chem 222 ch 9 Thermochemistry

Enthalpy (H): definition & equation -exo vs endothermic -fusion & vaporization

heat at constant pressure ΔH = ΔE + PΔV = q -Exothermic (heat is released): ΔH < 0 -Endothermic (heat is absorbed): ΔH > 0 -Enthalpy of fusion (ΔHfus): heat (or enthalpy change) needed to melt 1 mole of a solid substance -Enthalpy of vaporization (ΔHvap): heat (or enthalpy change) needed to evaporate 1 mole of a liquid substance -When a process is reversed, ΔH has the same number but changes sign

Specific Heat Capacity (cp): definition, unit & equation

heat required to raise the temperature of 1 g of a substance by 1 °C Unit: J/(g·°C) q = mcpΔT

Molar Heat Capacity(cp,n): definition, unit, equation

heat required to raise the temperature of 1 mol of a substance by 1 °C

endo vs exothermic

• Exothermic process: energy is released from the system to the surroundings • Endothermic process: energy from the surroundings is consumed by the system

First law of thermodynamics: If the reaction (system) releases or consumes energy

• If the reaction (system) releases energy: energy goes to the surrounding • If the reaction (system) consumes energy: energy comes from the surrounding ΔEuniv = ΔEsys + ΔEsurr = 0

ΔT = Tf - Ti if a system is heated up _ if a system is cooled down _

• Tf >Ti:thesystemisheatedup,ΔT>0,q>0:heatflowsintothe system • Tf <Ti:thesystemiscooleddown,ΔT<0,q<0:heatflowsoutof the system

Use the bond energies in the table below to calculate the enthalpy change associated with the chlorination of methane: CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g) Cl - Cl = 243 kJ/mol C - Cl = 328 kJ/mol C - H = 413 kJ/mol H - Cl = 431 kJ/mol

-103 kJ

Hot-water heaters fueled by natural gas need to be vented because incomplete combustion can produce toxic carbon monoxide: Reaction A: 2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(g) ΔHA° = ? Calculate ΔHA° using the following thermochemical information: Reaction B: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) ΔHB° = -802 kJ Reaction C: 2CO(g) + O₂(g) → 2CO₂(g) ΔHc° = -566 kJ

-1038 kJ

Determine the standard enthalpy of reaction (ΔHrxn°) for the following reaction:2NO(g) + O₂(g) → 2NO₂(g), given that ΔHf°(NO) = 90.3 kJ/mol; ΔHfo(NO₂) = 33.2 kJ/mol

-114 kJ

Given the following reactions, what is the overall enthalpy change for the reaction: C₄H₄(g) + 2H₂(g) → C₄H₈(g)? C₄H₄(g) + 5O₂(g) → 4CO₂(g) + 2H₂O(l) ΔHrxn = -2341 kJ H₂(g) + 12O₂(g) → H₂O(l) ΔHrxn = -286 kJ C₄H₈(g) + 6O₂(g) → 4CO₂(g) + 4H₂O(l) ΔHrxn = -2755 kJ

-158 kJ

Calculate the standard heat of reaction for the following methane-generating reaction of methanogenic bacteria: 4CH₃NH₂(g) + 2H₂O(l) → 3CH₄(g) + CO₂(g) + 4NH₃(g) Given that ΔHfo(CH₃NH₂, g) = -22.97 kJ/mol; ΔHfo(H₂O, l) = -285.8 kJ/mol; ΔHfo(CH₄, g) = -74.8 kJ/mol; ΔHfo(CO₂, g) = -393.5 kJ/mol ΔHfo(NH₃, g) = -46.1 kJ/mol

-138.82 kJ

Determine the change in enthalpy for the following reaction from the enthalpies of formation for the reactants and products. 4NH₃(g) + 7O₂(g) → 4NO₂(g) + 6H₂O(l) ΔHf(NH₃, g) = -46 kJ/mol; ΔHf(NO₂, g) = +33 kJ/mol; ΔHf(H₂O,l) = -286 kJ/mol

-1400 kJ/mol

N₂(g) + 3H₂(g) → 2NH₃(g) ΔHrxn = -92 kJ H₂(g) + Cl₂(g) → 2HCl(g) ΔHrxn = -185 kJ N₂(g) + 4H₂(g) + Cl₂(g) → 2NH₄Cl(s) ΔHrxn = -629 kJ Calculate ΔHrxn for the reaction: NH₃(g) + HCl(g) → NH₄Cl(s)

