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Although the actual size of an atom cannot be measured exactly, it is possible to measure the distance between the nuclei of two atoms. For example, the 'covalent radius' of the Cl atom is assumed to be half of the distance between the nuclei in a Cl₂ molecule. Similarly, the 'metallic radius' is half of the distance between two metal atoms in the crystal lattice of a metal. These two types of radius are generally known as 'atomic radii'. The table below contains the resulting atomic radii for the elements of period three of the Periodic Table, Na to Cl.
[Table_1]
(a) (i) Explain qualitatively this variation in atomic radius.
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(ii) Suggest why it is not possible to use the same type of measurement for argon, Ar.
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(b) (i) Use the Data Booklet to complete the following table of radii of the cations and anions formed by some of the period three elements.
[Table_2]
(ii) Explain the differences in size between the cations and the corresponding atoms.
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(iii) Explain the differences in size between the anions and the corresponding atoms.
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(c) Each of the elements Na to Cl forms at least one oxide. Na₂O is an ionic oxide, SO₂ is a covalent oxide. Both oxides react with water.
(i) Write an equation for the reaction of each of these oxides with water.
Na₂O ..........................................................................................
SO₂ ..........................................................................................
(ii) What is the pH of the resulting solution in each case?
Na₂O ............ SO₂ ............
(iii) Write an equation for the reaction that occurs between the products of your reactions in (i).
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597
600
579
Washing soda is hydrated sodium carbonate, $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$.
A student wished to determine the value of $x$ by carrying out a titration, with the following results.
(a) (i) Write a balanced equation for the reaction between $\text{Na}_2\text{CO}_3$ and $\text{HCl}$.
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(ii) Calculate the amount, in moles, of $\text{HCl}$ in the 35.8 $\text{cm}^3$ of solution used in the titration.
(iii) Use your answers to (i) and (ii) to calculate the amount, in moles, of $\text{Na}_2\text{CO}_3$ in the 25.0 $\text{cm}^3$ of solution used in the titration.
(iv) Use your answer to (iii) to calculate the amount, in moles, of $\text{Na}_2\text{CO}_3$ in the 250 $\text{cm}^3$ of solution in the standard volumetric flask.
(v) Hence calculate the mass of $\text{Na}_2\text{CO}_3$ present in 5.13 g of washing soda crystals. [6]
(b) Use your calculations in (a) to determine the value of $x$ in $\text{Na}_2\text{CO}_3\cdot x\text{H}_2\text{O}$ [2]
582
582
510
With the prospect that fossil fuels will become increasingly scarce in the future, many compounds are being considered for use in internal combustion engines. One of these is DME or dimethyl ether, CH$_3$OCH$_3$. DME is a gas which can be synthesised from methanol. Methanol can be obtained from biomass, such as plant waste from agriculture.
(a) Define, with the aid of an equation which includes state symbols, the standard enthalpy change of combustion, $\Delta H_c^\circ$, for DME at 298 K.
equation .................................................................................................................
definition ..................................................................................................................
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(b) DME may be synthesised from methanol. Relevant enthalpy changes of formation, $\Delta H_f^\circ$, for this reaction are given in the table below.
[Table_1]
Use these values to calculate $\Delta H_{reaction}^\circ$ for the synthesis of DME, using the following equation. Include a sign in your answer.
2CH$_3$OH(l) $\rightarrow$ CH$_3$OCH$_3$(g) + H$_2$O(l)
$\Delta H_{reaction}^\circ$ = .................. kJ mol$^{-1}$ [2]
(c) DME and ethanol are isomers with the molecular formula C$_2$H$_6$O.
(i) Draw the displayed formula of DME and of ethanol.
(ii) What type of isomerism do DME and ethanol show?
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(d) DME is a gas at room temperature while ethanol is a liquid.
(i) Which intermolecular force exists between ethanol molecules, which causes ethanol to be a liquid at room temperature?
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(ii) Draw a diagram that clearly shows this intermolecular force. Your diagram should show any lone pairs or dipoles present that you consider to be important. You should represent at least two molecules in your diagram. [4]
507
501
568
But-2-ene, $\text{CH}_3\text{CH}=\text{CHCH}_3$, is an important compound which is obtained from the cracking of hydrocarbons present in crude oil.
(a) Give \textbf{two} different conditions under which long chain hydrocarbons may be cracked.
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(b) Dodecane, $\text{C}_{12}\text{H}_{26}$, is a long chain hydrocarbon which is present in crude oil and which can be cracked to form but-2-ene and an alkane.
Write a balanced equation for this reaction.
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(c) Some reactions of but-2-ene are shown below.
In the boxes below, give the \textbf{structural} formulae of the organic compounds $\text{A to D}$.
[4]
(d) (i) Draw the \textbf{skeletal} formula of compound $\text{D}$.
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(ii) By using the letters $\text{A to D}$ as appropriate, identify those compounds which are chiral. If there are none, write 'none'.
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(e) But-2-ene can be polymerised to give poly(butene).
Draw the \textbf{structural} formula of a portion of the polymer chain in poly(butene) showing \textbf{two} repeat units.
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(f) Compound $\text{C}$ is a liquid which can be reacted with concentrated sulfuric acid to give a gas, $\text{E}$, which will decolourise aqueous bromine when passed through it.
(i) Suggest the \textbf{structural} formula of $\text{E}$.
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(ii) Suggest the \textbf{structural} formula of the product of the reaction between $\text{E}$ and an excess of bromine.
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(iii) What \textbf{type of reaction} occurs between $\text{E}$ and an excess of bromine?
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501
552
565
Compound F is a white solid which has the molecular formula $C_3H_6O_3$.
Compound F is soluble in water. Addition of $NaHCO_3$ to this solution produces a colourless gas, G, which turns lime water milky.
(a) (i) What is the identity of the gas G?
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(ii) What functional group does this test show to be present in F?
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(b) When F is heated with concentrated sulfuric acid, a colourless liquid H is produced. When cold dilute acidified $KMnO_4$ is shaken with H, the solution becomes colourless.
(i) What type of reaction occurs when H is formed from F?
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(ii) Use your answers to (a)(ii) and (b)(i) to deduce the structural formula of the colourless liquid H.
(c) Compound F will react with sodium.
Calculate the volume of $H_2$, measured at room temperature and pressure, which will be produced when 0.600 g of F is reacted with an excess of Na.
(d) There are two structural isomers of F that give the reactions described in (a) and (b).
(i) Suggest two structural formulae for these isomers.
(ii) Isomers J and K can both be oxidised. What will be produced when each of the isomers J and K is heated under reflux with acidified $K_2Cr_2O_7$?
555
514
509
