Enthalpy Changes

The standard enthalpy change of combustion is

• energy released when one mole of substance is completely burnt in excess oxygen under 298K and 1 bar

• energy released when one mole of substance is completely burnt in excess oxygen under 298K and 1 atm

• energy released when one mole of substance is burnt in oxygen under 298K and 1 bar

• energy released when one mole of substance is burnt in oxygen under 298K and 1 atm

Answer explanation

The elaboration on "completely burnt in excess oxygen" is important as complete combustion must take place.

Standard conditions is 298K and 1 bar.

An example of a Hess' cycle using enthalpy changes of combustion is shown. ΔHr​ can be calculated by

• ∑​ΔHc​(products) - ∑​ΔHc​(reactants)

• ∑​ΔHc​(reactants) - ∑​ΔHc​(products)

Now that we know ΔHr = ∑​ΔHc​(reactants) - ∑​ΔHc​(products), in what scenario is ΔHr exothermic?

• |∑​ΔHc​(reactants)| < |∑​ΔHc​(products)|

• |∑​ΔHc​(reactants)| > |∑​ΔHc​(products)|

Here instead, we have a Hess' cycle involving the enthalpy changes of formation of the reagents. ΔHr​ can be calculated by

• ∑​ΔHf​(products) - ∑​ΔHf​(reactants)

• ∑​ΔHf​(reactants) - ∑​ΔHf​(products)

Now that we know ΔHr = ∑​ΔHf​(products) - ∑​ΔHc​(reactants), in what scenario is ΔHr exothermic?

• |∑​ΔHf​(products)| < |∑​ΔHf​(reactants)|

• |∑​ΔHf​(products)| > |∑​ΔHf​(reactants)|

Which standard enthalpy change(s) are ALWAYS exothermic?

[1 or more ans can be correct]

• Standard enthalpy change of formation

• Standard enthalpy change of combustion

• Standard enthalpy change of neutralisation

• Standard enthalpy change of hydration

• Standard enthalpy change of solution

Which standard enthalpy change(s) are ALWAYS endothermic?

[1 or more ans can be correct]

• Standard enthalpy change of formation

• Standard enthalpy change of atomisation

• Ionisation Energy

• Electron Affinity

• Standard enthalpy change of solution

The standard enthalpy change of atomisation of a compound is

• energy required when 1 mole of gaseous atoms is formed from the constituent elements of the compound at 298 K and 1 bar.

• energy required to convert 1 mole of the compound into gaseous atoms at 298 K and 1 bar.

• energy required when 1 mole of the compound is formed from its gaseous atoms at 298 K and 1 bar.

• equals to the sum of the bond energies of the compound in any state

The second electron affinity is endothermic as

• The electrostatic forces of attraction between the outermost electron and the anion is overcome

• The bond between the outermost electron and the anion is broken

• The doubly charged anion formed is less stable than the singly charged anion.

• The electron and negatively charged anion repel each other