-176 kJ

use bond energies to calculate ΔHrxn° for the synthesis of ammonia: N₂(g) + 3H₂(g) → 2NH₃(g) N≡N = 945kJ/mol H-H = 436kJ/mol N-H = 391kJ/mol

-93 kJ

The enthalpy of the neutralization reaction: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) is ΔHrxn = -55.6 kJ/mol. When 0.100 L of 0.200 M HCl is mixed with 0.100 L of 0.200 M NaOH in a coffee-cup calorimeter, the temperature of the mixture increases from 22.5 °C to 23.7 °C. If the density of the mixed solution is 1.00 g/mL, and the specific heat capacity of the mixed solution is 4.2 J/(g.°C) 1. The ___ lost heat 2. The ___ gained heat. Its initial temperature was ___ °C; its final temperature was ___ °C 3. The amount of heat gained (qgain) is ___ J 4. The amount of heat lost (qloss) is ___ J 5. The specific heat capacity of the metal is (amount) (units)

1.

Given that ΔHfus = 6.01 kJ/mol for water, how much heat does it take to melt 0.20 moles of water? How much heat is released when 2.5 moles of water freeze completely into ice?

1.2 kJ required for melting -15 kJ released for freezing

An expanding gas does 150.0 J of work on its surroundings at a constant pressure of 1.01 atm. If the gas initially occupied 68 mL, what is the final volume (in L) of the gas?

1.53 L

Given the Melting point for methanol:-94 °C _ kJ heat is released/absorbed when 20 g melts at melting point

1.98 kJ absorbed

Calculate ΔE for the combustion of a gas that releases 210.0 kJ of heat to its surroundings and does 65.5 kJ of work on its surroundings.

-275.5 kJ

Use the ΔHrxn values of the following reactions: 2SO₂(g) + O₂(g) → 2SO₃(g) ΔHrxn = -196 kJ 2S(s) + 3O₂(g) → 2SO₃(g) ΔHrxn = -790 kJ to calculate the ΔHrxn value of this reaction: S(s) + O2(g) → SO₂(g) ΔHrxn= ?

-297 kJ

How much heat is released when 124 g water is frozen to ice? (ΔHfus(H2O) = 6.01 kJ/mol)

-41.4 kJ

How much heat is released when 20.0 g water vapor condenses? (∆Hvap = 40.7 kJ/mol)

-45.2 kJ

A tank of compressed helium is used to inflate 100 balloons for sale at a carnival on a day when the atmospheric pressure is 1.01 atm. If each balloon is inflated with 4.8 L, how much P-V work (in J) is done by the compressed helium?

-49,100 J

How much P-V work does a gas system do on its surroundings at a constant pressure of 1.00 atm if the volume of gas triples from 250.0 mL to 750.0 mL? Express your answer in joules (J).

-50.7 J

Given the following thermochemical data: N₂(g) + O₂(g) → NO(g) ΔHrxn = +90.3 kJ NO(g) + Cl₂(g) → NOCl(g) ΔHrxn = -38.6 kJ What is the value of ΔHrxn for the decomposition of NOCl? 2NOCl(g) → N₂(g) + O₂(g) + Cl₂(g) ΔHrxn = ?

-51.7 kJ

when 0.200 L of 0.200 M HCl is mixed with 0.200 L of 0.200 M NaOH in a coffee-cup calorimeter, the temperature of the mixture increases from 22.15 °C to 23.48 °C. If the densities of the mixed solution is 1.00 g/mL, and the heat capacity of the calorimeter is 1.0 J/°C, what is ΔHrxn (in kJ/mol)?

-55.6 kJ/mol

If a gas releases 30 J of heat, and expanded from 0.25 L to 0.50 L at a constant pressure of 1.00 atm, what is the change of its energy? (1 atm∙L = 101.325 J)

-55J

Fertilizer (ammonium nitrate) and fuel oil (a mixture of long-chain hydrocarbons similar to decane, C10H22) can form an explosive mixture. Determine the enthalpy change of the following explosive reaction: 3NH₄NO₃(s) + C₁₀H₂₂(l) + 14O₂(g) → 3N₂(g) + 17H₂O(g) + 10CO₂(g) Given that ΔHf°(C₁₀H₂₂, l) = 249.7 kJ/mol, ΔHf°(NH₄NO₃, s) = -365.6 kJ/mol, ΔHf°(H₂O, g) = -241.8 kJ/mol, ΔHf°(CO₂, g) = -393.5 kJ/mol.

