Using Mean Bond Enthalpies

Using Mean Bond Enthalpies

What is the equation used to calculate an enthalpy change of reaction from mean bond enthalpies?

ΔH = Σ(bond enthalpies of bonds broken) − Σ(bond enthalpies of bonds made).

What is the first step when calculating ΔH using mean bond enthalpies?

Calculate the sum of the mean bond enthalpies of all bonds broken in the reactants.

What is the second step when calculating ΔH using mean bond enthalpies?

Calculate the sum of the mean bond enthalpies of all bonds made in the products.

What is the final step when calculating ΔH using mean bond enthalpies?

Subtract the total energy of bonds made from the total energy of bonds broken.

What does Σ mean in a bond enthalpy calculation?

The sum of all the relevant bond enthalpies.

Are bonds broken in the reactants or products?

Reactants.

Are bonds made in the reactants or products?

Products.

Is breaking bonds endothermic or exothermic?

Endothermic because energy is required to break bonds.

Is making bonds endothermic or exothermic?

Exothermic because energy is released when bonds form.

If more energy is released making bonds than absorbed breaking bonds, what is the sign of ΔH?

Negative, so the reaction is exothermic.

If more energy is absorbed breaking bonds than released making bonds, what is the sign of ΔH?

Positive, so the reaction is endothermic.

How do you calculate the energy required to break several identical bonds?

Multiply the mean bond enthalpy by the number of those bonds broken.

How do you calculate the energy released when several identical bonds are formed?

Multiply the mean bond enthalpy by the number of those bonds formed.

When calculating ΔH using bond enthalpies, what should you do with bonds that are present unchanged in both reactants and products?

They can be cancelled because the same bonds are both broken and made.

For H₂(g) + Cl₂(g) → 2HCl(g), which bonds are broken?

One H–H bond and one Cl–Cl bond.

For H₂(g) + Cl₂(g) → 2HCl(g), which bonds are made?

Two H–Cl bonds.

Using E(H–H) = 436, E(Cl–Cl) = 244 and E(H–Cl) = 432 kJ mol⁻¹, what is Σ(bonds broken) for H₂ + Cl₂ → 2HCl?

436 + 244 = 680 kJ mol⁻¹.

Using E(H–H) = 436, E(Cl–Cl) = 244 and E(H–Cl) = 432 kJ mol⁻¹, what is Σ(bonds made) for H₂ + Cl₂ → 2HCl?

2 × 432 = 864 kJ mol⁻¹.

What is ΔH for H₂(g) + Cl₂(g) → 2HCl(g) using these bond enthalpies?

680 − 864 = −184 kJ mol⁻¹.

Why are enthalpy changes calculated using mean bond enthalpies only estimates?

Mean bond enthalpies are average values obtained from bonds in many different molecules.

Why can a calculated ΔH using mean bond enthalpies differ from an experimental value?

The actual bond enthalpies in specific molecules may differ from the mean bond enthalpy values used.

What additional limitation applies when comparing mean-bond-enthalpy calculations with reactions involving liquids?

Bond enthalpies are measured for gaseous species, so changes of state can cause differences from experimental values.

How can mean bond enthalpies be calculated from a known enthalpy change of reaction?

Substitute the known bond enthalpies and ΔH into ΔH = Σ(bonds broken) − Σ(bonds made), then rearrange to find the unknown bond enthalpy.

When calculating an unknown bond enthalpy, why must you count all bonds carefully?

Every bond broken and made contributes to the total enthalpy change.

What is a useful strategy if you are unsure which bonds actually change during a reaction?

Calculate the total bond enthalpies of all bonds in both reactants and products; unchanged bonds will cancel.

How can bond enthalpy data indicate which bond is most likely to break first?

Bonds with lower bond enthalpies require less energy to break and are generally easier to break.

What does a high bond enthalpy indicate about a bond?

The bond is strong and requires a large amount of energy to break.

What does a low bond enthalpy indicate about a bond?

The bond is weaker and requires less energy to break.

How can bond enthalpy affect the temperature required for a reaction?

Reactions requiring the breaking of high-bond-enthalpy bonds are more likely to require heating or a catalyst.

How can bond enthalpy data give an indication of reaction rate?

A reaction involving easier-to-break, lower-enthalpy bonds may occur more readily, whereas breaking stronger bonds requires more energy and may make the reaction more difficult.

Does bond enthalpy alone completely determine the rate of a reaction?

No. It only provides an indication because reaction rate also depends on factors such as activation energy, temperature and reaction mechanism.

What is the standard enthalpy change of atomisation?

The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.

What symbol is used for standard enthalpy change of atomisation?

ΔatH°.

What are the standard conditions used for standard enthalpy change of atomisation?

100 kPa and a stated temperature, usually 298 K.

What must be formed in a standard enthalpy change of atomisation?

Exactly one mole of gaseous atoms.

What physical state must the product of an atomisation reaction have?

Gaseous atoms.

Is standard enthalpy change of atomisation normally positive or negative?

Positive because energy is required to separate particles and form gaseous atoms.

What equation represents the standard enthalpy change of atomisation of carbon?

C(s) → C(g).

What is the standard enthalpy change of atomisation of carbon shown in the textbook?

+717 kJ mol⁻¹.

What equation represents the standard enthalpy change of atomisation of sodium?

Na(s) → Na(g).

What is the standard enthalpy change of atomisation of sodium shown in the textbook?

+107 kJ mol⁻¹.

What equation represents the standard enthalpy change of atomisation of hydrogen?

½H₂(g) → H(g).

What is the standard enthalpy change of atomisation of hydrogen shown in the textbook?

+218 kJ mol⁻¹.

What equation represents the standard enthalpy change of atomisation of chlorine?

½Cl₂(g) → Cl(g).

What is the standard enthalpy change of atomisation of chlorine shown in the textbook?

+122 kJ mol⁻¹.

Why is ½H₂(g) → H(g) used for the standard enthalpy change of atomisation of hydrogen rather than H₂(g) → 2H(g)?

The definition requires the formation of exactly one mole of gaseous atoms.

For an element that exists as a diatomic molecule, how is its atomisation equation usually written?

½X₂(g) → X(g).

What is the difference between bond enthalpy and standard enthalpy change of atomisation for a diatomic element?

Bond enthalpy breaks one mole of molecules into two moles of gaseous atoms, whereas atomisation forms exactly one mole of gaseous atoms.

How is the atomisation enthalpy of a gaseous diatomic element related to its bond enthalpy?

The atomisation enthalpy is half its bond enthalpy.

Why is atomisation of a solid element not simply a bond enthalpy?

It includes converting the element from its solid standard state into separate gaseous atoms rather than breaking one specific covalent bond.

Define standard enthalpy change of atomisation.

The enthalpy change measured at a stated temperature, usually 298 K, and 100 kPa when one mole of gaseous atoms is formed from an element in its standard state.