Covid Assignment 3: Energetics

Question 1

  1. Write an equation, including state symbols, for the reaction with enthalpy change equal to the standard enthalpy of formation for CF4(g).
  2. Explain why CF4 has a bond angle of 109.5°.
  3. Some values of standard enthalpies of formation (ΔfHϴ) are given in the table below:

 

 

Substance F2(g) CF4(g) HF(g)
ΔfHϴ / kJ mol−1 0 −680 −269

 

The enthalpy change for the following reaction is −2889 kJ mol−1.

C_2H_{6(g)} + 7F_{2(g)} \rightarrow 2CF_{4(g)} + 6HF_{(g)}

Use this value and the standard enthalpies of formation in the table above to calculate the standard enthalpy of formation of C2H6(g).

  1. Methane reacts violently with fluorine according to the following equation.

CH_{4(g)} + 4F_{2(g)} \rightarrow CF_{4(g)} + 4HF_{(g)} \: \: \: \Delta H=-1904

Some mean bond enthalpies are given in below

 

Bond C−H C−F H−F
Mean bond enthalpy / kJ mol−1 412 484 562

 

A student suggested that one reason for the high reactivity of fluorine is a weak F−F bond.

Is the student correct? Justify your answer with a calculation using these data.

 

Question 2

An equation for the combustion of cyclopropane, C3H6, is shown below.

C_3H_{6(g)} + 4.5O_{2(g)} \rightarrow 3CO_{2(g)} + 3H_2O_{(g)}

The standard enthalpy of combustion of cyclopropane can be calculated either from standard enthalpies of formation or by using mean bond enthalpies.

  1.  Use the standard enthalpies of formation given below to calculate the standard enthalpy of combustion of cyclopropane.

ΔH C3H6(g) = +53 kJ mol–1

ΔH CO2(g) = –393 kJ mol–1

ΔH H2O(g) = –242 kJ mol–1

  1. State what is meant by the term mean bond enthalpy.
  2. The following mean bond enthalpies have been obtained from a data book. Use these values to calculate the standard enthalpy of combustion of cyclopropane.

C–C             347 kJ mol–1
C–H            413 kJ mol–1
C=O            805 kJ mol–1
O=O            498 kJ mol–1
O–H            464 kJ mol–1

 

  1. Explain, by reference to the structure of cyclopropane, why the enthalpy of combustion calculated in part (a) is more exothermic than that calculated in part (c).

Question 3

A student added 50.0 cm3 of hydrochloric acid to 50.0 cm3 of sodium hydroxide solution in a polystyrene cup. The temperature rose by 6.5 oC. The initial concentration of each solution was 1.00 mol dm-3.

  1. Write an ionic equation for the reaction occurring.
  2. Calculate the number of moles of acid used in the reaction.
  3. Calculate the heat energy evolved in the reaction. (Assume that the final solution has a specific heat capacity of 4.18 J g-1 K-1 and a density of 1.00 g cm3.)
  4. Calculate the molar enthalpy change for the reaction.

Question 4

The table below contains some entropy data relevant to the reaction used to synthesise methanol from carbon dioxide and hydrogen. The reaction is carried out at a temperature of 250 °C.

 

Substance CO2(g) H2(g) CH3OH(g) H2O(g)
Entropy (SƟ) / J K−1 mol−1 214 131 238 189

 

CO_{2(g)} + 3H_{2(g)} \rightarrow CH_3OH_{(g)} + H_2O{(g)} \: \: \: \: \Delta H = -49 kJ mol^-^1

Use this enthalpy change and data from the table to calculate a value for the free-energy change of the reaction at 250 °C. Give units with your answer.

Question 5

  1. Define the term standard enthalpy of formation.
  2. Write an equation, including state symbols, for a reaction for which the enthalpy change is the standard enthalpy of formation of liquid CH3NO2
  3. Give the name of the principle or law which enables enthalpies of formation to be calculated from enthalpies of combustion.
  4. In the presence of a catalyst, gaseous hydrogen cyanide, HCN, burns in an excess of oxygen as shown by the equation below.

2HCN_{(g)} + 4.5O_{2(g)} \rightarrow H_2O_{(g)} + 2CO_{2(g)} + 2NO_{2(g)}

Some standard enthalpies of combustion, H are given in the table below.

Substance HCN(g) H2(g) C(s) N2(g)
H/kJ mol–1 –611 –242 –394 +68

 

Use these data to calculate a value for the standard enthalpy of formation for gaseous hydrogen cyanide.

Question 6

Born–Haber cycles can be used to determine lattice enthalpies of ionic compounds.

  1. Define, in words, the term lattice enthalpy.
  2. The Born–Haber cycle below can be used to determine the lattice enthalpy of calcium oxide. The cycle includes the values for the enthalpy changes of the steps labelled A–G.

  1. Complete the Born–Haber cycle by adding the species present on the two dotted lines. Include state symbols.
  2. Name the enthalpy changes for the following steps in the Born–Haber cycle.
    • step A
    • step C
    • step G
  3.  Calculate the lattice enthalpy of calcium oxide
  1. Describe and explain the factors that affect the values of lattice enthalpies.