Enthalpy Change & Hess's Law

Transcription

1 Enthalpy hange & Hess's Law Question Paper 1 Level International Level Subject hemistry Exam oard IE Topic hemical Energetics Sub-Topic Paper Type Enthalpy hange & Hess's Law Multiple hoice ooklet Question Paper 1 Time llowed: Score: 51 minutes /42 Percentage: /100 Grade oundaries: * E U >85% 777.5% 70% 62.5% 57.5% 45% <45%

2 Section For each question there are four possible answers,,,, and. hoose the one you consider to be correct. 1 Which quantity gives the best indication of the relative strengths of the hydrogen bonds between the molecules in liquid hydrogen halides? bond dissociation energies enthalpy changes of formation enthalpy changes of solution enthalpy changes of vaporisation 2 The standard enthalpy changes of combustion of glucose and ethanol are given as 2820 and 1368 kj mol 1 respectively. Glucose, 6 H 12 O 6, can be converted into ethanol. 6 H 12 O 6 (s) 2 2 H 5 OH(l) + 2O 2 (g) What is the standard enthalpy change for this reaction? 1452 kj mol 1 84 kj mol kj mol kj mol 1

3 3 Enthalpy changes that are difficult to measure directly can often be determined using Hess Law to construct an enthalpy cycle. Which enthalpy change is indicated by X in the enthalpy cycle shown? (s) + 2H 2 (g) + 2O 2 (g) X H 4 (g) + 2O 2 (g) O 2 (g) + 2H 2 O(l) 4 the enthalpy of combustion of hydrogen +4 the enthalpy of combustion of hydrogen 2 the enthalpy of formation of water +2 the enthalpy of formation of water 4 student mixed 25.0 cm 3 of mol dm 3 sodium hydroxide solution with 25.0 cm 3 of mol dm 3 hydrochloric acid. The temperature rose by ssume that no heat was lost to the surroundings. The final mixture had a specific heat capacity of 4.20 J cm 3 K 1. What is the molar enthalpy change for the reaction? 150 kj mol kj mol kj mol kj mol 1

4 5 The diagram shows the skeletal formula of cyclopropane. The enthalpy change of formation of cyclopropane is kj mol 1 and the enthalpy change of atomisation of graphite is +717 kj mol 1. The bond enthalpy of H H is 436 kj mol 1 and of H is 410 kj mol 1. What value for the average bond enthalpy of the bond in cyclopropane can be calculated from this data? 187 kj mol kj mol kj mol kj mol 1 6 Ethanol is increasingly being used as a fuel for cars. The standard enthalpy change of formation of carbon dioxide is 393 kj mol 1. The standard enthalpy change of formation of water is 286 kj mol 1. The standard enthalpy change of formation of ethanol is 277 kj mol 1. What is the standard enthalpy change of combustion of ethanol? 1921 kj mol kj mol kj mol kj mol 1 7 In an experiment to calculate the enthalpy change of combustion of a fuel, 1.5 g ( mol) of the fuel was used to heat 200 g of water. The temperature of the water rose from 25 to 55. The specific heat capacity of water is 4.18 J g 1 K 1. There is significant heat loss in this experiment. Therefore, the experimental value for the enthalpy change of combustion, H c, of the fuel will be different from the theoretical value. Using the information above, what is the experimental value for the enthalpy change of combustion, H c, of the fuel? 1410 kj mol kj mol kj mol kj mol 1

5 8 For which equation is the enthalpy change correctly described as an enthalpy change of formation? (g) + O 2 (g) O 2 (g) (s) + 21 O 2 (g) O(g) 2N(g) + 4O(g) N 2 O 4 (g) 2NO(g) + O 2 (g) 2NO 2 (g) 9 The reaction pathway for a reversible reaction is shown below. energy / kj mol extent of reaction Which statement is correct? The activation energy of the reverse reaction is +80 kj mol 1. The enthalpy change for the forward reaction is +30 kj mol 1. The enthalpy change for the forward reaction is +50 kj mol 1. The enthalpy change for the reverse reaction is +30 kj mol Use of the ata ooklet is relevant to this question. This question should be answered using bond enthalpy data. The equation for the complete combustion of methane is given below. H 4 + 2O 2 O 2 + 2H 2 O What is the enthalpy change of combustion of methane? 1530 kj mol kj mol kj mol kj mol 1

6 11 Use of the ata ooklet is relevant to this question. student carried out an experiment to determine the enthalpy change for the combustion of methanol. The following results were obtained by the student. start temperature of the water final temperature of the water mass of alcohol burner before burning mass of alcohol burner after burning mass of glass beaker plus water mass of glass beaker g g g g How much of the heat energy produced by the burning of methanol went into the water? 209 J J J J 12 The equation for a reaction is shown. 1 H 2 (g) + 2 O 2 (g) H 2 O(l) ; H = x kj mol 1 Which pair of descriptions is fully correct for this reaction? type(s) of enthalpy change formation only formation only combustion, formation combustion, formation value of x positive negative positive negative

