5.1. Enthalpy change, \(\Delta H\)
A subsection of Chemistry, 9701, through 5. Chemical energetics
Listing 10 of 305 questions
Calcium nitride, Ca3N2 , reacts readily with water to form a white precipitate suspended in an alkaline solution. The oxidation number of nitrogen does not change during the reaction. Construct an equation for the reaction of Ca3N2 with water. The enthalpy changes of solution, , of the hydroxides of the Group2 elements become less endothermic down the group. State and explain the trend in the solubilities of the Group2 hydroxides. Complete the energy cycle to show the enthalpy changes that occur in the transformations between aqueous ions, gaseous ions and an ionic solid. On your diagram label each enthalpy change with its appropriate symbol; lattice energy, , enthalpy change of hydration, , or enthalpy change of solution, . Complete the three arrows showing the correct direction of each enthalpy change. aqueous ions gaseous ions ionic solid  The numerical value of the solubility product, Ksp, of CaF2 is 3.45×10–11 at 298 K. Write an expression for the solubility product of CaF2. Include its units. Ksp =  units =  Calculate the solubility of CaF2 at 298 K.  solubility = mol dm–3 
9701_w18_qp_42
THEORY
2018
Paper 4, Variant 2
View
Hydrogen iodide, HI, is a colourless gas at room temperature. Explain why HI has a higher boiling point than HCl and HBr. The bar chart shows the boiling points of HCl, HBr and HI. The boiling point of HF is not shown. boiling point / K HF HCl HBr HI Hydrogen bonds form between HF molecules. Draw a bar on the bar chart to predict the boiling point of HF. Explain your answer. The standard enthalpy change of formation, , of HIis +26.5 kJ mol–1. Define the term standard enthalpy change of formation. HIcan be formed by reacting H2with I2. The reaction is reversible, and an equilibrium forms quickly at high temperatures. H2+ I22HIConstruct an expression for the equilibrium constant, Kp, for the reaction of H2and I2to form HI. Kp =  The equilibrium partial pressures of the gases at 200 °C are as follows. pH2= 895 Pa pI2= 895 Pa pHI= 4800 Pa Calculate Kp for this reaction.  Kp = State how the value of Kp would change, if at all, if the reaction were carried out at 100 °C rather than 200 °C. Explain your answer. HI reacts with oxygen to form iodine and water. Construct an equation for the reaction of HI with oxygen. Explain, with reference to oxidation numbers, why this reaction is a redox reaction. HIcan also be formed by the reaction of I2with hydrazine, N2H4. 2I2+ N2H4→ 4HI+ N2State the change in pressure that would occur when 2 mol I2fully reacts with 1 mol N2H4in a sealed container at constant temperature. Explain your answer. In the laboratory, HIcan be formed in a two-step process. step 1 3I2+ 2P→ 2PI3step 2 PI3+ 3H2O→ H3PO3+ 3HIDraw a ‘dot-and-cross’ diagram of a PI3 molecule.  Name the type of reaction in step2. H3PO3and HIare both strong Brønsted–Lowry acids. Give the meaning of the term strong Brønsted–Lowry acid. Give the formula of the conjugate base of H3PO3. HIreacts with propene, CH3CH=CH2to form a mixture of 1-iodopropane and 2-iodopropane. Identify which of 1-iodopropane and 2-iodopropane is the major product of this reaction. Explain your answer. Complete the diagram to show the mechanism of the reaction between HI and CH3CH=CH2 that forms the major product identified in . Include curly arrows, lone pairs of electrons and charges as necessary. C C H H H I H3C H + –  
9701_w21_qp_22
THEORY
2021
Paper 2, Variant 2
View
