiCl}_4 + 2\text{CO} + \text{CO}_2$ In this reaction, what is reduced?

Electrochemistry

What is the mean oxidation number of sulfur in each compound?

Electrochemistry

Sulfur dioxide and oxygen react to form sulfur trioxide. This reaction can proceed in both directions. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g)$ $K_p = 2.96 \times 10^{-9}\ \text{Pa}^{-1}$ at $700\,^{\circ}\text{C}$. The mixture is left to reach equilibrium at $700\,^{\circ}\text{C}$. The partial pressure of O$_2$(g) is $375\ \text{kPa}$ and the partial pressure of SO$_3$(g) is $20.3\ \text{kPa}$. What is the partial pressure of SO$_2$(g)?

Equilibria

The Haber process is used to produce ammonia. The reactants are nitrogen and hydrogen. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \qquad \Delta H \text{ is negative}$ Which change would increase the rate of the forward reaction?

Reaction kinetics

Element X is in Period 3 and shows these properties. • Its oxide has a giant structure. • It makes covalent bonds with chlorine. • Its oxide can neutralise $\text{HCl}(aq)$. Which element is X?

The Periodic Table: chemical periodicity

Which row could describe barium metal and barium hydroxide?

Group 2

$\text{AgNO}_3(aq)$ is introduced into a solution containing a halide ion, $\text{X}^-(aq)$, and then aqueous ammonia is added. The ionic equations for the two reactions that take place are shown. $\text{Ag}^+(aq) + \text{X}^-(aq) \rightleftharpoons \text{AgX}(s)$ (equilibrium 1) $\text{Ag}^+(aq) + 2\text{NH}_3(aq) \rightleftharpoons \text{Ag(NH}_3)_2^+(aq)$ (equilibrium 2) Which statement is correct?

Equilibria

Water and ammonia react to form the ammonium ion. Which statement about this reaction is correct?

Equilibria

Because lithium and magnesium have closely matched ionic radii, lithium and magnesium, together with their respective compounds, show very similar reactions. Which statement about the reactions of lithium, or of its compounds, could be predicted from this?

The Periodic Table: chemical periodicity

Which of the following statements about Group 17 elements and their compounds is correct?

Group 17

Which substance, when heated with aqueous ammonium chloride, would generate an alkaline gas?

Nitrogen compounds

Compound X is an oxide of an element in Period 3. When Compound X is added to water, an acidic solution is formed. A solution is made by reacting $0.100\,\text{g}$ of compound X with an excess of water. Exactly $25.0\,\text{cm}^3$ of $0.100\,\text{mol dm}^{-3}$ sodium hydroxide solution is required to neutralise this solution. What is the possible identity of compound X?

Atoms, molecules and stoichiometry

An ore of manganese is made up of $4\%$ by mass of MnO$_2$ and contains no other manganese compound. What mass of manganese could be obtained from $1$ tonne of this ore?

Atoms, molecules and stoichiometry

The unsaturated hydrocarbon octa-1,3,5,7-tetraene, $\text{C}_8\text{H}_{10}$, is capable of showing geometric isomerism. octa-1,3,5,7-tetraene $\text{CH}_2=\text{CHCH}=\text{CHCH}=\text{CHCH}=\text{CH}_2$ How many isomers are possible?

Chemical bonding

What is the correct name of the compound displayed?

Atoms, molecules and stoichiometry

This polymer has the repeat unit shown below. It is formed from two different monomers. $\left( {-\text{CH}_2-\text{CHCl}-\text{CH}_2-\text{CH}=\text{CH}-\text{CH}_2-} \right)_n$ Which two monomers could be combined to produce this polymer?

Chemical bonding

Compound X has two functional groups. compound X: $\text{CH}_3-\text{C}(=\text{O})-\text{CHO}$ Which reagent will react with just one of the functional groups?

Carbonyl compounds

Ethanol may be changed into ethene in one reaction step. Ethanol may be changed into bromoethane in one reaction step. At standard laboratory conditions, is a catalyst needed in these reactions?

Hydroxy compounds

Diols in which both hydroxy groups are attached to the same carbon atom may lose a molecule of water spontaneously to form a carbonyl compound. Which compound produces a silver mirror with Tollens’ reagent after complete hydrolysis?

