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Important concepts, Example

Important concepts, Example 1. 100 ml of water has a temp of 30 °C. When ammonium nitrate is added to it the temp becomes 27 °C. Is the reaction exothermic or endothermic? The temp you measure is the surroundings , NOT the ammonium nitrate itself.

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Important concepts, Example

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  1. Important concepts, Example • 1. 100 ml of water has a temp of 30°C. When ammonium nitrate is added to it the temp becomes 27°C. Is the reaction exothermic or endothermic? • The temp you measure is the surroundings, NOT the ammonium nitrate itself. • Temp drops, showing heat is absorbed from the surroundings, hence reaction is endothermic. • Enthalpy change for bond breaking is positive, as it is endothermic. • Enthalpy change for bond making is negative, as it is exothermic.

  2. Students tend to be confused when it comes to calculations and are not sure whether to include sign. Here are some tips: • If questions ask for enthalpy change (H)/heat of reaction or energy involved, then answer must include sign, even if answer is positive. • If questions ask for energy given out or energy taken in, then no sign is required.

  3. Answers to Energy Change • MCQ • D • C • B • B • C • A

  4. Section B, Question 1 • exothermic; endothermic • Given out; more; taken in • Endothermic; exothermic • Combustion

  5. Section B, Question 2 • a) • NH4NO3 -10 (Final temp minus initial temp) • CaCl2 +20 • (sign must be included, question says “temp change”) • Endothermic • When ammonium nitrate dissolves in water, the temperature of solution decreases, showing that heat is absorbed from the surroundings. • (Note you are measuring the temp of solution, which is the surroundings, not ammonium nitrate itself. Remember the temp you detect is always from the surroundings.)

  6. Section B, Question 3 • Mass of 1 mol of CH4 = 1x (12+4x1) = 16 g • 16 g of CH4 releases 890 kJ of energy when burnt. • Hence 0.32 g of methane release 0.32/16 x 890 • = 17.8 kJ • *b) Bond breaking takes in energy while bond making gives out energy. • The energy given out during bond making is more than the energy taken in during bond breaking. • Thus reaction is overall exothermic. • (CANNOT write energy “used” in bond making is …; the term used is not appropriate here, should be energy given out during bond making)

  7. Section B, Question 3 c) • 4 sigma bonds, 0 pi bonds. • Recall sigma bonds are single bond and there are 4 C-H single bonds in CH4 • Methane has a • simple molecular structure. • Weak induced dipole-induced dipole forces/dispersion forces exist between the • molecules of methane. • Hence only a small amount of energy is needed to overcome these weak forces and methane has a low boiling point.

  8. Section B, Question 3 *f) The second member of alkane would have the following formula: C2H6 The boiling point of second member would have a higher boiling point than CH4 as it has a larger molecular size and has more electrons per molecule. Thus the induced dipole-induced dipole/dispersion forces between the molecules would be stronger and more energy would be needed to overcome these forces. Hence the 2nd member has a higher boiling point. (Recall strength of induced dipole-induced dipole forces increases with increasing no. of electrons per molecule!)

  9. Misconceptions in students’ answers - Section B, Question 3 *f) The second member of alkane would have the following formula: C2H6 The boiling point of second member would have a higher boiling point than CH4 as it has more covalent bonds. Hence more energy is needed to overcome the greater no. of bonds. Concept seriously wrong!!!!!! During boiling of simple molecular substances, it is the intermolecular forces such as induced dipole-induced dipole forces that is overcome, NOT covalent bonds!

  10. Question 4 • C-H bond, Cl-Cl bond • C-H bond, C-Cl bond, H-Cl bonds • 4 mol C-H bonds and 3 mol Cl-Cl bonds are broken • Hence energy involved in bond breaking • = 4 x 412 + 3 x 242 = +2374 kJ • (When question is phrased this way: energy involved, it is not clear whether energy is taken in or given out, hence sign must be shown here.)

  11. Question 4 • d) 1 mol C-H bond, 3 mol C-Cl bond and 3 mol H-Cl bonds are made • Energy involved in bond making • = -412 + 3 x (-338) + 3 x (-431) = -2719 kJ. (sign important) •  H = 2374 + (-2719) = -345 kJ (sign important) • Exothermic (since  H is negative)

  12. Question 5 Always start by writing the equation using structural formulae Bonds broken: 8 mol C-H bonds, 1 mol of C=C bond,, 2 mol C-C bonds, 2 mol O-H bonds Bonds made: 9 mol C-H bonds, 3 mol C-C bond, 1 mol C-O bond, 1 mol O-H bond

  13. Section B, Question 5 H for bond breaking=8(412) + 612 + 2(348)+ 2(463) = +5530 kJ (Unit here is kJ, NOT kJ/mol) H for bond making = 9(-412) +3 (-348) + (-360) + (-463) = -5575 kJ (Unit here is kJ, NOT kJ/mol) H = 5530 + (-5575) = - 45.0 kJ

  14. Question 6 Potential energy Activation energy Fe2O3 + 3CO H = -27 kJ 2Fe + 3CO2 Progress of reaction MUST label axes! Arrow pointing in the correct direction must be shown for activation energy and enthalpy change. Be specific in reactants and products, as equation is given to you!!

  15. Section B, Question 7 • An exothermic reaction is a reaction which gives out energy to the surroundings • A rocket has to be propelled at very great speed and • hence the reaction has to be fast to supply energy at a • fast rate. • c) 2H2O2 (l) + N2H4 (l)  N2 (g) + 4H2O (g) • d) No. of mol in 680 kg of H2O2 • = 680000/(2+2x16) = 20000 • [Must convert 680 kg to g before you divide by Mr] • 2 mol of H2O2 react with 1 mol of N2H4 • 20000 mol of H2O2 react with 10000 mol of N2H4 • Mass of N2H4 = 10000 x (2x14+4) = 320 000 g = 320 kg

  16. Question 7 e) f) Bonds broken: 4 mol O-H bonds, 2 mol of O-O bond, 1 mol N-N bonds, 4 mol N-H bonds Bonds made: 1 mol NN bond, 8 mol O-H bonds H for bond breaking = 4(463) + 2(142) + 163 + 4(388)=+3851 kJ H for bond making = -945 + 8 (-463) = -4649 kJ H = 3851 + (-4649) = -798 kJ

  17. Question 8 • a) No. The bond energy of a C-C bond is 348 kJ. Twice • the bond energy of a C-C bond would be 2x348 = 696 kJ. • The bond energy of a C=C bond is only 612 kJ and is less • than 696 kJ. • H = 4 x 412 = +1648 kJ (sign important)

  18. Question 8 c) Energy taken in during the breaking of bonds in 1 mol of H2 = 436 kJ (no need sign here, question stated ‘taken in’) Energy taken in during the breaking the bonds in 1 mol Cl2 = 242 kJ. (no need sign here, question stated ‘taken in’) Total amount of energy taken in = 436 + 242 = 678 kJ Energy given out during making of 2 mol of H-Cl bonds = 2x 431 = 862 kJ. (no need sign here, question stated ‘given out’) H = 678 – 862 = -184 kJ. (Sign important here as question asks for H)

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