Hess's Law (DP IB Chemistry: SL)

The first stage in the industrial production of nitric acid from ammonia can be represented by the following equation. 

4NH₃(g)  +  5O₂(g)  ⇌  4NO(g)  +  6H₂O(g) 

Using the following standard enthalpy change of formation data, what is the value of the standard enthalpy change  for this reaction?

Compound	/kJ mol– 1
NH3 (g)	-46.1
NO (g)	+90.3
H2O (g)	-241.8

Titanium occurs naturally as the mineral rutile, TiO₂. One possible method of extraction of titanium is to reduce the rutile by heating with carbon. 

TiO₂(s) + 2C(s)  →  Ti(s) + 2CO(g) 

The standard enthalpy changes of formation of TiO₂(s) and CO(g) are –890 kJ mol^-1 and –110.5 kJ mol^-1 respectively. 

What is the standard enthalpy change of the extraction of titanium?

The combustion of ethanol (C₂H₅OH) is increasingly being used to fuel cars. The combustion reaction is shown below 

                         C₂H₅OH(l)  +   3O₂ (g)   →  2CO₂(g)    +  3H₂O(l) 

The standard enthalpy change of formation of carbon dioxide is a kJ mol^–1.

The standard enthalpy change of formation of water is b kJ mol^–1.

The standard enthalpy change of formation of ethanol is c kJ mol^–1. 

What is the standard enthalpy change of combustion for ethanol?

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

Hess’s Law can be used to calculate the enthalpy change for reactions that are difficult to measure experimentally, such as the conversion of graphite to diamond. 

Which enthalpy data could be used to calculate the enthalpy change for the conversion of graphite to diamond? 

1. Enthalpy of reaction
2. Enthalpy of combustion
3. Enthalpy of formation

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 equations below show the formation of sulfur oxides from sulfur and oxygen. 

S(s) + O₂(g) → SO₂(g)                    = –297 kJ mol^–1 

S(s) + 1½O₂(g) →SO₃(g)                = –395 kJ mol^–1 

What is the enthalpy change of reaction,,of 2SO₂(g) + O₂(g) → 2SO₃(g) in kJ mol^–1?

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? 

A student calculated the standard enthalpy change of formation of propane, C₃H₈, using a method based on standard enthalpy changes of combustion. 

He used correct values for the standard enthalpy change of combustion of propane

 (–2220 kJ mol^–1) and hydrogen (–286 kJ mol^–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol^–1. 

What did he use for the standard enthalpy change of combustion of carbon?

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

Lithium iodide solution can be produced by two different reaction paths, according to the following diagram:

Which labels could be added to complete the diagram

	ΔH1	ΔH2	ΔH3
A	+364 kJ mol-1	ΔHhyd	+82 kJ mol-1
B	ΔHhyd	ΔHsol	+82 kJ mol-1
C	ΔHhyd	-307 kJ mol-1	ΔHsol
D	-364 kJ mol-1	ΔHsol	ΔHhyd

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	+20.0	-393.5	-285.8	-241.8

  

Using the enthalpy of formation data, which calculation correctly shows 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?