5.6 Standard Enthalpies of Formation

If we calculate the change in enthalpy of a reaction by using heat of formation data, the correct calculation using the heats of formation of the reactants and products would be:

(sum of products) - (sum of reactants)

A reaction that absorbs heat is endothermic. What will be the sign of ΔH?

+

Consider the following chemical equations. C(s)+O2(g)→CO2(g)CO2(g)→CO(g)+12O2(g)ΔHΔH=−394kJmol=283kJmol Use the equations above and Hess's Law to calculate the standard enthalpy of formation for CO(g). The reaction for the formation of CO is represented by the equation below. C(s)+12O2(g)⟶CO(g)ΔH=?

-111

Calculate the change in enthalpy for the reaction, given the associated enthalpies of formation. 2NO(g)+O2(g)⟶2NO2(g)ΔHf,NO2=33.2 kJΔHf,NO=90.2 kJ

-114

Calculate the change in enthalpy for the given reaction. NaOH(s)+HCl(g)⟶NaCl(s)+H2O(g) ΔH∘f,NaClΔH∘f,H2OΔH∘f,NaOHΔH∘f,HCl=−411.2 kJ=−241.8 kJ=−425.6 kJ=−92.3 kJ

-135.1

Consider the reaction for the combustion of ethanol. C2H5OH(l)+3O2(g)⟶3H2O(l)+2CO2(g) Use the enthalpies of formation given below to calculate the molar enthalpy of combustion, in kilojoules per mole, of ethanol, C2H5OH(l). C2H5OH(l):H2O(l):CO2(g):ΔH∘f=−278 kJ/molΔH∘f=−286 kJ/molΔH∘f=−394 kJ/mol

-1368

If the enthalpy of combustion of ethanol (C2H6O) is −1,366.8 kJmole, what is the enthalpy change associated with the combustion of 5.00 g of ethanol in kilojoules?

-148

If we burn a 0.545 g sample of carbon and the the enthalpy of combustion for carbon is −393.5 kJ/mol, what is the qrxn?

-17.9

Given that the standard enthalpy of formations for NH3=−46.1kJmol, HCl=−92.3kJmol, and NH4Cl=−314.4kJmol, calculate the standard enthalpy change for the reaction. NH3(g)+HCl(g)⟶NH4Cl(s)

-176

NH3(g)+HCl(g)⟶NH4Cl(s) Given the standard enthalpies of formation (below), calculate the enthalpy change for the reaction in kilojoules. NH3(g)HCl(g)NH4Cl(s)ΔH∘f=−46.1kJmolΔH∘f=−92.3kJmolΔH∘f=−314.4kJmol

-176

Hydrogen gas, H2, reacts explosively with gaseous chlorine, Cl2, to form hydrochloric acid, HCl(g). What is the enthalpy change, in kilojoules, for the reaction of 1 mole of H2(g) with 1 mole of Cl2(g) if both the reactants and products are at standard state conditions? The standard enthalpy of formation of HCl(g) is −92.30 kJ/mol.

-184.6

Given that CH4 (g)+Cl2 (g)→CH3Cl (g)+HCl (g) has an enthalpy change of −99.6 kJ, and CH3Cl (g)+Cl2 (g)→CH2Cl2 (g)+HCl (g) has an enthalpy change of −105.8 kJ, what is the change in enthalpy for CH4 (g)+2Cl2 (g)→CH2Cl2 (g)+2HCl (g)?

-205

Calculate the change in enthalpy associated with the combustion of 8.00 g of carbon. Report your answer with three significant figures. C(s)+O2(g)⟶CO2(g)ΔH∘c=−393.5kJ/mol

-262

2S (s)+3O2 (g)→2SO3 (g)2SO2 (g)+O2 (g)→2SO3 (g) has an enthalpy change of −790.4 kJ has an enthalpy change of −198.2 kJ What is the heat of formation of SO2 in kilojoules? S (s)+O2 (g)→SO2 (g)

-296.1

If 1.48 g of Cl2 reacts with excess P, what is the total change in enthalpy in kilojoules? 2P+5Cl2⟶2PCl5ΔH=−886kJ

-3.70

Consider the following thermochemical equations. PCl5 (s)→PCl3 (g)+Cl2 (g)2P (s)+3Cl2 (g)→2PCl3 (g)ΔH∘rxnΔH∘rxn=87.9kJmol=−574kJmol