Calciumchloride, CaCl 2, is an ionic solid. The values of some energy changes are shown in Table1.1. Table 1.1 energy change value / kJ mol–1 lattice energy, , CaCl 2–2237 standard enthalpy change of atomisation of calcium +193 first ionisation energy of calcium +590 second ionisation energy of calcium +1150 standard enthalpy change of atomisation of chlorine +121 first electron affinity of chlorine –364 Define lattice energy. Use the data in Table1.1 to calculate the standard enthalpy change of formation, , of calciumchloride. It may be helpful to draw an energy cycle. Show all your working.  (CaCl 2) = kJ mol–1 Three possible values for the first electron affinity of bromine are shown in Table1.2. One of them is correct. Place a tick by the correct value. Explain your choice. Table 1.2 possible values place one tick (✓) in this column –342 kJ mol–1 –364 kJ mol–1 –386 kJ mol–1 explanation The enthalpy change of hydration of the chloride ion can be calculated using the lattice energy of calciumchloride and the data shown in Table1.3. Table 1.3 energy change value / kJ mol–1 standard enthalpy change of solution of CaCl 2–83 standard enthalpy change of hydration of Ca2+–1650 Define the following terms. enthalpy change of solution enthalpy change of hydration  Calculate the standard enthalpy change of hydration of the chloride ion, Cl –. It may be helpful to draw an energy cycle. Show all your working.  (Cl –) = kJ mol–1 Calciumfluoride, CaF2, can be synthesised directly from its elements. The value of (CaF2) is –1214 kJ mol–1. Predict the sign of the entropy change, ∆S o, for this synthesis. Explain your answer. The sign of the entropy change is . explanation  Use the value of (CaF2) given in and your answer to to predict how the feasibility for this synthesis will change with increasing temperature. 
9701_w22_qp_42
THEORY
2022
Paper 4, Variant 2
View
Phosphoric(acid, H3PO4, is used in both inorganic and organic reactions. H3PO4 is made in a two-step process from phosphorus. step 1 Phosphorus reacts with an excess of oxygen to form a white solid. step 2 The white solid then reacts with water to form H3PO4. Write an equation for each step. step 1 step 2 H3PO4 is a weak Brønsted–Lowry acid. Define weak Brønsted–Lowry acid. H3PO4 is also formed in the process shown in reaction 1. reaction 1 4H3PO3 3H3PO4 + PH3 Table 3.1 shows some relevant thermodynamic data. Table 3.1 compound enthalpy change of formation, ΔHf / kJ mol–1 H3PO3 –972 H3PO4 –1281 PH3 +9 Define enthalpy change of formation. Use the data in Table 3.1 to calculate the enthalpy change, ΔH r , of reaction 1. ΔH r = kJ mol–1 Explain why reaction 1 is a disproportionation reaction. Explain your reasoning with reference to relevant oxidation numbers. shows a reaction scheme that involves H3PO4 in several reactions. reaction 2 reaction 3 oxidation A and H3PO4 KBr and H3PO4 CH3COOH B OH O O O and H3PO4 Identify A, which reacts with propene in the presence of H3PO4 in reaction 2. Draw the structure of B. Name the type of reaction that occurs in reaction 3. Reaction 3 is monitored using infrared spectroscopy. It is not possible to use the O—H absorption frequency to monitor the reaction. Use Table 3.2 to identify a suitable bond whose absorption frequency can be used to monitor the progress of reaction 3. State the change you would see in the infrared spectrum during reaction 3. bond change in infrared spectrum Table 3.2 bond functional groups containing the bond characteristic infrared absorption range (in wavenumbers) / cm–1 C–O hydroxy, ester 1040–1300 C=C aromatic compound, alkene 1500–1680 C=O amide carbonyl, carboxyl ester 1640–1690 1670–1740 1710–1750 C–H alkane 2850–2950 H3PO4 also reacts with alcohols to form organophosphates. Organophosphates are compounds similar to esters. They have the general structure shown in . O O O P R = alkyl group R R R O Complete the equation to suggest the products of the reaction of H3PO4 with methanol, CH3OH. H3PO4 + 3CH3OH Compound T is a simple organophosphate. The mass spectrum of T shows a molecular ion peak at m / e = 182. This peak has a relative intensity of 12.7. The relative intensity of the M +1 peak is 0.84. Deduce the number of carbon atoms in T. Hence suggest the molecular formula of T. Assume that phosphorus and oxygen exist as single isotopes. Show your working. number of carbon atoms in T = molecular formula of T =
9701_w23_qp_22
THEORY
2023
Paper 2, Variant 2
View
Questions Discovered
305