Carbonyl compounds

Which alcohol produces a yellow precipitate when it is warmed with alkaline aqueous iodine?

Hydroxy compounds

How many structural isomers of molecular formula $\text{C}_5\text{H}_{10}\text{O}$ produce a red precipitate with Fehling’s solution?

Carbonyl compounds

Which of the following statements about the use of alkane fuels in internal combustion engines is correct?

Chemical energetics

The structure of compound Z is shown. What product is formed in good yield when compound Z is reacted with an excess of sodium carbonate solution at room temperature?

Carboxylic acids and derivatives

Which atomic orbitals are filled in a phosphorus atom?

Atomic structure

The infra-red spectrum shown was collected for compound G. What is compound G likely to be?

Chemical bonding

The symbol for a phosphorus ion is $^{33}_{15}\text{P}^{3-}$. The symbol for a potassium ion is $^{37}_{19}\text{K}^+$. What do these two ions share in common?

Atomic structure

The repeat unit of a polymer is shown. Which kinds of intermolecular forces are present in the solid polymer?

Chemical bonding

The diagram shows the enthalpy changes for a group of reactions. Which of the following statements is correct?

Chemical energetics

Which of the following statements about a reaction at dynamic equilibrium are correct?

Equilibria

Group 2 elements and the compounds they form display patterns in their physical and chemical properties. Barium is located above radium in Group 2 of the Periodic Table. Which of the following statements are likely to be correct?

Group 2

Which of the statements are correct?

Atoms, molecules and stoichiometry

Which reagents could be employed, under suitable conditions, to oxidise $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$ to $\text{CH}_3\text{CH}_2\text{CHO}$?

Electrochemistry

During a free-radical substitution reaction between chlorine and ethane, which free radicals can be formed?

Halogen compounds

Which substances possess the molecular formula $\text{C}_4\text{H}_8\text{O}$?

Atoms, molecules and stoichiometry

The diagram shows the structure of compound A. In compound A, some carbon atoms are arranged tetrahedrally around their bonds. Other carbon atoms in compound A have a trigonal planar bond arrangement. How many carbon atoms are there of each type?

Chemical bonding

Which reactions can form pentanoic acid?

Carboxylic acids and derivatives

It is known that a white powder is a mixture of magnesium oxide and aluminium oxide. $100\ \text{cm}^3$ of $2\ \text{mol dm}^{-3}$ NaOH(aq) is exactly sufficient to dissolve the aluminium oxide in $x$ grams of the mixture. The equation for this reaction is: $\text{Al}_2\text{O}_3 + 2\text{OH}^- + 3\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_4^-$ $800\ \text{cm}^3$ of $2\ \text{mol dm}^{-3}$ HCl(aq) is exactly sufficient to dissolve all the oxide in $x$ grams of the mixture. The equations are shown: $\text{Al}_2\text{O}_3 + 6\text{H}^+ \rightarrow 2\text{Al}^{3+} + 3\text{H}_2\text{O}$ $\text{MgO} + 2\text{H}^+ \rightarrow \text{Mg}^{2+} + \text{H}_2\text{O}$ What number of moles of each oxide are contained in $x$ grams of the mixture?

Atoms, molecules and stoichiometry

A graph showing $pV$ against $T$ is given for a fixed mass of gas ($p$ = pressure, $V$ = volume and $T$ = temperature in K). Which gas produces this graph across the broadest range of temperatures and pressures?

States of matter

A weather balloon contains $12.0\ \text{kg}$ of helium. When the balloon is at a height of $20\ \text{km}$, the pressure inside it is $6000\ \text{Pa}$ and the temperature is $216\ \text{K}$. Calculate the volume of the weather balloon at this height, correct to three significant figures.

States of matter

Which two enthalpy changes will always have the same sign, that is, both will always be exothermic or both will always be endothermic?

Chemical energetics

Chlorine dioxide, ClO$_2$, undergoes reaction with sodium hydroxide as shown below. $2\text{ClO}_2 + 2\text{OH}^- \rightarrow \text{ClO}_2^- + \text{ClO}_3^- + \text{H}_2\text{O}$ Which statement gives the correct description of this redox reaction?

Electrochemistry

Which of the following statements is correct?