7 13 Propanone has the molecular formula 3 H 6 O. The enthalpy change of combustion of hydrogen is 286 kj mol 1. The enthalpy change of combustion of carbon is 394 kj mol 1. The enthalpy change of combustion of propanone is 1786 kj mol 1. Using this information, what is the enthalpy change of formation of propanone? 1106 kj mol kj mol kj mol kj mol 1 14 Slaked lime, a(oh) 2, may be made from limestone, ao 3. On heating in a lime kiln at 1000, limestone decomposes as follows. reaction 1 ao 3 (s) ao(s) + O 2 (g) Water is then reacted with calcium oxide, ao, as follows. reaction 2 ao(s) + H 2 O(l) a(oh) 2 (s) What are the enthalpy changes of these reactions?

8 15 Use of the ata ooklet is relevant to this question. reaction which causes the presence of oxides of nitrogen in car exhausts is the formation of NO. N2 + O 2 2NO H = +180 kj mol 1 What is the bond energy in kj mol 1 of the bond between the atoms in NO? In the table below, + means that this type of standard enthalpy change can only have positive values, means that this type of standard enthalpy change can only have negative values, + / means that either positive or negative values are possible. Which row is correct? atomisation formation solution / + + / / + / + 17 student calculated the standard enthalpy change of formation of ethane, 2 H 6, using a method based on standard enthalpy changes of combustion. He used correct values for the standard enthalpy change of combustion of ethane ( 1560 kj mol 1 ) and hydrogen ( 286 kj mol 1 ) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was 158 kj mol 1. What did he use for the standard enthalpy change of combustion of carbon? 1432 kj mol kj mol kj mol kj mol 1

9 18 Which process could be used to calculate the bond energy for the covalent bond X-Y by dividing its H by n? XY n (g) X(g) + ny(g) 2XY n (g) 2XY n-1 (g) + Y 2 (g) Y(g) + XY n-1 (g) XY n (g) nxy(g) nx(g) + n Y 2 (g) 2 19 Ethyne, 2 H 2, completely combusts, as shown in the equation. 1 H H O O H 2 O + 2O 2 Using the average bond enthalpies in the table, what is the enthalpy change of combustion of ethyne? bond average bond enthalpy / kj mol 1 H O O 49 O 74 O H 46 O kj mol kj mol kj mol kj mol 1 20 The equation below represents the combination of gaseous atoms of non-metal X and of hydrogen to form gaseous X 2 H 6 molecules. 2X(g) + 6H(g) X 2 H 6 (g) H = 2775 kj mol 1 The bond energy of an X H bond is 395 kj mol 1. What is the bond energy of an X X bond? kj mol kj mol kj mol kj mol 1

10 21 50 cm 3 of 2.50 mol dm 3 hydrochloric acid was placed in a polystyrene beaker of negligible heat capacity. Its temperature was recorded and then 50 cm 3 of 2.50 mol dm 3 NaOH at the same temperature was quickly added, with stirring. The temperature rose by 17. The resulting solution may be considered to have a specific heat capacity of 4.2 J g 1 K 1. What is an approximate value for the molar enthalpy change of neutralisation of hydrochloric acid and sodium hydroxide from this experiment? (50 x 4.2 x17) 1 J mol (0.050 x 2.5) (50 x 4.2 x17) 1 J mol (0.10 x 2.5) (100 x 4.2 x17) 1 J mol (0.050 x 2.5) (100 x 4.2 x17) 1 J mol (50 x 2.5) 22 In the conversion of compound X into compound Z, it was found that the reaction proceeded by way of compound Y, which could be isolated. The following steps were involved. Which reaction profile fits these data? X Y ; H, positive Y Z ; H, negative energy X Y Z energy X Y Z energy X Y Z energy X Y Z progress of reaction progress of reaction progress of reaction progress of reaction

11 23 The standard enthalpy change for the reaction 2NF 3 (g) 2N(g) + 6F(g) is H o = kj What is the bond energy of the N F bond? 556 kj mol kj mol kj mol kj mol 1 24 Which reaction is endothermic? 2Hr H 2 + r 2 N 2 + 3H 2 2NH 3 2SO 2 + O 2 2SO 3 SO 3 + H 2 O H 2 SO 4 25 Hess s Law can be used to calculate the average -H bond energy in methane. = standard enthalpy change of atomisation = standard enthalpy change of formation = standard enthalpy change of combustion Which data values are needed in order to perform the calculation? (), (H), 4) (), (H 2 ), ( 4) (), (H 2 ), ( 4) (H 4 ) only, as (), and (H 2 ), are defined as zero