-7198.5 kJ

Types of Systems: isolation, closed, open

-isolated: a thermos bottle containing hot soup with the lid screwed on tightly -closed: a cup of hot soup with a lid -open: an open cup of hot soup

types of motions: -transitional -rotational -vibrational

-transitional: molecule moving in space -rotational: molecule rotating -vibrational: bonds stretch

A 64.5 g sample of an unidentified metal was placed in a boiling water bath and heated to 100 °C. The metal was quickly transferred to a coffee-cup calorimeter containing 120 g water at 25.0 °C. The final temperature of everything was 27.0 °C. If the heat capacity of the calorimeter is 48.0 J/°C, what is the specific heat capacity of the metal? [cp(H2O,l) = 4.18 J/(g.°C)]

0.233 J/g∙°C

To measure the specific heat capacity of an unknown metal sample, 10.695 g sample of the metal was heated to 100 °C, then placed into a coffee cup calorimeter with 100 g of water [cp = 4.18 J/(g.°C)] that was initially at 22.0 °C. The final temperature of both came to 23.7 °C. If the heat capacity of the calorimeter is 15 J/°C: 1. ___ lost heat. Its initial temperature was ___ °C; its final temperature was ___ °C 2. ___ gained heat. Its initial temperature was ___ °C; its final temperature was ___ °C 3. The amount of heat gained (qgain) is ___ J 4. The amount of heat lost (qloss) is ___ J 5. The specific heat capacity of the metal is (amount) (units)

1. 2. 3. 736.1 4. -736.1 5. 0.902

Suppose 275 g snow, initially at -18 °C is heated by a winter hiker to make soup: Step 1: heat the snow to its melting point (0 °C), given that cp(ice) = 2.11 J/(g·°C): Step 2: heat is required to melt the snow into water (ΔHfus = 6.01 kJ/mol): Step 3: heat the water to its boiling point (100 °C), given that cp(water) = 4.18 J/(g·°C): Suppose the hiker left the boiling water unattended and it vaporized completely: Step 4: heat is required to vaporize the water (ΔHvap = 40.7 kJ/mol): • Total heat required to turn 275 g, -18 °C snow into water vapor:

10.4 kJ 91.7 kJ 115 kJ 621 kJ 838.1 kJ

A 29.5 g piece of concrete (cp = 0.880 J(g∙°C)) is heated to 100°C and quickly transferred to 50.0 g water in a coffee cup calorimeter at 20.1 °C. The final temperature of everything is 28.5 °C. What is the heat capacity of the calorimeter? [cp(H₂O,l) = 4.18 J/g ∙ °C)]

12.0 J/°C

Use the information in the following two thermochemical equations: NO₂Cl(g) → NO₂(g) + 1/2Cl₂(g) ∆Hrxn° = -114 kJ 1/2N₂(g) + O₂(g) → NO₂(g) ∆Hrxn° = 33.2 kJ to calculate the value of ∆Hrxn° for the following reaction: 1/2N₂(g) + O₂(g) + 1/2Cl₂(g) → NO₂Cl(g)

147 kJ

given that cp = 4.18 J/(g·°C) for water, if 1.4 g water is heated from 20 °C to 50 °C, how much heat is required?

175.56 J

given that cp,n = 75.3 J/(mol·°C) for water, if 0.078 mol water is heated from 20 °C to 50 °C, how much heat is required?

176.2 J

Calculate ΔE for a system that absorbs 726 kJ of heat from its surroundings and does 526 kJ of work on its surroundings.

200 kJ

Given the Molar heat capacity (l) for methanol:2.53 J/(g·°C) _ J heat is released/absorbed when 20 g is heated from 0 °C to 40 °C.

2024 J absorbed

Given the Boiling point for methanol: 65 °C _ kJ heat is released/absorbed when 20 g gas condenses at the boiling point

22.03 kJ released when 20.0 g methanol gas condenses at the boiling point.

Use average bond energies given at the beginning of this homework to estimate the enthalpy change of the following reaction: N₂(g) + O₂(g) → 2NO(g) N≡N 945 O=O 498 N=O 607

229 kJ

Given that ΔHvap = 40.7 kJ/mol at 100 °C for water, how much heat does it take to vaporize 0.600 moles of water? How much heat is released when 1.20 moles of water vapor condense completely into liquid?

