5.2 Hess's Law

Some bond energy values are listed below.

These bond energy values relate to the following four reactions. 

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

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

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?