CHEM 14B flash cards
48 cards for Thermodynamics, Electrochemistry and Kinetics, laid out week by week and timed around your exams. Free on iPhone and the web.
Exam dates
Unblank times your reviews around these. You can always set your own dates in the app.
- FinalWed, Mar 17, 2027A sample date until the Registrar posts the real one
The first 20 cards
Read the question, think of your answer, then open the card.
Week 1 · The first law and workWhat does the first law of thermodynamics say?Show the answerHide the answer
Energy is conserved: ΔU = q + w, where q is heat added to the system and w is work done on it.
This is the sign convention most chemistry courses use. Some physics texts write ΔU = q - w, with w as work done by the system.
Week 1 · The first law and workA gas absorbs 250 J of heat and does 100 J of work on its surroundings. What is ΔU?Show the answerHide the answer
+150 J. ΔU = q + w = (+250 J) + (-100 J); work done by the gas is negative.
Week 1 · The first law and workWhat is a state function?Show the answerHide the answer
A property that depends only on the system's current state, not on the path taken to reach it.
U, H, S and G are state functions. Heat and work are not: they depend on the path.
Week 1 · EnthalpyWhat is the enthalpy change, ΔH, of a reaction?Show the answerHide the answer
The heat the reaction absorbs or releases at constant pressure. Negative ΔH is exothermic; positive ΔH is endothermic.
Week 1 · EnthalpyFor 2H₂(g) + O₂(g) → 2H₂O(l), ΔH = -572 kJ. How much heat is released when 10.0 g of H₂ burns?Show the answerHide the answer
1.42 × 10³ kJ. 10.0 g ÷ 2.016 g/mol = 4.96 mol H₂, and each mole releases 572/2 = 286 kJ.
Week 2 · CalorimetryMixing 50.0 mL of 1.00 M HCl with 50.0 mL of 1.00 M NaOH raises the temperature 6.8 °C. What is ΔH per mole of water formed?Show the answerHide the answer
About -57 kJ/mol. q = (100. g)(4.184 J/(g·°C))(6.8 °C) = 2.8 kJ released, for 0.0500 mol of water.
This assumes the solution has water's density and specific heat, and the cup absorbs no heat.
Week 2 · CalorimetryWhy does a bomb calorimeter measure ΔU rather than ΔH?Show the answerHide the answer
Its sealed steel vessel has a fixed volume, so no expansion work is done and the heat measured equals ΔU.
A coffee-cup calorimeter is open to the air, at constant pressure, so it measures ΔH.
Week 2 · Hess's law and bond enthalpiesWhat does Hess's law say?Show the answerHide the answer
The ΔH of an overall reaction equals the sum of the ΔH values of any series of steps that add up to it.
Week 2 · Hess's law and bond enthalpiesGiven ΔHf° of CH₄(g) -74.8, CO₂(g) -393.5 and H₂O(l) -285.8 kJ/mol, what is ΔH° for CH₄ + 2O₂ → CO₂ + 2H₂O(l)?Show the answerHide the answer
-890.3 kJ. ΔH° = [-393.5 + 2(-285.8)] - [-74.8]: products minus reactants.
O₂(g) is an element in its standard state, so its ΔHf° is zero.
Week 2 · Hess's law and bond enthalpiesEstimate ΔH for H₂ + Cl₂ → 2HCl from bond enthalpies: H-H 436, Cl-Cl 242, H-Cl 431 kJ/mol.Show the answerHide the answer
-184 kJ. Bonds broken minus bonds formed: (436 + 242) - 2(431).
Breaking bonds costs energy and forming them releases it. Bond enthalpies are averages, so the result is an estimate.
Week 3 · Entropy and microstatesWhat is entropy, in terms of microstates?Show the answerHide the answer
A measure of how many microscopic arrangements (microstates) fit a system's state: S = k ln W.
k is Boltzmann's constant and W is the number of microstates.
Week 3 · Entropy and microstatesWhat is the sign of ΔS for 2SO₂(g) + O₂(g) → 2SO₃(g)?Show the answerHide the answer
Negative. Three moles of gas become two, so there are fewer ways to arrange the particles.
Week 3 · Entropy and microstatesWater's ΔH_vap is 40.7 kJ/mol at its boiling point, 373 K. What is ΔS_vap?Show the answerHide the answer
+109 J/(mol·K). At a phase change at equilibrium, ΔS = ΔH/T = 40,700 J/mol ÷ 373 K.
Week 3 · The second lawWhat does the second law of thermodynamics say?Show the answerHide the answer
In any spontaneous process, the entropy of the universe increases: ΔS_univ = ΔS_sys + ΔS_surr > 0.
Week 3 · The second lawA reaction releases 100. kJ of heat (ΔH = -100. kJ) at 298 K. What is ΔS of the surroundings?Show the answerHide the answer
+336 J/K, from ΔS_surr = -ΔH_sys/T = 100,000 J ÷ 298 K.
Week 4 · Gibbs free energy and spontaneityWhat is the Gibbs free energy change, ΔG?Show the answerHide the answer
ΔG = ΔH - TΔS. At constant temperature and pressure, a negative ΔG means the process is spontaneous.
Week 4 · Gibbs free energy and spontaneityWhy does a negative ΔG mean a process is spontaneous?Show the answerHide the answer
At constant temperature and pressure, ΔG = -TΔS_univ, so a negative ΔG means the universe's entropy increases.
ΔS_surr = -ΔH/T, so ΔS_univ = ΔS - ΔH/T, and multiplying by -T gives ΔH - TΔS.
Week 4 · Gibbs free energy and spontaneityA reaction has ΔH° = +178 kJ and ΔS° = +161 J/K. Above what temperature is it spontaneous?Show the answerHide the answer
Above about 1.11 × 10³ K (about 830 °C), where TΔS° overtakes ΔH°: T = 178,000 J ÷ 161 J/K.
These are close to the values for CaCO₃ → CaO + CO₂. The estimate assumes ΔH° and ΔS° barely change with temperature.
Week 4 · Gibbs free energy and spontaneityFor N₂ + 3H₂ → 2NH₃, ΔH° = -92.2 kJ and ΔS° = -198.7 J/K. What is ΔG° at 298 K?Show the answerHide the answer
-33.0 kJ. ΔG° = -92.2 kJ - (298 K)(-0.1987 kJ/K) = -92.2 + 59.2.
Week 4 · Gibbs free energy and spontaneityDoes a negative ΔG mean a reaction is fast?Show the answerHide the answer
No. ΔG says whether a reaction can happen, not how quickly. Diamond turning into graphite is spontaneous but immeasurably slow.
Speed is set by the activation energy, which thermodynamics says nothing about.
Week by week
The whole deck: 48 cards over 10 weeks. Every week is open from day one.
- 1Energy and enthalpy5 cards
- The first law and work 3
- Enthalpy 2
- 2Thermochemistry5 cards
- Calorimetry 2
- Hess's law and bond enthalpies 3
- 3Entropy5 cards
- Entropy and microstates 3
- The second law 2
- 4Gibbs free energy5 cards
- Gibbs free energy and spontaneity 5
- 5Free energy and equilibrium4 cards
- Free energy and equilibrium 4
- 6Redox and electrochemical cells5 cards
- Oxidation and reduction 2
- Galvanic and electrolytic cells 3
- 7Cell potential5 cards
- Standard cell potential and free energy 3
- The Nernst equation 2
- 8Rate laws3 cards
- Rates and rate laws 3
- 9Integrated rate laws5 cards
- Integrated rate laws and half-life 5
- 10Activation energy and mechanisms6 cards
- Activation energy and catalysis 4
- Reaction mechanisms 2