24.4 kJ required for vaporizing -48.8 kJ released for condensation

How much heat (in J) must be absorbed by 100.0 grams of water to raise its temperature from 30.0 °C to 100 °C? (cp(water) = 4.18 J/(g·°C))

29260 J

Hess's law

2x(1) = (2) + (3) so/or (2) = 2x(1) - (3)

Use average bond energies given at the beginning of this homework to estimate the enthalpy change of the following reaction: CO₂(g) + H₂(g) → H₂O(g) + CO(g) C=O 799 H-H 436 H-O 463 C≡O 1072 N≡N 945 O=O 498

36 kJ

The flavor is anise is due to anethole, a compound with a molecular formula of C₁₀H₁₂O. Combustion of 1 mol of anethole produces 5541 kJ of heat. If 0.950 g of anethole is combusted in a bomb calorimeter whose heat capacity (Ccalorimeter) is 7.854 kJ/°C, what s the change in temperature of the calorimeter?

4.525 °C

A sample of gas is compressed under a constant pressure of 0.500 atm from a volume of 0.35 L to a volume of 0.050 L, while releasing 10.0 J of heat to the surroundings. What is the change of its energy? (Make sure your answer has the correct sign!) (1 atm.L = 101.325 J).

5.2 J

The value of ΔHrxn for the reaction: 2H₂S(g) + 3O₂(g) → 2SO₂(g) + 2H₂O(g) is -1036 kJ. Calculate the average bond energy of S=O in SO₂. H-H 436 H-O 463 O=O 498 H-S 347

516 kJ/mol

The value of ΔHrxn for the reaction: 2H₂S(g) + 3O₂(g) → 2SO₂(g) + 2H₂O(g) = -1036 kJ. H-H 436 H-S 347 O=O 498 H-O 463 Calculate the average bond energy of S=O in SO₂.

516 kJ/mol

A cup of hot water at 79.1 °C is poured into a coffee-cup calorimeter with cold water at 23.1 °C. The temperature rises to 39.6 °C. The volume of the cold water is 100.0 mL, and the volume of hot water is also 100.0 mL. Calculate the heat capacity of the calorimeter. (density of water = 1.00 g/mL, cp = 4.18 J/(g.°C))

582.7 J/°C

We need to cool 72 aluminum cans (each containing 355 mL) of beverages, initially at 25 °C, by covering them with ice cubes (initially at -18 °C) in an insulated cooler. Assuming that (a) the mass of each aluminum can is 12.5 g; (b) each can contains mostly water with a density of 1.00 g/mL and cp = 4.18 J/(g·°C) (all other ingredients can be ignored); (c) the beverages need to be cooled to 0 °C (ice-cold), and given that cp of Al is 0.897 J/(g·°C), how many kg of ice is needed?

7.27 kg

Which of the following are formation reactions? a. H₂(g)+1⁄2O₂(g)→H₂O(g) b. C(s,graphite) + 2H₂(g) + 1⁄2O₂(g) → CH₃OH(l) c. CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) d. P₂(s, white) + 6Cl₂ → 4PCl₃(l) e. C₂H₂(g) → 2C(graphite, s) + H₂(g)

A. yes - formation reaction of water vapor B. yes C. no - can only have 1 product D. no - can only have 1 mole of product E. no - reverse of formation

Given the bond energies in the table below: calculate ΔHrxn for the reaction: 2N₂(g) + O₂(g) → 2N₂O(g) N≡N = 945 kJ/mol O=O = 498 kJ/mol N-O = 201 kJ/mol

BOND ENERGIES = reactant - product 96 kJ

Which of the following processes are exothermic? A. Rubbing alcohol evaporates from the skin B. Ice cubes in a frost-free freezer slowly lose mass C. Ice cubes solidify in the freezer. D. Water droplets condenses on the outside of a cold drink. E. Gallium melts in your hand.

C. D.

Calorimetry

Changes in the temperature of a device with a known heat capacity (calorimeter) is used to measure the heat released or absorbed by a process qloss = -qgain o If measuring cp of an unknown substance in a coffee-cup calorimeter filled with water: qloss = m(substance)cp(substance)[Tf - Ti(substance)]; qgain = m(water)cp(water)[Tf - Ti(water)] + Ccalorimeter[Tf - Ti(water)] o If measuring enthalpy of a chemical reaction: qloss = ΔHrxn; qgain = m(water)cp(water)[Tf - Ti(water)] + Ccalorimeter[Tf - Ti(water)]

Why is the sign of ΔH negative for an exothermic process?