12 26 Some bond energy values are listed below. These bond energy values relate to the following four reactions. P Q R S 2 2r 2 l l l H 3 l 4 H 3 + H What is the order of enthalpy changes of these reactions from most negative to most positive? P Q R S Q R S P R Q P S S P Q R 27 Given the following enthalpy changes, I2(g) + 3l2(g) 2Il 3 (s) H o = 214 kj mol 1 I2(s) I 2 (g) H o = +38 kj mol 1 What is the standard enthalpy change of formation of iodine trichloride, Il 3 (s)? +176 kj mol 1 88 kj mol kj mol kj mol 1

13 28 Equations involving four enthalpy changes are shown. Na(g) Na + (g) + e Na(g) Na 2+ (g) + 2e Na(s) Na(g) Na(s) Na 2+ (g) + 2e H = W H = X H = Y H = Z What is the second ionisation energy of sodium? 2W X W Y W Z Y 29 Enthalpy changes of combustion can be used to determine enthalpy changes of formation. The following equation represents the enthalpy change of formation of butane. 4(s) + 5H 2 (g) 4 H 10 (g) y using the following standard enthalpy of combustion data, what is the value of the standard enthalpy change of formation,, for this reaction? 5883 kj mol kj mol kj mol kj mol 1 30 Which equation represents the change corresponding to the enthalpy change of atomisation of iodine? 1 I 2 2 (g) I(g) I 2 (g) 2I(g) 1 I 2 2 (s) I(g) I 2 (s) 2I(g)

14 31 In a calorimetric experiment 1.60 g of a fuel is burnt. 45 % of the energy released is absorbed by 200 g of water whose temperature rises from 18 to 66. The specific heat capacity of water is 4.2 J g 1 K 1. What is the total energy released per gram of fuel burnt? 25 2 J J J J 32 Hydrogen peroxide slowly decomposes into water and oxygen. The enthalpy change of reaction can be calculated using standard enthalpies of formation. (hydrogen peroxide(l)) = kj mol 1 (water(l)) = kj mol 1 Using a Hess cycle, what is the enthalpy change of reaction for this decomposition? +98 kj mol 1 2H 2 O 2 (l) 2H 2 O(l) + O 2 (g) 98 kj mol kj mol kj mol 1

15 33 The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation. 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) Using the following standard enthalpy change of formation data, what is the value of the standard enthalpy change, H o, for this reaction? kj mol kj mol kj mol kj mol 1 34 Pl 5 dissociates as follows. Pl 5 (g) Pl 3 (g) + l 2 (g) The extent of dissociation is 13 % at 160 and 100 % at 300. Which pair of statements about this formation of Pl 3 is correct? shape of Pl 3 molecule pyramidal pyramidal trigonal trigonal the reaction is endothermic exothermic endothermic exothermic

16 35 For which equation does the enthalpy change correspond to the enthalpy change of atomisation of iodine? ½ I 2 (s) I(s) ½ I 2 (s) I(g) I 2 (g) 2I(g) I 2 (s) 2I(g) 36 Titanium occurs naturally as the mineral rutile, TiO 2. One possible method of extraction of titanium is to reduce the rutile by heating with carbon. TiO 2 (s) + 2(s) Ti(s) + 2O(g) The standard enthalpy changes of formation of TiO 2 (s) and O(g) are 940 kj mol kj mol 1 respectively. and What is the standard enthalpy change of this reaction? 830 kj mol kj mol kj mol kj mol 1 37 Which quantity would best indicate the relative strengths of the hydrogen bonds between the molecules in liquid hydrogen halides? bond dissociation energies enthalpy changes of solution enthalpy changes of formation enthalpy changes of vaporisation

17 Section For each of the questions in this section, one or more of the three numbered statements 1 to 3 may be correct. ecide whether each of the statements is or is not correct (you may find it helpful to put a tick against the statements that you consider to be correct). The responses to should be selected on the basis of 1, 2 and 3 are correct 1 and 2 only are correct 2 and 3 only are correct 1 only is correct No other combination of statements is used as a correct response. 38 Which statements are correct for all exothermic reactions? H for the reaction is negative. On a reaction pathway diagram the products are shown lower than the reactants. The reaction will happen spontaneously. 39 Use of the ata ooklet is relevant to this question. The bond energy of the r O bond is 235 kj mol 1. Which reactions are exothermic? OH + Hr H 2 + ro OH + Hr H 2 O + r H + Hr H 2 + r

18 40 The reaction pathway for a reversible reaction is shown below. 50 energy / kj mol 1 20 Which statements are correct? extent of reaction The enthalpy change for the backward reaction is 20 kj mol 1. The forward reaction is endothermic. The activation energy for the forward reaction is +70 kj mol The diagram represents the oltzmann distribution of molecular energies at a given temperature. number of molecules energy Which of the factors that affect the rate of a reaction can be explained using such a oltzmann distribution? increasing the concentration of reactants increasing the temperature the addition of a catalyst

19 42 For which reactions does the value of H o represent both a standard enthalpy change of combustion and a standard enthalpy change of formation? 1 2 (s) + O 2 (g) O 2 (g) 2(s) + O 2 (g) 2O(g) 1 3 O(g) + 2 O 2 (g) O 2 (g)