Unit 4 - Lesson 4

The enthalpy change of solution for NaOH with differing amounts of water is shown.

NaOH(s) + 3H2O(L) = NaOH(aq) , ΔHsol = -28.9 KJ/mol

NaOH(s) + 300H2O(L) = NaOH(aq) , ΔHsol = -42.3 KJ/mol

What is the enthalpy change of dilution, ΔHdil?

• -13.4 KJ/mol

• +13.4 KJ/mol

• -253.4 KJ/mol

• -333.4 KJ/mol

The process of dissolution can be considered to involve three steps. Which of the following is not one of these steps?

• The separation of solvent–solute interactions

• The separation of solute–solute attractions

• The formation of solute–solvent interactions

• The separation of solvent–solvent intermolecular attractions

Which of the following statements best defines the standard enthalpy of formation (ΔH_f) ?

• It is the change in enthalpy for the formation of one mole of a compound from its elements, under standard conditions, and with all elements in their standard state.

• It is the change in enthalpy for the formation of elements from one mole of a compound, under standard conditions, and with all elements in their standard state.

• It is the change in enthalpy for the decomposition of one mole of a compound to its elements, under standard conditions, and with all elements in their standard state.

• It is the change in enthalpy for the reaction of one mole of a compound with one mole of another compound to form a mole of products, under standard conditions, and with all elements in their standard state.

• It is the change in enthalpy for the formation of one mole of crystal from a saturated solution, under standard conditions, and with all elements in their standard state.

In the Haber process, nitrogen and hydrogen gases react reversibly to produce gaseous ammonia (NH3) .There are no other products. The energies of selected bonds are listed in the table.

• 1. N2 + 6H2 = 4NH3

• N2 + 3H2 = 2NH3

• 2N2 + 3H2 = 4NH2

The substitution reaction between methane and fluorine gases is represented by the following equation:

CH4 + F2 = CH3F + HF

The bond enthalpies of the reactants and products are listed in the table.

What is the enthalpy change for the substitution reaction?

• -484 KJ/mol

• -534 KJ/mol

• +154 KJ/mol

• +184 KJ/mol

The complete combustion of heptene (C₇H₁₄) produces water and carbon dioxide gases as the only products. Using the bond energies listed in the table, calculate the total change in bond energy for this reaction, per mole of heptene combusted.

• -4339 KJ/mol

• -4459 KJ/mol

• -542 KJ/mol

• -145 KJ/mol

In which of these reaction profile diagrams is the energy required to break chemical bonds the greatest?

• a

• b

• c

• d