-375

Calculate ΔH∘298 in kilojoules for the process: Sb(s)+52Cl2(g)⟶SbCl5(g) Given the information below: Sb(s)+32Cl2(g)⟶SbCl3(g)ΔH∘298=-314 kJSbCl3(g)+Cl2(g)⟶SbCl5(g)ΔH∘298=-80.0 kJ

-394

In carbon combustion, the oxidation of solid carbon to produce carbon monoxide is represented below: 2C(s)+O2(g)⟶2CO(g) Use the following reaction enthalpies to find the enthalpy of the first reaction. C(s)+O2(g)2CO2(g)→CO2(g)→2CO(g)+O2(g)ΔHΔH=−394 kJ=+283 kJ

-505

CO(g)+H2(g)+O2(g)⟶CO2(g)+H2O(g) What is ΔH for the reaction above, given the following reaction enthalpies 2C(s)+O2(g)C(s)+O2(g)2H2(g)+O2(g)→2CO(g)→CO2(g)→2H2O(g)ΔH1=−222 kJΔH2=−394 kJΔH3=−484 kJ

-525

The chemical equation below shows the formation of SO2. S(g)+O2(g)⟶SO2(g)ΔH=−297 kJ

-594

Calculate the change in enthalpy associated with the combustion of 14.6 g of isooctane. C8H18(l)+25O2(g)⟶8CO2(g)+9H2O(l)ΔHc=−5461kJmol

-698

What is the enthalpy of formation for C+2H2→CH4? C+O2→CO2,ΔH=−393 kJH2+12O2→H2O,ΔH=−286 kJCH4+2O2→CO2+2H2O,ΔH=−892 kJ

-73

Use the reaction below to answer the question that follows. a.b.c.2Al(s)+32O2(g)⟶Al2O3(s)ΔH=−1676 kJ2Fe(s)+32O2(g)⟶Fe2O3(s)ΔH=−824 kJFe2O3(s)+2Al(s)⟶Al2O3(s)+2Fe(s)

-852

Given that I2(g)+3Cl2(g)→2ICl3(g) has an enthalpy change of −214 kJ and I2(s)→I2(g) has an enthalpy change of 38 kJ, what is the enthalpy of formation for ICl3(g)?

-88

Consider the following chemical equations to answer the question that follows. CaCO3(s)Ca(OH)2(s)Ca(OH)2(s)+2HCl(g)→CaO(s)+CO2(g)→H2O(l)+CaO(s)→CaCl2(s)+2H2O(l)ΔHΔHΔH=175kJ=67kJ=−198kJ Using the information above, determine the change in enthalpy for the following chemical reaction. CaCO3(s)+2HCl(g)⟶CaCl2(s)+H2O(l)+CO2(g)

-90

If 250 kJ of energy are absorbed in the following reaction, how many moles of CO2 reacted: 6CO2+6H2O→C6H12O6+6O2ΔH=2803 kJ

.54

Consider the reaction described by the following chemical equation. 2Ag2S(s)+2H2O(l)⟶4Ag(s)+2H2S(g)+O2(g)ΔH∘rxn=595.5kJmol

0

What is the standard enthalpy of formation for an element in its most stable form?

0

If 3.500 moles of water react completely according to the following equation, how much heat must have been absorbed in kilojoules? 6CO2+6H2O→C6H12O6+6O2ΔH=2803 kJ

1,635 kJ

In the heat formation equation for sodium chloride (NaCl), what would be the coefficient for chlorine (Cl2)? Remember fractional coefficients are possible when balancing equations for the purpose of determining heat of formation.

1/2

In the heat of formation equation for H3PO4, what would be the coefficient for the most stable state of phosphorus (P4)?

1/4

Consider the chemical reaction described by the following equation. HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH=−58 kJ What would be the enthalpy, in kilojoules, for the reaction 2NaCl(aq)+2H2O(l)→2HCl(aq)+2NaOH(aq)ΔH=?

116 kJ

The standard heat of formation for HCl is −92.3kJmol. If 45.3 kJ of heat is released during the formation of HCl from its elements, how many grams of HCl have formed?

17.9

2C(s)+O2(g)2H2(g)+O2(g)H2O(l)→2CO(g)→2H2O(g)→H2O(g)ΔHΔHΔH=−222kJ=−484kJ=+44kJ Use the thermochemical data above to calculate the change in enthalpy for the reaction below. H2O(l)+C(s)→CO(g)+H2(g)

175

What is the heat of formation reaction for acetic acid (CH3COOH)?

