Thermochemistry Answers

SG 09 Thermochemistry Heat is the energy that transfers from one place to another and raises a substance’s _temperature_ (mass/ temperature/ moles). The symbol or variable for heat is __Q__ (m/ ΔG / ΔT/ Q), and in this class the units that we use for heat are usually _Joules or kilojoules_ (degrees Celsius or Kelvin/ moles/ Joules or kilojoules/ grams or kilograms), which are abbreviated as ___J or kJ___ (˚C or K/ mol/ J or kJ/ g or kg). Other units for heat include calories and British thermal units, BTU. Our nerves only sense heat transfers in or out of our body. Because temperature is a measure of the average kinetic energy of a substance’s atoms or molecules, there is really no defined boundary between hot and cold – only substances with higher or lower temperatures. Specific heat, ___Cp___ (ΔG/ ΔS / ΔH/ Cp), is defined as the ____heat____ (mass/ heat / temperature) required to raise the temperature of a 1 gram of substance by ____1 ˚C____ (1 ˚C/ 2 ˚C/ 5 ˚C/ 10 ˚C). Each substance has its own unique specific heat. The formula that uses specific heat can tell us a substance’s change in temperature for a known amount of heat and mass. Change in temperature, ΔT, is defined as the final temperature __minus__ (times/ plus/ minus) the initial temperature. When used in math and science the Greek letter delta, Δ, means “change in”. Enthalpy, __ΔH__ (ΔG/ ΔS / ΔH/ Cp), is a measure of a substance’s ___internal heat___ (internal heat/ disorder) per mole. It is usually measured in kJ/mol. Changes in enthalpy are known for most substance’s phase changes, going from one state of matter to another. The heat of fusion, ΔHfus, is the heat required per mole to change from _a solid to a liquid_ (a liquid to a solid/ a solid to a liquid/ a liquid to a gas). The heat of solidification, ΔHsolid, is the heat required per mole to change from _a liquid to a solid_ (a liquid to a solid/ a solid to a liquid/ a liquid to a gas). heat of vaporization, ΔHvap, is the heat required per mole to change from _a liquid to a gas_ (a liquid to a solid/ a gas to a liquid/ a liquid to a gas). The heat of condensation, ΔHcond, is the heat required per mole to change from _a gas to a liquid_ (a liquid to a solid/ a liquid to a gas / a gas to a liquid). All chemical reactions have a change in enthalpy, ΔHr, which is sometimes called a __heat of reaction__ (heat of vaporization/ heat of reaction/ specific heat). A heat of combustion is the heat of reaction for a specific combustion reaction. For processes that are the reverse of each other, such as the melting and freezing for a substance or a reaction and the reverse reaction, the changes in enthalpy are ____opposite____ (the same/ double/ half/ opposite). If a process exothermic then it ___releases___ (absorbs/ releases) heat and its change in enthalpy, ΔH, will be ___negative___ (positive/ negative). If a process endothermic then it ____absorbs____ (absorbs/ releases) heat and its change in enthalpy, ΔH, will be ____positive____ (positive/ negative). Entropy, __ ΔS__ (ΔG/ ΔS / ΔH/ Cp), is a measure of a substance’s _____disorder____ (internal heat/ disorder). It is usually measured in J/(mol·K). When something becomes more disordered or random there is a ____positive___ (positive/ negative) change in entropy. When something becomes more organized or less random disordered there is a __negative__ (positive/ negative) change in entropy. An example of a process with increasing entropy is _an ionic compound dissolving in water_ (steam condensing into a liquid/ an ionic compound dissolving in water). An example of a process with decreasing entropy is _steam condensing into