3 G belongs to a group of compounds called ethers. [Figure 3.1] (a) When G, C4H10O, is heated, thermal decomposition occurs. C4H10O(g) → C₂H₄(g) + CO(g) + CH₄(g) ΔΗ = −7.00 kJ mol-1 (i) The atoms in a molecule of CO are held together by a triple covalent bond. One of these bonds is a coordinate (dative covalent) bond. Draw a dot-and-cross diagram to show the arrangement of outer electrons in a CO molecule. Use • to represent electrons from an oxygen atom. Use × to represent electrons from a carbon atom. (ii) Calculate the bond energy of C=O using the bond energy values in Table 3.1 and the enthalpy change, ΔH, for the thermal decomposition of G. Show your working. Table 3.1 | bond | bond energy/kJmol-1 | |---|---| | C-C | 350 | | C-O (in G) | 360 | | C-H | 410 | bond energy (C=O) = (iii) When G, C4H10O, is heated in a sealed container, an equilibrium mixture is produced. C4H10O(g) ⇌ C₂H₄(g) + CO(g) + CH₄(g) Complete the expression for the equilibrium constant, Kc, for this reaction. State the units of Kc. Kc = units = (iv) Thermal decomposition of G in the presence of I₂ affects the activation energy, Ea, for the reaction. Table 3.2 shows Ea for the thermal decomposition of G with and without I₂. Table 3.2 | reaction | Ea (with I₂)/kJ mol-1 | Ea/kJ mol-1 | |---|---|---| | C4H10O(g) → C₂H₃(g) + CO(g) + CH₄(g) | 143 | 224 | State what effect adding I₂ to the reaction mixture has on the value of Kc. Explain your answer. (v) Fig. 3.2 shows the Boltzmann distribution of energies for molecules of G at constant temperature, T°C. Sketch, on Fig. 3.2, the Boltzmann distribution of energies for molecules of G at a higher temperature, (T+100)°C. [Figure 3.2] (b) The functional group in G is an oxygen atom bonded to two carbon atoms. [Figure 3.3] G, H and J are structural isomers with molecular formula C4H10O. H and J are straight chain molecules. Table 3.3 shows the boiling points and reactions of G, H and J when heated under reflux with excess acidified K2Cr2O7. Table 3.3 | | G | H | J | |---|---|---|---| | boiling point/°C | 35 | 117 | 98 | | heat under reflux with excess acidified K2Cr2O7 | remains orange | orange to green | orange to green | (i) Identify the type of structural isomerism shown between G and H using the information in Table 3.3. (ii) Identify the type of structural isomerism shown between H and J using the information in Table 3.3. (iii) Draw a possible structure for H and for J. State the systematic name for each structure. [Space for H and J structures and names] (c) K has molecular formula C3H6O. When K is added to 2,4-dinitrophenylhydrazine, an orange precipitate forms. When K is warmed with Tollens' reagent, a silver mirror forms. Draw the displayed formula of K. [Space for K structure]