Energy Cycles in Reactions (SL IB Chemistry)

Propanone has the molecular formula C₃H₆O. Use the following information to calculate the enthalpy change for the formation of propanone? 

The enthalpy change of combustion of carbon is x kJ mol ^-1

The enthalpy change of combustion of hydrogen is y kJ mol ^-1

The enthalpy change of combustion of propanone is z kJ mol ^-1

Combustion of ethene proceeds via the reaction shown in the Hess cycle diagram.

 What are the correct elements missing from the Hess cycle?

Combustion of propene proceeds via the reaction shown in the Hess cycle diagram.

Which are the correct labels for the enthalpies shown in the Hess cycle?

	P	Q	R	S
A	ΔHӨc	ΔHӨr	ΔHӨc	ΔHӨc
B	ΔHӨc	ΔHӨf	ΔHӨr	ΔHӨr
C	ΔHӨc	ΔHӨf	ΔHӨc	ΔHӨf
D	ΔHӨr	ΔHӨr	ΔHӨf	ΔHӨf

A basic definition of Hess’s Law states that the overall enthalpy change for a reaction is the same independent of the route taken. 

The addition of which of the following statements makes the definition of Hess’s Law more complete? 

1. Providing that the reactants are the same
2. Providing that the products are the same
3. Providing that the conditions at the start and the end of the reaction are the same

The hydration enthalpy of anhydrous copper(II) sulfate cannot be measured directly. It can be found indirectly by determining the solution enthalpies of anhydrous and hydrated copper(II) sulfate.

Which are the correct labels for the enthalpies shown in the Hess cycle? 

	1	2	3
A	ΔHӨsol	ΔHӨhyd	ΔHӨhyd
B	ΔHӨr	ΔHӨhyd	ΔHӨhyd
C	ΔHӨhyd	ΔHӨsol	ΔHӨhyd
D	ΔHӨr	ΔHӨsol	ΔHӨsol

The hydration enthalpy of anhydrous copper(II) sulfate, labelled as ΔH_exp, cannot be measured directly. It can be found indirectly by determining the solution enthalpies of anhydrous and hydrated copper(II) sulfate.

Which of the following statements correctly explains why the value for ΔH_exp for this reaction cannot be measured directly?

The standard enthalpy change, ΔH^Ө, for the following reaction is -246 kJ. 

N₂(g)  +  3F₂(g)  →  2NF₃(g)     

The bond energy of  N≡N is 945 kJ mol^-1 and F–F is 159 kJ mol^-1 

What is the bond energy of the N–F bond?

The complete combustion of ethyne, C₂H₂, is shown in the equation below. 

H−C≡C−H + 2½ O═O  →  H−O−H + 2O═C═O 

Using the average bond enthalpies given in the table, what is the enthalpy change of combustion of ethyne?

bond	average bond enthalpy / kJ mol-1
C−H	a
C≡C	b
O=O	c
C=O	d
O−H	e

The incomplete combustion of methanol can produce carbon monoxide and water: 

 

Some enthalpy data is given in the table.

	C-H	C-O	O-H	O=O	C=O
Mean bond dissociation enthalpy / kJ mol−1	414	358	463	498	804

 

What is the enthalpy change for this incomplete combustion of methanol?

Which of the following statements about bond enthalpy are true? 

1. A bond enthalpy may be inaccurate when compared to data tables because it is not an average value
2. Average bond enthalpies are valid for all states of matter
3. Bond enthalpies may be inaccurate as they do not account for intermolecular forces

Some bond energy values are listed below.

bond	bond energy / kJ mol-1
Br–Br  Cl–Cl C–H C–Cl	193 242 414 324

These bond energy values relate to the following four reactions. 

W	Br2 → 2Br
X	2Cl → Cl 2
Y	CH3 + Cl → CH3Cl
Z	CH4 → CH3 + H

What is the correct order of enthalpy changes of the above reactions from most negative to most positive? 

Given the following enthalpy changes:

I₂ (s) → I₂ (g)   ∆H^Ɵ = +38 kJ mol^–1 

I₂ (g) + 3Cl₂ (g) → 2ICl₃ (s)   ∆H^Ɵ = –214 kJ mol^–1                  

What is the correct value for ∆H_f^Ɵ of iodine trichloride, ICl₃ (s)? 