In an exothermic process, the system releases heat to the surroundings, so q is negative. ΔH is q under constant pressure, so ΔH must also be negative.

P-V Work: definition & pressure

P-V work: w = -PΔV • Definition of work: w = force × distance = F × d • Pressure: force per unit area, P = F/A

The racing cars are powered by V8 engines in which the motion of each piston in its cylinder displaces a volume of 0.733 L. If combustion of the mixture of gasoline vapor and air in one cylinder releases 1.68 kJ of energy, and if 33% of the energy does P-V work, how much pressure, on average, does the combustion reaction mixture exert on each piston? How much heat flows from the reaction mixture to its surroundings?

P=7.5 atm q=-1.13kJ

Calculate ΔHrxn° from ΔHf°

Standard enthalpy of formation (ΔHf°) products - reactants

Standard enthalpy of reaction vs. Standard enthalpy of formation

Standard enthalpy of reaction (ΔHrxn°): ΔHrxn under standard conditions Standard enthalpy of formation (ΔHfo): the enthalpy of the reaction in which 1 mole of a compound is formed from its constituent elements in their standard states

System vs. surrounding

System: what you are studying; Surroundings: everything else

For the reaction: C₇H₆O(s) + 8O₂(g) → 7CO₂(g) + 3H₂O(l), given that: ∆Hf°(C₇H₆O) = -87.1 kJ/mol; ∆Hf°(CO₂,g) = -393.5 kJ/mol; ∆Hf°(H₂O,l) = -285.8 kJ/mol a) Calculate the standard enthalpy of reaction (ΔHrxn°) b) Is the reaction endothermic or exothermic?

a) b)

Kinetic energy on the atomic level

atoms/molecules move faster as the temperature increases

Heat Capacity (Cp): definition, unit & equation

heat required to raise the temperature of an object by 1 °C Unit: J/°C q = CpΔT

Heat required/released when temperature changes (without any phase transition)

q=ncp,nΔT, ΔT = final temperature - initial temperature = Tf - Ti

equation for heat of fusion & heat of vaporization

qfus = nΔHfus; qvap = nΔHvap

Heat of a phase transition

qfus = nΔHfus; qvap = nΔHvap o n: number of moles; ΔHfus: enthalpy of melting; ΔHvap: enthalpy of vaporization o For freezing (reverse of melting): qfus = -nΔHfus o For condensation (reverse of vaporization): qvap = -nΔHvap

Calculate ΔHrxn from bond energies

reactants - products

sublimation melting freezing vaporization condensation deposition

s → g s → l l → s l → g g → l g → s

Energy

the ability to do work - can be transferred between a system and its surroundings

thermochemistry

the study of energy changes that occur during chemical reactions and changes in state

energy equation (total change of a system)

ΔE = q + w ΔE (change in energy) = heat (thermal energy) + work o Heat flows into the system or work done on the system: energy of the system increases, + sign on heat and work o Heat flows out of the system or work done by the system: energy of the system decreases, -sign on heat and work

bond energy/strength is_ energy is ___ when a bond is formed/broken

• Bond energy/strength = energy required to break 1 mole of bonds in the gas phase -energy is released when bond is formed -energy is required when a bond is broken -higher bond energy = stronger bond

breaking bond: energy is _ forming bond: energy is _

• Breaking bond: energy is consumed • Forming bond: energy is released • Net effect: the energy consumed or released by the whole reaction

Standard Enthalpy of Formation (ΔHf°)

• The enthalpy of the reaction in which 1 mole of a compound is formed from its constituent elements in their standard states

P-V Work: • Work done to/by the system

• Work done to the system (compression): ΔV < 0, w > 0, ΔE > 0 • Work done by the system (expansion): ΔV > 0, w < 0, ΔE < 0

solid/liquid/gas movement

• s → l → g: to move atoms faster/break intermolecular forces; energy is consumed • g → l → s: to move atoms slower/form intermolecular forces: energy is released

as temperature increases/decreases, atoms/molecules move _

•increases, atoms/molecules move faster •decreases, atoms/molecules move slower

Calculate ΔHrxn° for the complete combustion of propane: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)

−𝟐𝟎𝟒𝟑. 𝟗 𝐤J

What happens to the internal energy of a gas when it expands (with no heat flow)? A. It increases. B. It decreases. C. It stays the same.

B. It decreases.