COTW9

Which statement is correct for the reaction with this enthalpy level diagram?

• Heat energy is released during the reaction and the reactants are more stable than the products.
• Heat energy is absorbed during the reaction and the reactants are more stable than the products.
• Heat energy is released during the reaction and the products are more stable than the reactants.
• Heat energy is absorbed during the reaction and the products are more stable than the reactants.

The enthalpy changes of three reactions are given below.
2HCOOH(l) + O₂(g) --> 2CO₂(g) + 2H₂O(l) ΔH = a
C₂H₅OH(l) + 3O₂(g) --> 2CO₂(g) + 3H₂O(l) ΔH = b
2HCOOC₂H₅(l) + 7O₂(g) --> 6CO₂(g) + 6H₂O(l) ΔH = c
What is the enthalpy change for the following reaction?
HCOOH(l) + C₂H₅OH(l) --> HCOOC₂H₅(l) + H₂O(l)

• a + b + c
• a + 2b - c
• ½a + b + ½c
• ½a + b - ½c

What is the correct definition of lattice enthalpy?

• Enthalpy change when one mole of a solid ionic compound is separated into gaseous ions.
• Enthalpy change when one mole of a solid ionic compound is separated into its ions in their standard state.
• Enthalpy change when one mole of a solid cionic compound is formed from gaseous elements.
• Enthalpy change when one mole of a compound is formed from the elements in their standard states.

Which reaction has the greatest increase in entropy?

• 2CH₃OH(l) + 3O₂(g) --> 2CO₂(g) + 4H₂O(l)
• N₂(g) + 3H₂(g) --> 2NH₃(g)
• 2HCl(aq) + MgCO₃(s) --> MgCl₂(aq) + H₂O(l) + CO₂(g)
• NH₃(g) + HCl(g) --> NH₄Cl(s)

Which change must be negative when a reaction occurs spontaneously?

• ΔH
• ΔG
• ΔS
• ΔT

THIS IS A FREE RESPONSE, NOT MULTIPLE CHOICE! So answer all the questions.

a) Two chemistry students wished to determine the enthalpy of hydration of anhydrous magnesium sulfate. They measured the initial and the highest temperature reached when anhydrous magnesium sulfate, MgSO₄(s), was dissolved in water. They presented their results in Table A.

a i) Calculate the amount, in mol, of anhydrous magnesium sulfate.

a ii) Calculate the enthalpy change, ∆H₁, for anhydrous magnesium sulfate dissolving in water, in kJ mol^-1. State your answer to the correct number of significant figures.

b) The students repeated the experiment using 6.16 g of solid hydrated magnesium sulfate, MgSO₄∙7H₂O(s), and 50.0 cm³ of water. They found the enthalpy change, ∆H₂, to be +18 kJ mol^-1. The enthalpy of hydration of solid anhydrous magnesium sulfate is difficult to determine experimentally, but can be determined using Diagram B.

b i) Determine the enthalpy change, ∆H, in kJ mol^-1, for the hydration of solid anhydrous magnesium sulfate, MgSO₄.

b ii) The literature value for the enthalpy of hydration of anhydrous magnesium sulfate is 103 kJ mol^-1. Calculate the percentage difference between the literature value and the value determined from experimental results, giving your answer to one decimal place. (If you did not obtain an answer for the experimental value in (b i) then use the value of -100 kJ mol^-1, but this is not the correct value.)

c) Another group of students experimentally determined an enthalpy of hydration of -95 kJ mol^-1. Outline two reasons which may explain the variation between the experimental and literature values.

d) Magnesium sulfate is one of the products formed when acid rain reacts with dolomitic limestone. This limestone is a mixture of magnesium carbonate and calcium carbonate.

d i) State the equation for the reaction of sulfuric acid and magnesium carbonate.

d ii) Deduce the Lewis (electron dot) structure of the carbonate ion, giving the shape, and the oxygen-carbon-oxygen bond angle.

d iii) There are three possible Lewis structures that can be drawn for the carbonate ion, which lead to a resonance structure. Explain, with reference to the electrons, why all carbon-oxygen bonds have the same length.

d iv) Deduce the hybridization of the carbon atom in the carbonate ion.

• I have answered ALL questions a-d iv.

• I did NOT answer all questions a-d iv.