Mastering Chemistry: Chapter 5- Post Lecture HW

Relating ΔH to Quantities of Reactants and Products The complete combustion of ethanol, C2H5OH (FW = 46.0 g/mol), proceeds as follows: C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l) ΔH = −555 kJ What is the enthalpy change for combustion of 15.0 g of ethanol? 1) -12.1 kJ 2) -181 kJ 3) -422 kJ 4) -555 kJ 4) -1700 kJ

-181 kJ To determine the enthalpy change for combustion of 15.0 g of ethanol follow these steps: determine moles of ethanol in 15 g : (mass ethanolFW ethanol = 15.0 g46.0 g = 0.3261 mol) find the enthalpy change for 0.3261 mol of ethanol: ΔH = -555 kJ for the combustion of 1 mol of ethanol, therefore for 0.3261 mol of ethanol −555 kJ/mol ethanol×0.03261 mol ethanol=−181 kJ.

When a 6.50-g sample of solid sodium hydroxide dissolves in 100.0 g of water in a coffee-cup calorimeter (the following Figure), the temperature rises from 21.6 ∘C to 37.8 ∘C. Calculate ΔH (in kJ/mol NaOH) for the solution process NaOH(s)→Na+(aq)+OH−(aq) Assume that the specific heat of the solution is the same as that of pure water.

-44.4 kJmol

Calcium carbide (CaC2) reacts with water to form acetylene (C2H2) and Ca(OH)2. From the following enthalpy of reaction data and data in Appendix C, calculate ΔH∘f for CaC2(s): CaC2(s)+2H2O(l)→Ca(OH)2(s)+C2H2(g)ΔH∘=−127.2kJ

-60.6 kJ

From the enthalpies of reaction 2C(s)+O2(g)→2CO(g)ΔH=−221.0kJ 2C(s)+O2(g)+4H2(g)→2CH3OH(g)ΔH=−402.4kJ calculate ΔH for the reaction CO(g)+2H2(g)→CH3OH(g)

-90.7 kJ

Which of the following objects has the greatest kinetic energy? 1) a 500-kg motorcycle moving at 100 km/h 2) a 1,000-kg car moving at 50 km/h 3) a 1,500-kg car moving at 30 km/h 4) a 5,000-kg truck moving at 10 km/h 5) a 10,000-kg truck moving at 5 km/h

1) a 500-kg motorcycle moving at 100 km/h

A stalk of celery has a caloric content (fuel value) of 9.0 kcal. If 1.0 kcal is provided by fat and there is very little protein, estimate the number of grams of carbohydrate and fat in the celery. 2 g carbohydrate and 1 g fat 32 g carbohydrate and 10 g fat 1 g carbohydrate and 2 g of fat 2 g carbohydrate and 0.1 g fat 2.2 g carbohydrate and 0.1 g fat

2 g carbohydrate and 0.1 g fat

If a balloon is expanded from 0.055 L to 1.403 L against an external pressure of 1.02 atm, how many L⋅atm of work is done? 1) -0.056 L⋅atm 2) -1.37 L⋅atm 3) 1.43 L⋅atm 4) 1.49 L⋅atm 5) 1.39 L⋅atm

2) -1.37 L⋅atm

The combustion of exactly 1.000 g of benzoic acid in a bomb calorimeter releases 26.38 kJ of heat. If the combustion of 0.550 g of benzoic acid causes the temperature of the calorimeter to increase from 22.01∘C to 24.27∘C, calculate the heat capacity of the calorimeter. 1) 0.660 kJ/∘C 2) 6.42 kJ/∘C 3) 14.5 kJ/∘C 4) 21.2 kJ/∘C 5) 32.7 kJ/∘C

2) 6.42 kJ/∘C

If the heat of formation of H2O(l) is -286 kJ/mol, which of the following thermochemical equations for the formation of H2O is correct? 1) 2 H(g) + O(g) → H2O(l) ΔH=−286 kJ 2) H2O(l) → H2(g) + 12 O2(g) ΔH=286 kJ 3)H2(g) + 12 O2(g) → H2O(l) ΔH=−286 kJ 4)2 H2(g) + O2(g) → 2 H2O(l) ΔH=−286 kJ 5)H2(g) + O(g) → H2O(g) ΔH=−286 kJ

3) H2(g) + 12 O2(g) → H2O(l) ΔH=−286 kJ

Calculate ΔH for 2 NO(g) + O2(g) → N2O4(g) using the following information: N2O4(g)2 NO(g) + O2(g)→→2 NO2(g)2 NO2(g)ΔHΔH==+57.9 kJ−114.1 kJ 1) 2.7 kJ 2) -55.2 kJ 3) -85.5 kJ 4) -171.0 kJ 5) +55.2 kJ

4) -171.0 kJ

When 0.243 g of Mg metal is combined with enough HCl to make 100 mL of solution in a constant-pressure calorimeter, the following reaction occurs: Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g) If the temperature of the solution increases from 23.0 ∘C to 34.1 ∘C as a result of this reaction, calculate ΔH in kJ/mol of Mg. Assume that the solution has a specific heat of 4.18 J/g∘C. 1) −19.1 kJ/mol 2) −111 kJ/mol 3) −191 kJ/mol 4) −464 kJ/mol 5) −961 kJ/mol

4) −464 kJ/mol

At 20 ∘C (approximately room temperature) the average velocity of N2 molecules in air is 1050 mph. Part 1-What is the average speed in m/s? Part 2- What is the total kinetic energy of 1 mol of N2 molecules moving at this speed? Part 3- What is the kinetic energy (in J) of an N2 molecule moving at this speed?