Atoms, molecules and stoichiometry

When sulfuric acid is in aqueous solution, it dissociates as follows. $\text{H}_2\text{SO}_4 \rightarrow \text{HSO}_4^- + \text{H}^+$ (this reaction goes to completion) $\text{HSO}_4^- \rightleftharpoons \text{SO}_4^{2-} + \text{H}^+$ (this reaction reaches equilibrium with constant $K_c$) An analysis of a $2.00\,\text{mol dm}^{-3}$ solution of $\text{H}_2\text{SO}_4$ showed that the $\text{HSO}_4^-$ concentration was $1.988\,\text{mol dm}^{-3}$. What is $K_c$?

Equilibria

An autocatalytic reaction is one where a product acts as a catalyst for the reaction. What shape of curve would be produced when the rate of an autocatalytic reaction is plotted against time?

Reaction kinetics

X and Y are two elements from Period 3 of the Periodic Table. They react together to produce compound Z. X produces a soluble acidic oxide. The oxidation number of X in this oxide is $+4$. Y produces an amphoteric oxide. What is the formula of compound Z?

The Periodic Table: chemical periodicity

This question concerns two elements in Group 2, Q and R. Three of the statements shown are true for metal Q. The single statement left is true for metal R. Which statement is correct for R?

Group 2

The electronic configurations of atoms of four elements are shown. Which element is the most powerful oxidising agent?

Electrochemistry

A student combines pairs of chemicals in separate test-tubes: • excess calcium (s) + water (l) • barium chloride (aq) + strontium hydroxide (aq) • calcium carbonate (s) + excess hydrochloric acid (aq) • magnesium sulfate (aq) + barium nitrate (aq) In how many of the mixtures is a white solid formed?

Atoms, molecules and stoichiometry

Which compound reacts with concentrated sulfuric acid in its roles as a strong acid and as an oxidising agent?

Electrochemistry

Ammonia can take part in an acid-base reaction with hydrogen chloride to produce ammonium chloride. Which of the statements below is correct?

Equilibria

What patterns are observed in the stated properties as Group 2 is descended from magnesium to barium?

Group 2

Sulfur dioxide, $\text{SO}_2$, reacts with calcium hydroxide in aqueous solution. Which main product is initially produced?

Nitrogen and sulfur

Strontium metal may be obtained from strontium oxide, $\text{SrO}$, by reduction with aluminium. One possible reaction is shown below. $6\text{SrO} + 2\text{Al} \rightarrow 3\text{Sr} + \text{Sr}_3\text{Al}_2\text{O}_6$ What is the greatest mass of strontium metal that can be made from the reduction of $100\,\text{g}$ of strontium oxide by means of this reaction?

Atoms, molecules and stoichiometry

Both structural isomerism and stereoisomerism need to be taken into account for this question. The compounds X, Y and Z are displayed. How many additional isomers of $\text{C}_3\text{H}_7\text{ClO}$ are alcohols?

Hydroxy compounds

Two students each make a claim about 2-methylbut-1-ene. Student 1 says that 2-methylbut-1-ene has geometrical isomers. Student 2 says that 2-methylbut-1-ene undergoes an addition reaction with HBr to produce 1-bromo-2-methylbutane as the major product. Which students are correct?

Hydrocarbons

Which statement is correct when PVC undergoes complete combustion?

Halogen compounds

Iodoethane, $\text{CH}_3\text{CH}_2\text{I}$, is heated with aqueous silver nitrate at $50^\circ\text{C}$. As the reaction proceeds, a precipitate appears. Which row in the table is the correct one for this reaction?

Halogen compounds

A student changes 1-iodopropane, $\text{C}_3\text{H}_7\text{I}$, into butanoic acid, $\text{C}_3\text{H}_7\text{CO}_2\text{H}$, using a two-step chemical synthesis. During the first of these two stages, which reagent is used to react with 1-iodopropane?

Chemical bonding

Heating 1-chloro-1-methylcyclohexane with $\text{NaOH(aq)}$ causes hydrolysis. The reaction scheme is shown below. A carbon atom in 1-chloro-1-methylcyclohexane is attached to three other carbon atoms. What charge is on this carbon atom at point Z?

