The structures of two compounds commonly found in food, lauric acid, C12H24O2, and sucrose, C12H22O1 — Thermodynamics Chemistry Question
Problem Context
The structures of two compounds commonly found in food, lauric acid, C12H24O2, and sucrose, C12H22O11, are shown above.
[VISUAL]
Model Answer
Sucrose is more soluble than lauric acid. Stronger interactions occur between sucrose and water molecules due to a greater capacity for hydrogen bonding as a result of a larger number of −OH groups in sucrose. Although lauric acid molecules have one site for hydrogen bonding, the long hydrocarbon chain causes London dispersion forces to be the predominant, yet weaker, interaction with water molecules.
1 point is earned for discussion of hydrogen bonding between each substance and water.
1 point is earned for a valid conclusion about solubility based on a comparison of these forces.
Model Answer
37 kJ/g × 1.5 g = 56 kJ released
q = mc∆T ⇒ ∆T = q / mc = 56,000 J / ((325 g)(4.18 J/(g K))) = 41 K (41°C)
Final temperature = 25°C + 41°C = 66°C
1 point is earned for heat released.
1 point is earned for a final temperature consistent with the heat released.
Model Answer
The temperature will be greater in the can made of Cu. Since Cu has a lower specific heat than Al, more heat will be absorbed by the water in the Cu can than the water in the Al can.
1 point is earned for the correct answer.
1 point is earned for an appropriate explanation. || Possible answers include:
Heat is lost to the environment. The temperature of the water is lower because less heat is transferred to the water.
OR Heat is transferred to the can or thermometer. The temperature of the water is lower because less heat is transferred to the water.
OR Incomplete combustion of the lauric acid. The temperature of the water is lower because less heat was released from the combustion reaction.
1 point is earned for a valid source of error.
1 point is earned for an acceptable explanation.
Model Answer
The reaction has a high activation energy. The ignition source provides energy to the molecules; some of them can then overcome the activation energy barrier. The reaction is exothermic, so the heat released allows the reaction to continue.
1 point is earned for a correct explanation. || ∆S ° for the reaction is positive; there are 23 moles of gaseous products for every 12 moles of gaseous reactant.
1 point is earned for the correct sign and justification.