 Using the following information: 

CO (g) + ½O₂ (g) → CO₂ (g)           ∆H^Ɵ  = –283 kJ mol^–1 

H₂ (g) + ½O₂ (g) → H₂O (I)            ∆H^Ɵ  = –286 kJ mol^–1 

H₂O (g) → H₂O (I)                        ∆H^Ɵ = –44 kJ mol^–1 

What is the enthalpy change, ∆H^Ɵ, for the following reaction? 

CO₂ (g) + H₂ (g) → CO (g) + H₂O (g)

Iodine trichloride, ICl₃, is made by reacting iodine with chlorine. 

                                    I₂(s) + Cl₂(g) → 2ICl(s)        ∆H^o = +14 kJ mol^–1

                                                ICl(s) + Cl₂(g) → ICl₃(s)      ∆H^o= –88 kJ mol^–1

By using the data above, what is the enthalpy change of the formation for solid iodine trichloride?

Shown below are three enthalpy changes: 

CH₄(g)     +    O₂(g)   →  HCHO(l)   +   H₂O(l)                 ΔH = x 

HCHO(l)   +  ½O₂(g)  → HCOOH(l)                                 ΔH = y 

2HCOOH(l)  +  ½O₂(g)  → (COOH)₂(l) + H₂O(l)             ΔH = z 

Use the information given to deduce the correct expression for the enthalpy change of the following reaction: 

2CH₄(g)   +   3½ O₂(g)    →   (COOH)₂(l)  + 3H₂O(l)

The hydration enthalpy of anhydrous copper(II) sulfate, labelled as ΔH_exp, cannot be measured directly. It can be found indirectly by determining the solution enthalpies of anhydrous and hydrated copper(II) sulfate.

Which of the following statements correctly explains why the value for ΔH_exp for this reaction cannot be measured directly? 

1. Hydrated copper(II) sulfate is not produced in a controlled manner
2. Dissolving of the solid is difficult to avoid
3. Heat energy is trapped inside the solid copper(II) sulfate

Bond energy calculations show the enthalpy of combustion for propene to be -1572.0 kJ mol^-1.

Compound	C3H6 (g)	CO2 (g)	H2O (l)	H2O (g)
ΔHӨf / kJ mol-1	+20	-394	-286	-242

  

Using the enthalpy of formation data, what is the percentage error between propene’s enthalpy of combustion values obtained from bond energy calculations and Hess’s Law calculations?

Assuming the bond energy calculation value is correct.

In the gas phase, phosphorus pentachloride can be thermally decomposed into gaseous phosphorus trichloride and chlorine.

PCl₅  → PCl₃ + Cl₂

The table below gives the relevant bond energies found in these compounds.

bond	bond energy / kJ mol–1
P–Cl (in both chlorides)	x
Cl–Cl	y

What is the enthalpy change in the decomposition of the reaction?

The diagram shows the skeletal formula of cyclobutane.
 

The enthalpy change of formation of cyclobutane is +75.1 kJ mol^–1, and the enthalpy change of atomisation of graphite is +712 kJ mol^–1.

The bond enthalpy of C–H is 414 kJ mol^–1 and of H–H is 436 kJ mol^–1.

What is the average bond enthalpy of the C–C bond in cyclobutane?

Butane can be produced by the hydrogenation of buta-1,3-diene.

C₄H₆ (g) + 2H₂ (g) → C₄H₁₀ (g)                 ΔH = -248 kJ mol^-1

Bond	C-C	C-H	H-H
Mean bond enthalpy / kJ mol-1	346	414	436

Using the information, which calculation shows the bond enthalpy for the C=C bond in buta-1,3-diene?

The equations to form methane and propane from their gaseous atoms are:

C (g) + 4H (g) → CH₄ (g)              ΔH_r = -1656 kJ mol^-1

3C (g) + 8H (g) → C₃H₈ (g)           ΔH_r = -4004 kJ mol^-1

What is the bond enthalpy of a C-C bond?

Water has two different bond enthalpies for the two O-H bonds that it contains:

H₂O (g) → H (g) + OH (g)             ΔH = +502 kJ mol^-1

OH (g) → H (g) + O (g)                 ΔH = +427 kJ mol^-1

The average O-H bond enthalpy from an accepted data table is 463 kJ mol^-1.

Which answer correctly shows the percentage difference between the average O-H bond enthalpy of water and the data table average bond enthalpy value?