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Thermochem Quick review

Thermochem Quick review. General Thermochemistry Terms Thermodynamics - Study of NRG changes & flow of NRG. Tells whether a reaction is possible . system - That portion of the universe on which we focus . surroundings - Everything outside the system .

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Thermochem Quick review

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  1. Thermochem Quick review

  2. General Thermochemistry Terms • Thermodynamics -Study of NRG changes & flow of NRG. • Tells whether a reaction is possible. • system - That portion of the universe on which • we focus. • surroundings- Everything outside the system. • Isothermal- A change that occurs at constant • temperature. • state functions Properties that depend on the initial and • final state, not on how the change was made. ex: DH, temp, but not work

  3. Energy changes can be measured using calorimetry. Often this involves heating water under controlled conditions (a bomb calorimeter). Three closely related terms are: 1. heat capacity is the amount of heat needed to change a system by 1°C. 2. molar heat capacity is the amount of heat needed to change a mole of a substance by 1°C. 3. specific heat is the amount of heat needed to change 1 gram of a substance by 1°C. (Water: 1 cal/g°C = 4.184 J/g°C) Note that heat capacity is an extensive property whereas the other two are intensive.

  4. Coffee cup calorimeter q=mcpDT

  5. Enthalpy • Heat content of a substance • Symbol H • Unit kilojoules/mole • D H : Hproducts -Hreactants

  6. Hess’ Law If several reactions add up to give an overall reaction, the DH’s of the reactions will add up to the overall DH. • Standard Heats of Formation, DHf ° , are useful for this • purpose. This is the energy involved in making one moleof a • substance from its elements at 25°C and 1 atm pressure. • This law is often written as: • DH° reaction = DHf ° products - DHf ° reactants • Note: DHf° for elements is 0. • If you are NOT using heats of formation, you need to write • out the equations to see how they combine.

  7. BOND ENERGY • Bond energy is the amount of energy needed to BREAK a • certain bond. • Breaking a bond is always endothermic • You can determine the approximate energy change in a • chemical reaction by adding the bond energies of the • reactants and subtracting from this the bond energies of the products. Reactants-products • Note: this is opposite to the Hess’s Law (products - • reactants) because bond energy involves breaking bonds • whereas DH°f involve forming bonds.

  8. Entropy • Symbol is S (DS) • Unit J/K mol or J.mol-1K-1 • The degree of disorder (randomness) in a system • Spontaneity tends toward +DS • The entropy of a pure crystalline substance at absolute zero is 0. • Larger and more complex molecules have greater entropies

  9. FREE ENERGY • There is an upper limit to how much work can be derived • from any system. That maximum work is called the Gibb’s • Free Energy, DG. • The theoretical maximum work can be achieved if the steps • are so small that they can go either way, if they are • reversible.

  10. DG = DH – TDS • This represents the theoretical maximum work that can be done by a system. • A reaction is spontaneous when DG is negative. • When DG = 0, the reaction is at equilibrium. • MAKE SURE THAT YOUR DH AND DS HAVE THE SAME UNITS (CONVERT J IN DS INTO kJ • DH DS Spontaneous… • - + … at all temperatures • - - … at LOW temperatures • + + … at HIGH temperatures • + - ..reverse rxn spontaneous

  11. MC 1 CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) For the reaction of methane represented above, H is -1,523 kJ mol-1. What is the value of H if the combustion produced liquid water H2O(l), rather than water vapor, H2O(g)? (H for the phase change H2O(g) H2O(l) is -44 kJ mol-1). a. -1,479 kJ/mol b. -1,567 kJ/mol c. -1,323 kJ/mol d. -1,611 kJ/mol e.-1,411 kJ/mol

  12. MC 2 • Which gives the thermodynamic parameters of the phase change in a system that begins as an open container of liquid water placed in a constant temperature environment of 230 K? • G < 0 • H < 0 • S < 0 • I only • III only • I and III only • II and III only • I, II and III

  13. MC 3 • Which gives the thermodynamic parameters of the phase change in a system that begins as an open container of liquid water placed in a constant temperature environment of 230 K? • G < 0 • H < 0 • S < 0 • I only • III only • I and III only • II and III only • I, II and III

  14. MC 4 I2(g) + 3 Cl2(g) 2 ICl3(g) According to the data in the table below, what is the value of Hof for the reaction represented above? Average Bond Energy Bond(kilojoules/mol) I-I 150 Cl-Cl 240 I-Cl 210 • +450 kJ/mol • +1,260 kJ/mo • -870 kJ/mol • -390 kJ/mol • +180 kJ/mol

  15. MC 5 H2(g) + O2(g) H2O(l)Ho = x 2 Na(s) + O2(g) Na2O(s) Ho= y Na(s) + O2(g) + H2(g)NaOH(s) Ho= z Based on the information above, what is the standard enthalpy change for the following reaction? Na2O(s) + H2O(l) 2 NaOH(s) • x + y + z • x + y - z • x + y - 2z • 2z - x - y • z - x - y

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