2,2

Complete the chemical equation for the formation reaction of ethyl alcohol (C2H5OH,l) from its constituent elements in their standard states. Remember fractional coefficients are possible when balancing equations for the purpose of determining heat of formation.

2,3,1/2

If the heat of combustion of magnesium is −601.6 kJmole, how many grams of magnesium must combust in order to release 5,000. kJ of energy?

202

If the heat of combustion of carbon monoxide (CO) is −283.0 kJmole, how many grams of carbon monoxide must combust in order to release 2,500.kJ of energy?

247.4g.

The standard state involved for standard enthalpy of combustion implies:

25∘ C and 1 atm

In the heat of formation equation for CH3CH2OH, what would be the coefficient for hydrogen gas?

3

In the heat of formation equation for Na2CO3, what would be the coefficient for oxygen gas?

3/2

The enthalpy of formation for ammonia (NH3) is −46.19 kJ/mol. What does this mean?

46.19 kJ of energy are released for every mole of ammonia that is formed from its elements in their most stable states under standard conditions.

Determine the standard enthalpy of reaction (ΔH∘rxn) given the enthalpies of formation below. 2Ag2S(s)+2H2O(l)⟶4Ag(s)+2H2S(g)+O2(g)ΔH∘fAg2S(s)ΔH∘fH2O(l)ΔH∘fAg(s)ΔH∘fH2S(g)ΔH∘fO2(g)=−32.6kJmol=−285.8kJmol=0kJmol=−20.6kJmol=0kJmol

595.6

Complete the heat of formation equation for glucose (C6H12O6).

6,6,3

Calculate the ΔH∘298 for the equation: N2(g)+2O2(g)⟶2NO2(g) Use the equations below to help you calculate your answer: Step 1:N2(g)+O2(g)⟶2NO(g)ΔH∘298=180.5 kJ/molStep 2:NO(g)+12O2(g)⟶NO2(g)ΔH∘298=−57.06kJ/mol

66.4

The standard heat of formation for NO2 is 33.2kJmol. How much energy is required to form 100.0 g of NO2 from its respective elements in kilojoules?

72.2

2NO (g)→N2 (g)+O2 (g)N2 (g)+O2 (g)+Cl2 (g)→2NOCl (g) has an enthalpy change of −180.6 kJ has an enthalpy change of 103.4 kJ What is the enthalpy change, in kilojoules, for 2NOCl (g)→2NO (g)+Cl2 (g)?

77.2

If the heat of combustion of hydrogen gas (H2) is −285.8kJmol, how many grams of H2 must combust in order to release 1.2×103kJ of heat?

8.5

"If a process can be written as the sum of several stepwise processes, the enthalpy change of the total process equals the sum of the enthalpy changes of the various steps." This is a statement of:

Hess's law

Which of the following is true regarding the enthalpy change (ΔH) of a reaction?

Increasing the coefficients of the reaction increases the enthalpy change, ΔH. & A change in the physical state of the reactants and products will affect the enthalpy change, ΔH.

Which of the following explains why internal energy is a state function?

Internal energy depends only on the state that a system is in.

What is the heat of formation reaction for sodium chloride?

Na+(1/2)Cl2→NaCl

_________ is the enthalpy change when 1 mole of a substance burns under standard state conditions.

Standard enthalpy of combustion

What can be said about enthalpies of formation?

The sign depends on the reaction.

During the combustion of a hydrocarbon:

a compound burns in the presence of oxygen, generates water and carbon dioxide as products, and is accompanied by a release of energy

When we add two chemical equations together, their changes in enthalpy must be:

added

Enthalpy of formation data is useful for calculating the enthalpy change of a reaction when that reaction is:

all of the above

A thermochemical equation is a balanced chemical equation that also lists the:

change in enthalpy

If a chemical equation has its stoichiometric coefficients doubled, the change in enthalpy must be:

doubled

A reaction that absorbs heat is ________ and has a _______ enthalpy.

endothermic, positive

What does the term "conservation of energy" mean with regards to the first law of thermodynamics?

energy is not lost, it just changes forms & energy can not be created or destroyed

A reaction that produces heat is ______ and has a _______ΔH.

exothermic, negative

If a chemical equation is reversed in direction, the change in enthalpy must:

have its sign reversed

Who is credited with formulating the chemical principle that states that the total enthalpy change for a reaction is the sum of all the enthalpy changes of each step of the reaction?

hess

The component of gasoline that gives it such a high enthalpy of combustion is:

hydrocarbons

The total sum of all kinds of energy present in a substance is called:

internal energy

Which of the following is a state function?

