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Thermochemistry

Learn about the relationship between chemical reactions and energy changes in thermochemistry. Calculate heat, work, enthalpy, and internal energy in different chemical systems.

jasonmartinez
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Thermochemistry

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  1. Thermochemistry The relationship between chemical reactions and energy changes is known as thermochemistry

  2. The relationship between chemical reactions and energy changes is known as thermochemistry 1 mv2 ek= 2

  3. The relationship between chemical reactions and energy changes is known as thermochemistry

  4. any energy entering the system carry positive (+) sign. Any energy leaving system, carry negative (-) sign.

  5. Heat flows from hotter to colder

  6. q = mcT

  7. question: How much heat is required to raise the temperature of 250g of water from 22 °C to 98 °C? (specific heat of water is 4.18 J g-1 K-1). q = (4.18 J g-1 K-1)*(250g)*(371-295 K) q = 79420 J

  8. determining the specific heat

  9. qlead = -qwater

  10. Question: a 150.0 g sample of lead is heated to 100 °C and added to 50.0g of water at 22.0 °C in thermally isolated beaker. the temperature of the system after lead addition changed to 28.8 °C, calculate the specific heat of lead.

  11. Question: a 150.0 g sample of lead is heated to 100 °C and added to 50.0g of water at 22.0 °C in thermally isolated beaker. the temperature of the system after lead addition changed to 28.8 °C, calculate the specific heat of lead. qwater = mcT = (50.0 g)(4.184 J/g °C)(28.8 - 22.0)°C qlead = -1.4x103 J = mcT = (150.0 g)(c)(28.8 - 100.0)°C clead = 0.13 Jg-1°C-1

  12. qrxn = -qcal

  13. Define the heat capacity of the calorimeter: qcal = miciT = CT qrxn = -qcal qcal = qbomb + qwater

  14. qrxn = -qcal Ccal= mwater x C water

  15. Question H2 + Cl2 → 2HCl Reaction studied in a bomb clorimeter when 1.00g sample of H2 reacts completely, the temperatures rises from 20.0 to 29.82. the heat capacity of calorimeter is 9.33kJ/°C, calculate the amount of heat evolved in the reaction Question: when 1.00g amonium nitrate dissolves in 50.0g of water in a coffee cup calorimeter. The temperature drops from 25.00 to 23.32 °C calculate q for the reaction system.

  16. heat effects chemical reactions may also do work. (Pressure- volume work.) Work is an energy transfer between a system and its surroundings. (Pressure- volume work.) Work = Force x distance F = P.A , w= P. A. h , ( V = (Vf-Vi) ) w = - P V negative sign shows that system loses energy.

  17. Question: how much work in joules is involved when 0.225 mol N2 at constant temperature of 23°C is allowed to expand 1.50L in volume againts an external pressure of 0.750atm w = - P V

  18. Internal energy (U) of a system is the total energy contained within the system. • Internal energy is a function of state depends on amount of matter, structure of matter, temperature and pressure. • A system contains only internal energy. • A system does not contain heat or work.These only occur during a change in the system.

  19. First law of thermodynamic in a chemical or physical change, energy can be exchanged between a system and its surroundings, but no energy can be created or destroyed. U = q + w or U = q - P V A system does not contain heat or work These only occur during a change in the system. • In a constant volume • U = qvqvis the quantity of heat at the constant volume

  20. any energy entering the system and work done on the system carry positive (+) sign. Any energy leaving system, and work done by system carry negative (-) sign.

  21. question: a gas expanding, absorbs 225 J of heat and does 243 J of work. What is the E for the gas?

  22. for a reaction, H =Hfinal – H initial or H = H(products) - H(reactants)

  23. When 1 mole butane is burned at 1 atm pressure, it makes 3kJ work and 2658kJ energy is produced. Calculate the enthalpy and E for the reaction of 1 mole butane.

  24. a reaction that gives off heat is an exothermic reaction and in this case H is negative CH4 + O2  CO2 + 2H2O H = -880.3 kJ • A reaction that absorb heat is an endothermic reaction. In this case H is positive. N2(g) + O2(g)  NO(g) H = + 180.5 kJ

  25. Properties of enthalpy: Enthalpy is an extensive property. The magnitude of DH is dependent upon the amounts of reactants consumed. Doubling the reactants, doubles the amount of enthalpy. N2(g) + O2(g) → 2 NO(g) H = +180.50 kJ ½N2(g) + ½O2(g) → NO(g) H = +90.25 kJ Reversing a chemical reaction results in the same magnitude of enthalpy but of the opposite sign. NO(g) → ½N2(g) + ½O2(g) H = -90.25 kJ the enthalpy change for a reaction depends upon the state of the reactants and products. The states (i.e. g, l, s or aq) must be specified.

  26. Calculate the internal energy changes of the following system at 25°C and 1atm • 2 CO + O2 → 2 CO2 ∆H= -566 kj

  27. Hess’s law, • if a reaction is carried out in a series of steps, ∆H for the reaction will be equal to the sum of the enthalpy changes for the individual steps. • the overall enthalpy change for the process is independent of the number of steps or the particular nature of the path by which the reaction is carried out.