a liquid_ (steam condensing into a liquid/ an ionic compound dissolving in water). In nature most things tend to __ increase__ (decrease/ increase) in entropy unless ___energy___ (mass/ energy) is used to reverse this trend. Gibbs energy, __ ΔG__ (ΔG/ ΔS / ΔH/ Cp), is a combined measure of a substance’s internal heat and its disorder. It is usually measured in kJ/mol. If a reaction has a positive change in Gibbs energy, then the reaction is _nonspontaneous_ (spontaneous/ nonspontaneous), which means the reaction ___cannot___ (can/ cannot) occur at those conditions. If a reaction has a negative change in Gibbs energy, then the reaction is _spontaneous_ (spontaneous/ nonspontaneous), which means the reaction ___can___ (can/ cannot) occur at those conditions. Just because a reaction can occur does not mean that it occurs very quickly. When ΔG equals __zero__ (zero/ one), a system is at equilibrium. Enthalpy, entropy, and Gibbs energy are all __state functions__ (nation functions/ state functions/ nationstates), which means that changes in them can be calculated from just the starting and ending conditions of a substance. We do not need to know the process involved in getting there. Heat Needed to Change Temperature Q = m × Cp × ΔT Heat of a Reaction ΔT = Tfinal – Tinitial ΔHr = ΔHproducts – ΔHreactants Change in Gibbs Free Energy ΔG = ΔH – ΔS × T Temperature in Kelvin Specific Heat Cp Substance water ethanol aluminum ice J/(g × ˚C) 4.18 2.44 0.90 2.11 Heats of Physical Change ΔHfus ΔHvap Substance (kJ/mol) (kJ/mol) ammonia ethanol water 5.65 4.60 6.01 23.4 43.5 40.7 Heats of Combustion at 25°C ΔH Substance Formula (kJ/mol) Hydrogen Carbon (coal) Methane Ethanol Glucose H2 (g) C (s) CH4 (g) C2H5OH (l) C6H12O6 (s) -286 -394 -890 -1,368 -2,808 1) How many Joules of heat are needed to raise the temperature of 4 grams of ethanol from 20°C to 60°C? Q = m × Cp × ΔT ΔT = 60 – 20 = +40 ˚C Q = (4) × (2.44) × (40) Q = 390.4 J 2) A 20 gram aluminum car part is cooled from 36°C to 28°C. How much heat was removed from the aluminum part? Q = m × Cp × ΔT ΔT = 28 – 36 = -8 ˚C Q = (20) × (0.90) × (-8) Q = -144 J 3) What is the change in temperature of a 40 gram sample of liquid ethanol if 2,400 Joules of heat are added to the sample? Q = m × Cp × ΔT +2,400 = 97.6 × ΔT +2,400 = (40) × (2.44) × ΔT 97.6 97.6 +2,400 = 97.6 × ΔT ΔT = -24.6 ˚C 4) If 3 moles of ethanol (C2H5OH) undergo complete combustion with oxygen, how many kilojoules (kJ) of heat will be released? Q = (3 mol) × (-1,368 kJ/mol) = -4,104 kJ 5) Photosynthesis is the reverse process combustion. If 0.5 moles of glucose (C6H12O6) and some oxygen are produced in a plant from water and carbon dioxide through photosynthesis, then how much energy was needed to form that much glucose? (Note: in real biochemistry with many enzymes and multi-step processes more energy is probably required, but this would be the bare minimum). Q = (0.5 mol) × (+2,808 kJ/mol) = +1,404 kJ N2 (g) + O2 (g)
2 NO (g) ΔHr = +180.5 kJ/mol ΔSr = +0.025 kJ/(mol·K) 6) The above reaction shows the reaction of nitrogen gas (N2) with oxygen gas (O2) to produce nitrogen oxide (NO). What is the change in Gibbs free energy for the reaction if it occurs at 300 K? ΔG = ΔH – ΔS × T ΔG = 180.5 – 0.025 × 300 ΔG = 173 kJ/mol CCl4 (l)
C (s) (graphite) + 2 Cl2 (g) ΔHr = +135 kJ/mol ΔSr = +0.235 kJ/(mol·K) 7) The above reaction shows the decomposition of carbon tetrachloride (CCl4) into graphite (C) and chlorine gas (Cl2). Use algebra to determine at what temperature the reaction will be at equilibrium. ΔG = ΔH – ΔS × T -135 = - 0.235 × T 0 = 135 – 0.235 × T - 0.235 - 0.235 – 135 – 135 -135 = - 0.235 × T T = 574.5 K