internal energy

In a thermochemical equation, if the chemical equation is reversed in direction, the change enthalpy must be:

reversed in sign

A quantity that has a value dependent only upon the state that a system is in and not on how that state was reached is called a

state function

Which of the statements below explain ΔH

the change in enthalpy for a process, ΔH=total enthalpy stored in the products−total enthalpy stored in the reactants

According to Hess's law, the sum of the enthalpy changes of the steps of a reaction will equal:

the enthalpy change of the reaction

If a physical or chemical process is carried out at constant pressure and volume, then the enthalpy change will be equal to:

the heat transferred

The internal energy of a system can be changed by:

work & heat flow

The chemical equation for the enthalpy of formation of C6H12O6 is as follows. xC(s)+yH2(g)+zO2(g)⟶C6H12O6(s)ΔH∘f=−1270kJmol

x=6, y=6 , z=3

What is the symbol for standard enthalpy of formation?

ΔHf∘

Which of the following summarizes Hess' law?

ΔH∘reaction=∑n×ΔH∘f(products)−∑n×ΔH∘f(reactants)

If the heat of combustion of methanol (CH3OH) is −726.1 kJ mole, what is the enthalpy change associated with the combustion of 500.g of methanol?

−11,330 kJ

When 1.34 g Zn(s) react with an excess of HCl, 3.14 kJ of heat are produced. Determine the enthalpy change per mole of zinc reacting for the reaction: Zn(s)+2HCl(aq)⟶ZnCl2(aq)+H2(g)

−153

What is the change in enthalpy associated with the combustion of 530 g of methane (CH4)?

−2.9×10^4kJ

Calculate the change in enthalpy associated with the combustion of 50.00 g of acetylene. C2H2(g)+52O2(g)⟶2CO2(g)+2H2O(l)ΔH∘c=−1301.1kJ/mol

−2498.46

Given the reaction: 2A+3B→C+2D ΔH=144.2 kJ What will be the ΔH for the following reaction, in kilojoules? 2C+4D→4A+6B

−288.4 kJ

Given the values for the standard formation enthalpies below, calculate the standard enthalpy change for the reaction below. Report your answer with three significant figures. C5H12(g)+8O2(g)⟶5CO2(g)+6H2O(g) ΔH∘f, C5H12(g)ΔH∘f,O2(g)ΔH∘f,CO2(g)ΔH∘f,H2O(g)=−146kJmol=0kJmol=−394kJmol=−242kJmol

−3,280

If the heat of formation for CO2 is −393.5 kJmol, what is the change in enthalpy when 5.00 g of CO2 are formed?

−44.706

If the heat of combustion of sulfur, S, is −296.8 kJmole, what is the enthalpy change associated with the combustion of 55.0 g of sulfur?

−509.075

What is the change in enthalpy associated with the combustion of 23.00g of methanol in kilojoules? CH3OH(l)+32O2(g)⟶CO2(g)+2H2O(l)ΔHc=−726.1kJmol

−521.2

If the heat of combustion of methane, CH4, is −890.8 kJ mol, what is the enthalpy change associated with the combustion of 1.000 kg of methane?

−55,530 kJ

Given the following reaction, calculate the enthalpy change when 6.40gH2 reacts with excess Cl2 to form HCl: H2(g)+Cl2(g)⟶2HCl(g)ΔH=−184.6 kJ

−586kJ

How much heat is produced by the combustion of 125 g of acetylene (C2H2)? C2H2(g)+52O2(g)⟶2CO2(g)+H2O(l)ΔH∘=−1301.1 kJ/mol

−6.25×10^3 kJ

What is the change in enthalpy associated with the combustion of 293 g of magnesium? Your answer should have three significant figures. Mg(s)+12O2(g)⟶MgO(s)ΔH∘c=−601.6kJ/mol

−7.25×103

How many kilojoules of heat are produced by the combustion of 75.00g of carbon monoxide? CO(g)+12O2(g)⟶CO2(g)ΔHc=−283.0 kJmol

−757.8

Consider the reaction described by the following chemical equation. 2HN3(l)+2NO(g)⟶H2O2(l)+4N2(g) What is the enthalpy change associated with the production of 1mol of H2O2 if a reaction that produces 2.50g of H2O2 releases 65.9kJ of heat?

−897

Calculate the change in enthalpy associated with the combustion of 322 g of ethanol. C2H5OH(l)+3O2(g)⟶2CO2(g)+3H2O(l)ΔH∘c=−1366.8kJ/mol

−9.55×103 kJ