1 Entropy

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Oxford Textbook Pages : 362 - 364

CGP Revision Guide Pages : 148 - 149

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Show Specification Ref : 5.2.2a, 5.2.2b, 5.2.2c

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Question 1 / 5

January 2010 QP – Unit 5 AQA Chemistry A-level_question_5 the simplified diagram below shows how the entropy of ammonia varies with temperature at a pressure of kpa in this diagram ammonia is a solid at point a and a gas at point f areas outside the box will not be scanned for marking s j k mol temperature k a state why the entropy value for ammonia is equal to zero at k mark b explain in terms of the movement of particles why the entropy value increases between points a and b on the diagram mark c temperature t is marked on the diagram what does the value of this temperature represent mark d explain why there is a large entropy change between points d and e on the diagram marks wmp jan chem e an equation for the reaction in the haber process is shown below together with some entropy data areas outside the box will not be scanned for marking n g h g nh g h kj mol s j k mol n g h g nh g e i calculate a value for the entropy change s for the formation of one mole of ammonia marks e ii give the equation that relates free energy change g to enthalpy change h and entropy change s use this equation to calculate the temperature at which the value of g for the formation of ammonia in the haber process if you have been unable to calculate an answer to part e i you may assume that s j k mol but this is not the correct value equation calculation marks extra space e iii what can you deduce about the formation of ammonia if the reaction mixture is heated to a temperature above the value that you have calculated in part e ii mark turn over wmp jan chem
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Question 2 / 5

January 2011 QP – Unit 5 AQA Chemistry A-level_question_2 do not write outside the box the balance between enthalpy change and entropy change determines the feasibility of a reaction the table below contains enthalpy of formation and entropy data for some elements and compounds hf kj mol s j k mol n g o g no g c graphite c diamond a explain why the entropy value for the element nitrogen is much greater than the entropy value for the element carbon graphite marks b suggest the condition under which the element carbon diamond would have an entropy value of zero mark c write the equation that shows the relationship between g h and s for a reaction mark d state the requirement for a reaction to be feasible mark wmp jan chem do not write outside the box e consider the following reaction that can lead to the release of the pollutant no into the atmosphere n g o g no g use data from the table on page to calculate the minimum temperature above which this reaction is feasible marks f at temperatures below the value calculated in part e decomposition of no into its elements should be spontaneous however in car exhausts this decomposition reaction does not take place in the absence of a catalyst suggest why this spontaneous decomposition does not take place mark g a student had an idea to earn money by carrying out the following reaction c graphite c diamond use data from the table on page to calculate values for h and s for this reaction use these values to explain why this reaction is not feasible under standard pressure at any temperature h s explanation marks turn over wmp jan chem
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Question 3 / 5

January 2013 QP – Unit 5 AQA Chemistry A-level_question_3 do not write outside the box the feasibility of a physical or a chemical change depends on the balance between the thermodynamic quantities of enthalpy change h entropy change s and temperature t a suggest how these quantities can be used to predict whether a change is feasible marks b explain why the evaporation of water is spontaneous even though this change is endothermic in your answer refer to the change in the arrangement of water molecules and the entropy change marks question continues on the next page turn over wmp jan chem c this table contains some thermodynamic data for hydrogen oxygen and water do not write outside the box s j k mol hf kj mol h g o g h o g h o i c i calculate the temperature above which the reaction between hydrogen and oxygen to form gaseous water is not feasible marks c ii state what would happen to a sample of gaseous water that was heated to a temperature higher than that of your answer to part c i give a reason for your answer what would happen to gaseous water reason marks wmp jan chem do not write outside the box d when hydrogen is used as a fuel more heat energy can be obtained if the gaseous water formed is condensed into liquid water use entropy data from the table in part c to calculate the enthalpy change when one mole of gaseous water is condensed at k assume that the free energy change for this condensation is zero marks turn over for the next question turn over wmp jan chem
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Question 4 / 5

June 2012 QP – Unit 5 AQA Chemistry A-level_question_3 do not write outside the box some thermodynamic data for fluorine and chlorine are shown in the table in the table x represents the halogen f or cl electronegativity electron affinity kj mol enthalpy of atomisation kj mol enthalpy of hydration of x g kj mol fluorine chlorine a explain the meaning of the term electron affinity marks b explain why the electronegativity of fluorine is greater than the electronegativity of chlorine marks extra space c explain why the hydration enthalpy of the fluoride ion is more negative than the hydration enthalpy of the chloride ion marks wmp jun chem do not write outside the box d the enthalpy of solution for silver fluoride in water is kj mol the hydration enthalpy for silver ions is kj mol d i use these data and data from the table to calculate a value for the lattice enthalpy of dissociation of silver fluoride marks d ii suggest why the entropy change for dissolving silver fluoride in water has a positive value mark d iii explain why the dissolving of silver fluoride in water is always a spontaneous process marks turn over wmp jun chem
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Question 5 / 5

June 2013 QP – Unit 5 AQA Chemistry A-level_question_3 a figure shows how the entropy of a molecular substance x varies with temperature do not write outside the box figure l t k l o s j k mol a i explain in terms of molecules why the entropy is zero when the temperature is zero kelvin marks extra space a ii explain in terms of molecules why the first part of the graph in figure is a line that slopes up from the origin marks extra space wmp jun chem do not write outside the box a iii on figure mark on the appropriate axis the boiling point tb of substance x mark a iv in terms of the behaviour of molecules explain why l is longer than l in figure marks extra space question continues on the next page turn over wmp jun chem do not write outside the box b figure shows how the free energy change for a particular gas phase reaction varies with temperature figure g kj mol t k b i explain with the aid of a thermodynamic equation why this line obeys the mathematical equation for a straight line y mx c marks b ii explain why the magnitude of g decreases as t increases in this reaction mark b iii state what you can deduce about the feasibility of this reaction at temperatures lower than k mark wmp jun chem do not write outside the box c the following reaction becomes feasible at temperatures above k h o g h g o g the entropies of the species involved are shown in the following table s j k mol h o g h g o g c i calculate the entropy change s for this reaction mark c ii calculate a value with units for the enthalpy change for this reaction at k if you have been unable to answer part c i you may assume that the value of the entropy change is j k mol this is not the correct value marks turn over wmp jun chem
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