Part 1- v = 469.4 m/s Part 2- K= 3.086 kJ/mol Part 3- K = 5.124×10^−21 J

Imagine a book that is falling from a shelf. At a particular moment during its fall, the book has a kinetic energy of 29 J and a potential energy with respect to the floor of 48 J. Part A-How do the book's kinetic energy and its potential energy change as it continues to fall? 1) As the book falls, potential and kinetic energy decrease. 2) As the book falls, potential and kinetic energy increase. 3) As the book falls, potential energy decreases and kinetic energy increases. 4) As the book falls, potential energy increases and kinetic energy decreases. Part B-What was the initial potential energy of the book, and what is its total kinetic energy at the instant just before it strikes the floor? Part C-If a heavier book fell from the same shelf, would it have the same kinetic energy when it strikes the floor? 1) A heavier book falling from the same shelf has the same kinetic energy. 2) A heavier book falling from the same shelf has lower kinetic energy. 3) A heavier book falling from the same shelf has greater kinetic energy.

Part A 2) As the book falls, potential energy decreases and kinetic energy increases. Part B Potential energy, kinetic energy = 77,77 J Part C 3) A heavier book falling from the same shelf has greater kinetic energy.

A gas is confined to a cylinder under constant atmospheric pressure, as illustrated in the following figure. When 0.460 kJ of heat is added to the gas, it expands and does 212 J of work on the surroundings. Part A- Based on the wording of the question, which values are positive? 1) q and w are both negative. 2) q is positive and w is negative. 3) w is positive and q is negative. 4) q and w are both positive. Part B- What is the value of ΔE for this process? Express the energy in kilojoules to three decimal places. Part C- What is the value of ΔH for this process?

Part A- 2) q is positive and w is negative. Part B- ΔE = 0.248 kJ Part C- ΔH = 0.460 kJ

The specific heat of octane, C8H18(l), is 2.22 J/g⋅K. Part A- How many J of heat are needed to raise the temperature of 70.0 g of octane from 10.0 ∘C to 24.8 ∘C? Part B- Which will require more heat, increasing the temperature of 1 mol of C8H18(l) by a certain amount or increasing the temperature of 1 mol of H2O(l) by the same amount? 1) Increasing the temperature of 1 mol of C8H18(l) by a certain amount 2) Increasing the temperature of 1 mol of H2O(l) by the same amount

Part A- 2300 J Part B- Increasing the temperature of 1 mol of C8H18(l) by a certain amount

Consider the following reaction: 2CH3OH(g)→2CH4(g)+O2(g),ΔH=+252.8 kJ Part A- Is this reaction exothermic or endothermic? Part B- Calculate the amount of heat transferred when 25.0 g of CH3OH(g) is decomposed by this reaction at constant pressure.

Part A- Endothermic Part B- 98.6 kJ When the enthalpy of the reaction is provided, it is always in the context of moles; i.e., ΔHrxn=+252.8 kJ when two moles of CH3OH (methanol) decomposes into two moles of CH4 and one mole of O2. Therefore, you can state that 252.8 kJ is transferred when 2 mol CH3OH decompose or when 1 mol O2 is formed (based on the coefficients of the balanced equation). To determine the amount of energy transferred, calculate the number of moles of CH3OH (methanol) that are present (based on the provided mass, 25.0 g ), then multiply it by the following ratio (conversion factor): 252.8 kJ2 mol CH3OH The denominator indicates two moles of methanol, which is derived from the balanced chemical equation.

The decomposition of ZnCO3(s) into ZnO(s) and CO2(g) at constant pressure requires the addition of 71.5 kJ of heat per mole of ZnCO3. Part A- Write a balanced equation for the reaction. Part B- What is the enthalpy of the reaction above?

Part A- ZnCO3(s)→ZnO(s)+CO2(g) Part B- ΔH = 71.5 kJ

Limestone stalactites and stalagmites are formed in caves by the following reaction: Ca2+(aq)+2HCO−3(aq)→CaCO3(s)+CO2(g)+H2O(l) If 1 mol of CaCO3 forms at 298 K under 1 atm pressure, the reaction performs 2.46 kJ of P−V work, pushing back the atmosphere as the gaseous CO2 forms. At the same time, 38.85 kJ of heat is absorbed from the environment. Part A What is the value of ΔH for this reaction? Part B What is the value of ΔE for this reaction? Express your answer using four significant figures

Part A- ΔH = 38.85 kJ Part B- ΔE = 36.39 kJ

Consider the following hypothetical reactions: A→BΔH=+15kJ B→CΔH=+69kJ Use Hess's law to calculate the enthalpy change for the reaction A→C.

ΔH = 84 kJ