Reactivity 1: What drives chemical reactions? practice questions.

Heating 100 g of water (c = 4.18 J/g/K) raises its temperature by 20 K. How much heat energy is absorbed? Use Q = mcΔT.

• 8.36 kJ
• 83.6 kJ
• 0.836 kJ
• 836 kJ

Answer: 8.36 kJ

Q = mcΔT = 100 × 4.18 × 20 = 8360 J = 8.36 kJ (divide by 1000 to convert J to kJ). Mass is in grams to match the units of c; a slip in the J-to-kJ conversion gives the other options.

Given C + O2 -> CO2 (delta H = -394 kJ) and CO + 1/2 O2 -> CO2 (delta H = -283 kJ), what is delta H for C + 1/2 O2 -> CO?

• -111 kJ
• -677 kJ
• +111 kJ
• -283 kJ

Answer: -111 kJ

By Hess's law, target = (C -> CO2) minus (CO -> CO2): delta H = (-394) - (-283) = -394 + 283 = -111 kJ. Reversing the second equation flips its sign.

Using bond enthalpies H-H = 436, Cl-Cl = 242, H-Cl = 431 kJ/mol, what is delta H for H2 + Cl2 -> 2HCl?

• -184 kJ
• +184 kJ
• -431 kJ
• -247 kJ

Answer: -184 kJ

delta H = (bonds broken) - (bonds formed) = (436 + 242) - (2 x 431) = 678 - 862 = -184 kJ. Breaking bonds is endothermic (positive); forming bonds is exothermic (negative).

Which of sodium chloride (NaCl) and magnesium oxide (MgO) has the larger (more endothermic) lattice enthalpy, and why?

• NaCl, because its singly charged ions are able to pack together more closely
• MgO, because of higher ionic charges and smaller ionic radii
• NaCl, because Cl- is smaller than O2-
• Both are equal

Answer: MgO, because of higher ionic charges and smaller ionic radii

Lattice enthalpy increases with higher ionic charges and smaller ionic radii (stronger electrostatic attraction). MgO has 2+ and 2- ions that are also small, so its lattice enthalpy is much larger than that of NaCl (1+ and 1- ions).

For a reaction, delta H = +50 kJ/mol and delta S = +200 J/K/mol. Using delta G = delta H - T delta S, at what temperature does the reaction become spontaneous (delta G < 0)?

• Above 250 K
• Below 250 K
• Above 400 K
• Never

Answer: Above 250 K

delta G = 0 at T = delta H / delta S = 50000 J / 200 J/K = 250 K. Since delta H and delta S are both positive, delta G becomes negative (spontaneous) when T delta S exceeds delta H, i.e. above 250 K.

In an exothermic reaction, the enthalpy change (ΔH) is

• positive
• negative
• zero
• always equal to the activation energy

Answer: negative

Exothermic reactions release energy to the surroundings, so the products have lower enthalpy than the reactants: ΔH < 0.