Review Sheet – Chemistry, Level 3 –
Ch. 12 & 23: Thermochemistry & Thermodynamics
Name: ______Date:______Period:___
The student should be able:
· Communicate effectively with thermochemistry and thermodynamics vocabulary, including enthalpy, entropy, heat, specific heat, heat, thermal energy, and temperature.
· Compare exothermic to endothermic reactions, including by using energy diagrams.
· Calculate heats given off by reactions when provided with enthalpies for a reaction
· Calculate enthalpies of reaction from calorimetry data, Hess’s Law situations, and heats of formations.
· Describe the factors that cause entropy to increase or decrease.
· Analyze the spontaneity of a chemical reaction, including Gibbs free energy.
1. Define each of the following terms clearly and completely.
(a) enthalpy –
(b) exothermic –
(c) endothermic –
(d) standard state –
(e) Hess’s law –
(f) Specific heat –
(g) Calorimetry –
(h) Heats of formation
(i) entropy –
(j) spontaneous chemical reaction –
(k) Gibbs Free Energy –
(l) heat –
(m) temperature –
(n) thermal energy –
2. How much heat is required to change 63.4 grams of liquid water to steam (vaporize)? The DHvap of water is 40.7 kJ/mol.
3. Using the balanced equation below, how much heat is given off when the following react:
2 C2H6(g) + 7 O2(g) à 4 CO2(g) + 6 H2O(l) DH = - 2860 kJ
A. 5.60 g of C2H6
B. 7.04 L of O2 (@ STP)
4. Using Hess’s law, calculate the DH for the reaction:
CaCO3 à CO2 + CaO
from the following enthalpy changes:
Ca + C + 3/2 O2 à CaCO3 DH = - 1206.9 kJ
2 Ca + O2 à 2CaO DH = - 1270.2 kJ
C + O2 à CO2 DH = - 393.5 kJ
CaCO3à Ca + C + 3/2 O2 DH = + 1206.9 kJ
Ca + ½ O2 à CaO DH = - 635.1 kJ
C + O2 à CO2 DH = - 393.5 kJ
______
CaCO3 à CO2 + CaO DH = + 178.3 kJ
5. What is the specific heat of carbon tetrachloride (CCl4) if it requires 77 J of heat to raise the temperature of 6.5-g sample from 21°C to 35°C?
6. Using heats of formation, calculate the DH for the reaction:
C2H4(g) + 3 O2(g) à 2 CO2(g) + 2 H2O(l)
DH (kJ) +52.3 0 -393.5 -285.8
DH = [2(-393.5)+2(-285.8)] – [1(52.3)] = - 1410 kJ
7. Propane burns in oxygen according to the following equation: 2 C2H6 + 7 O2 à 4 CO2 + 6 H2O. Draw an enthalpy diagram for this reaction and label the enthalpies of the products, the reactants, and the change in enthalpy. How does this diagram tell you the reaction is exothermic?
See Slide # 17 on the Ch. 12 & 23 Powerpoint. It is very similar to that one.
8. Some reaction has a DH = -106 kJ and DS = + 58 J/K. Calculate its Gibbs free energy (DG) at 500 °C. Is this reaction spontaneous at this temperature?
DG is negative, so this reaction is spontaneous at this temperature.
9. Calculate the Gibbs free energy (DG) at 25°C for
3 H2(g) + CO(g) à CH4(g) + H2O(l)
A. Given that DH = -206 kJ and DS = - 214 J/K for this reaction. Is this reaction spontaneous at this temperature?
DG is negative, so this reaction is spontaneous at this temperature.
B. At what temperature would it go from being spontaneous to not-spontaneous?
DG =0 when the reaction changes from spontaneous to non-spontaneous, so solving 0=DG-TDS for T, we get:
10. Using the standard tables, calculate the DH°, DS°, and DG° at 25°C for the reaction:
A. 2 CCl4(l) + H2(g) à 2 CHCl3(l) + 2 Cl2(g)
NOT POSSIBLE (DG and DS for CHCl3 are not available).
11. The explosion of dynamite, a mixture of nitroglycerin (liquid) and diatomaceous earth (solid), gives gaseous products such as water and carbon dioxide. Much heat is given off as well. Give the algebraic (+,-) signs of DH, DS, and DG. Explain your answers.
DH is negative because the explosion of dynamite is exothermic (heat is given off). DS is positive because gaseous products are produced from a liquid and solid reactant. DG is negative, because the reaction happens without constant outside intervention.
12. Why does Gibbs free energy determine a reaction’s spontaneity?
DG takes into account the three factors that determine spontaneity: enthalpy(DH), entropy(DS), and temperature.