Bond Enthalpy (DP IB Chemistry: SL)

The standard enthalpy change, ΔH^Ө, for the following reaction is -246 kJ. 

N₂(g)  +  3F₂(g)  →  2NF₃(g)     

The bond energy of  N≡N is 945 kJ mol^-1 and F–F is 159 kJ mol^-1 

What is the bond energy of the N–F bond?

The complete combustion of ethyne, C₂H₂, is shown in the equation below. 

H−C≡C−H + 2½ O═O  →  H−O−H + 2O═C═O 

Using the average bond enthalpies given in the table, what is the enthalpy change of combustion of ethyne?

Compound R into compound T, it was found that the reaction proceeded by way of compound S, which could be isolated. The following steps were involved. 

R → S;  ΔH is positive

S → T; ΔH is negative

Which reaction profile fits these data?

The reaction pathway for an endothermic reaction is shown.

 Which arrow represents the activation energy for the reverse reaction?

The reaction pathway shows the energies of the reactants, the products and the transition state of a reaction.

Which expression correctly represents how to calculate the activation energy of the forward reaction?

The incomplete combustion of methanol can produce carbon monoxide and water: 

Some enthalpy data is given in the table.

What is the enthalpy change for this incomplete combustion of methanol?

Which of the following statements about bond enthalpy are true? 

1. A bond enthalpy may be inaccurate when compared to data tables because it is not an average value
2. Average bond enthalpies are valid for all states of matter
3. Bond enthalpies may be inaccurate as they do not account for intermolecular forces

Four possible reactions (A, B, C and D) of the following equation are measured at the same temperature. 

                                                X (g) + Y (g) → Z (g) 

Which reaction pathway diagram shows the reaction occurring rapidly with an overall negative enthalpy value?

Which combination about bond breaking and bond formation energy changes is correct?

Which reactions are involved in maintaining the concentration of ozone in the upper atmosphere? 

1. O₃ → O₂ + O^●
2. O₂ → 2O^●
3. O₂ + O^● → O₃

In the gas phase, phosphorus pentachloride can be thermally decomposed into gaseous phosphorus trichloride and chlorine.

PCl₅  → PCl₃ + Cl₂

The table below gives the relevant bond energies found in these compounds.

What is the enthalpy change in the decomposition of the reaction?

Which equation correctly shows how the bond energy for the covalent bond Y–Z can be calculated by dividing ΔH by n?

The diagram shows the skeletal formula of cyclobutane.
 

The enthalpy change of formation of cyclobutane is +75.1 kJ mol^–1, and the enthalpy change of atomisation of graphite is +712 kJ mol^–1.

The bond enthalpy of C–H is 414 kJ mol^–1 and of H–H is 436 kJ mol^–1.

What is the average bond enthalpy of the C–C bond in cyclobutane?

Butane can be produced by the hydrogenation of buta-1,3-diene.

C₄H₆ (g) + 2H₂ (g) → C₄H₁₀ (g)                 ΔH = -248 kJ mol^-1

Using the information, which calculation shows the bond enthalpy for the C=C bond in buta-1,3-diene?

The equations to form methane and propane from their gaseous atoms are:

C (g) + 4H (g) → CH₄ (g)              ΔH_r = -1656 kJ mol^-1

3C (g) + 8H (g) → C₃H₈ (g)           ΔH_r = -4004 kJ mol^-1

What is the bond enthalpy of a C-C bond?

Ultraviolet radiation is split into three regions:

• UV A (wavelength 400 - 320 nm)
• UV B (wavelength 320 - 280 nm)
• UV C (wavelength < 280 nm)

High energy photons are present in the solar spectrum at high altitude. The maximum wavelength of a photon that has enough energy to break the O=O bond in oxygen is 240 nm, which is in the UV C region of ultraviolet radiation..

The energy per mole required to break an O=O bond can be calculated using the following equation:

 = Energy per bond

Using the supplementary equations c = νλ and E = hν, which is the correct calculation to determine the bond energy of an O=O in kJ mol^-1?

Avogadro’s constant = 6.02 x 10²³

Planck’s constant = 6.63 x 10^-34 J s

Speed of light = 3.00 x 10⁸ m s^-1

Which of the following statements about the average bond enthalpy of the halogens are correct?

1. Fluorine has the highest average bond enthalpy
2. Average bond enthalpy generally decreases as the size of the atoms increases
3. In general, increased shielding results in a lower bond enthalpy

What is the correct order to show the decreasing strength of the F-H, N-H and O-H bonds?

Water has two different bond enthalpies for the two O-H bonds that it contains:

H₂O (g) → H (g) + OH (g)             ΔH = +502 kJ mol^-1

OH (g) → H (g) + O (g)                 ΔH = +427 kJ mol^-1

The average O-H bond enthalpy from an accepted data table is 463 kJ mol^-1.

Which calculation correctly shows the percentage difference between the average O-H bond enthalpy of water and the data table average bond enthalpy value?

Some of the reactions involved in the formation and depletion of ozone are:

1. O₂ → 2O^●
2. O₂ + O^● → O₃
3. O₃ + O^● → 2O₂

Which reactions are exothermic?