Chemical bonding

An alcohol having the molecular formula $\text{C}_5\text{H}_{12}\text{O}$ turns warm acidified potassium manganate(VII) from colourless. It also produces a yellow precipitate with alkaline aqueous iodine. What could the alcohol be?

Hydroxy compounds

Which two test results would show that substance X is a ketone?

Carbonyl compounds

Carvone occurs in spearmint oil. Which product is produced when carvone is treated with NaBH$_4$?

Carbonyl compounds

Which compound is chiral and will react with Na$_2$CO$_3$ to release CO$_2$?

Carboxylic acids and derivatives

One $^{32}_{15}\text{P}$ nucleus may be formed when a $^{32}_{16}\text{S}$ nucleus combines with particle $X$. During this process, a proton is released. What is particle $X$?

Atomic structure

The skeletal structure of compound X is shown. What is the molecular formula of compound X?

An introduction to AS Level organic chemistry

Nitrogen can form several oxides. Their standard enthalpies of formation are listed as: $\Delta H_f^\circ[\text{NO}(g)] = +90\,\text{kJ mol}^{-1}$, $\Delta H_f^\circ[\text{N}_2\text{O}(g)] = +82\,\text{kJ mol}^{-1}$, $\Delta H_f^\circ[\text{NO}_2(g)] = +33\,\text{kJ mol}^{-1}$. Which of these statements are correct?

Chemical energetics

Which of the statements below are correct?

Atoms, molecules and stoichiometry

For an equilibrium reaction, the units of $K_c$ are $\text{mol}^{-1}\,\text{dm}^3$. Which equation could represent the equilibrium?

Equilibria

Methanol, CH$_3$OH, can be manufactured on an industrial scale by reacting CO with H$_2$: $\text{CO}(g) + 2\text{H}_2(g) \rightleftharpoons \text{CH}_3\text{OH}(g)$, $\Delta H = -91\,\text{kJ mol}^{-1}$. The process may be carried out at $4 \times 10^3\,\text{kPa}$ and $1150\,\text{K}$. Which of the statements about this reaction are correct?

Equilibria

Which rows correctly display the relative electrical conductivities of the three Period 3 element sets?

The Periodic Table: chemical periodicity

Three test tubes, X, Y and Z, are each filled with water. A small quantity of NaCl is then put into test tube X. A small quantity of SiCl$_4$ is then put into test tube Y. A small quantity of AlCl$_3$ is then put into test tube Z. After a brief period, two drops of universal indicator solution are added to each test tube. Which of these statements could be correct?

Equilibria

The structure of compound R is shown. Which statements concerning compound R are correct?

An introduction to AS Level organic chemistry

In the bromination of methane, the free radical CH$_3\!\cdot$ is formed. One termination step in this reaction is the creation of C$_2$H$_6$ when two free radicals combine. What could be formed in a termination step during the bromination of propane?

Reaction kinetics

Three reactions of primary alcohols are shown. Which of these reactions produces water as one of the products?

Hydroxy compounds

In which of the following substances, when they are in liquid form, are the only forces between the particles intermolecular forces due to temporary dipoles?

States of matter

The diagram illustrates a section of the structure of polymer X. Which reagents will react with polymer X?

Chemical bonding

Copper has a high melting point. Why does copper have a high melting point?

Chemical bonding

Which pair of standard enthalpy changes has the same numerical value?

Chemical energetics

An energy cycle diagram is shown for this reaction. $\text{Br}_2(l) + 3\text{F}_2(g) \rightarrow 2\text{BrF}_3(l)$ The standard enthalpy of formation of $\text{BrF}_3(l)$ is $-301\,\text{kJ mol}^{-1}$. The enthalpy change for $\text{BrF}_3(l)$ changing to $\text{BrF}_3(g)$ is $+44\,\text{kJ mol}^{-1}$. Calculate the average bond energy of the $\text{Br--F}$ bond in $\text{BrF}_3$?

Chemical energetics

In which reaction is the largest change in sulfur’s oxidation number observed?

Electrochemistry

In the chloride process used to manufacture titanium, the initial stage involves this reaction. $2\text{TiO}_2 + 4\text{Cl}_2 + 3\text{C} \rightarrow 2\text{TiCl}_4 + 2\text{CO} + \text{CO}_2$ What substance is reduced in this reaction?

Electrochemistry