  28. İf you reverse reaction equation, change the sign of its ∆ H, if you multiple a reaction equation, multiple ∆ H value by same factor. question: ½ N2(g) + ½ O2(g)  NO(g) ∆H1= + 90.25 kJ NO(g) + 1/2O2(g)  NO2(g) ∆H2= -57.07 kJ • What is the ∆ H of the reaction of ½ N2(g) + O2(g)  NO2(g)

  29. Standart enthalpies of formation The standard enthalpy of formation of a substance is the enthalpy change that occurs in the formation of 1mol of the substance from its elements. ∆ H0=  ∆ Hf (products) -  ∆ Hf(reactants)

  30. The standard enthalpy of formation of a pure element in its reference from is 0

  31. question: use the standard formation enthalpies from the table to calculate the standard enthalpy change for the reactions ? 3CH4 (g) + 2H2O (s) + CO2 (g)  4 CO (g) + 8 H2 (g) C2H4 (g) + H2O (l)  CH3CH2OH (g)∆H0 = ? H+(suda) + OH-(suda)  H2O (l) ∆H0 = ?

  32. question: • C2H4 (g) + H2O (l)  CH3CH2OH (g)∆H0 = ? • question: • H+(aq) + OH-(aq)  H2O (l) ∆H0 = ?

  33. question: Calcium carbide (CaC2) reacts with water to form acetylene (C2H2) a gas used as a fuel in welding. CaC2 (s) + H2O (l)  C2H2(g) + Ca(OH)2 (s)∆ H = -128,0kJ how many kilojoules of heat are evolved in the reaction of 3.5g CaC2 reacts with 125ml H2O? question: determine the enthalpy change for the oxidation of ammonia 4NH3(g) + 5O2(g)  4NO(g) + 6 H2O(l) from the following data • N2(g) + 3H2(g)  2 NH3(g)∆H = -92,22kJ • N2(g) + O2(g)  2NO(g) ∆H = +180,5kJ • 2H2(g) + O2(g)  2H2O (l) ∆H = -571,6kJ

  34. Dimethylhidrazine (N2H2(CH3)2) is used as a rocket fuel. When it reacts with oxygen, the thermochemical equation for the reaction is ½ N2H2(CH3)2(l) + 2O2(g) → CO2(g) + 2H2O(g) + ½ N2(g)∆H = -898,1kJ a- the heat formation of CO2(g) and H2O(g) are -393,5 kJ/mol and – 241,8 kJ/mol respectively. calculate the heat of formation of N2H2(CH3)2(l) (H:1,0 g/mol N: 14,0 g/mol O: 16,0 g/mol C: 12,0 g/mol) (R= 8,314J/mol K R=0,082 L atm /mol K R= 1,987 cal/mol K)

  35. Dimethylhidrazine (N2H2(CH3)2) is used as a rocket fuel. When it reacts with oxygen, the thermochemical equation for the reaction is ½ N2H2(CH3)2(l) + 2O2(g) → CO2(g) + 2H2O(g) + ½ N2(g)∆H = -898,1kJ b- how much heat is evolved when 1,00g N2H2(CH3)2(l) is burned in an open container? (H:1,0 g/mol N: 14,0 g/mol O: 16,0 g/mol C: 12,0 g/mol) (R= 8,314J/mol K R=0,082 L atm /mol K R= 1,987 cal/mol K)

  36. Dimethylhidrazine (N2H2(CH3)2) is used as a rocketfuel. When it reactswithoxygen, thethermochemicalequationforthereaction is ½ N2H2(CH3)2(l) + 2O2(g) → CO2(g) + 2H2O(g) + ½ N2(g) ∆H = -898,1kJ forthereaction ,∆H = qp= -898,1kJ calculatre ∆U at 25°C (H:1,0 g/mol N: 14,0 g/mol O: 16,0 g/mol C: 12,0 g/mol ) (R= 8,314J/mol K R=0,082 L atm /mol K R= 1,987 cal/mol K)

  37. 0,212 g sample of pure Na2C2O4 (Mw=134g/mol) was titrated with 43,31ml of KMnO4 what is the molarity of the KMnO4? Write the balanced eauation for this reaction? MnO4- + C2O42- + H+ → Mn2+ + CO2 Calculate the root mean square speed of H2 molecules at 20°C The spectral lines of hydrogen in infrared region represent electron transitions to the n=5 from higher levels. What is the electron transition that corresponds to the 3740nm spectral line? Rh=2.179x10-18 J h= 6,626x10-34Js c=3,0x108m/s Find the emprical and molecular formula of A molecule that has mass of 182 amu and a mass percent composition of 39,56 %C, 7,74%H and 52,70% O. In the Combustion of glucose, if the reaction is carried out in an expandable container at 35°C and 780 torr, what volume of CO2 is produced from 18,0g glucose and excess O2? C6H12O6 + 6O2 → 6CO2 + 6H2O An 8,07g Ag2O (88,3% by mass) decomposes into solid silver and O2(g). The O2(g) is collected over water at 25°C and 749,2mmHg pressure. The vapor pressure of water at 25°C is 23,8mmHg. What is the volume of gas collected? Rank the ions in each of the following sets in order of increasing size, and explain your ranking. a) Li+, K+, Na+ b) Se2-, Rb+, Br - c) O2-, F -, N 3- How much energy, in joule, must be absorbed to convert to K+ all atoms present in 10 mg of gaseous K? The first ionization energy of K is 418.8 kJ/mol.

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