Enthalpy change reactions

Hi,

Why is that when a reaction takes place, the temperature rises above e.g. 298k (25 degrees) to e.g. 328K and then after a while the temperature falls back to 298k - room temperature?

Reply 1

charco

Study Forum Helper

Original post by SANTR

Hi,

Why is that when a reaction takes place, the temperature rises above e.g. 298k (25 degrees) to e.g. 328K and then after a while the temperature falls back to 298k - room temperature?

During the chemical reaction energy is transformed from chemical potential to kinetic i.e. heat energy.

The zeroth law of thermodynamics says that heat energy flows from hot to cold. The reaction is hot and the surroundings are (relatively) cold.

Energy flows from reaction to surroundings until they are both the same temperature.

Reply 2

SANTR

Original post by charco

Thanks!
One other question, is the enthalpy change in the formation of liquid water greather than the enthalpy change for the formation of steam because the formation of liquid water involved forming more bonds e.g. hydrogen bonds etc whereas in steam, fewer bonds or hardly any bonds are formed between the water molecules and so less energy is released (i.e fewer bonds formed, therefore less energy released in total for bond formation)?

Reply 3

charco

Study Forum Helper

Original post by SANTR

Yes, it's greater by (an amount equal to) the enthalpy of condensation.

Reply 4

SANTR

Original post by charco

Yes, it's greater by (an amount equal to) the enthalpy of condensation.

I'm not too sure what you mean by enthalpy of condensation.
Also, if a reactant was in solid state instead of gaseous state, would the same logic apply i.e. more energy would have to be used towards break the bonds in the solid than that would be required for the reactant in gaseous state and therefore the energy change for the same reaction in gaseous would be larger i.e. more exothermic?

Reply 5

charco

Study Forum Helper

Original post by SANTR

Enthalpy of condensation is the energy change when 1 mole of a vapour is changed to 1 mole of a liquid. It is the opposite of the enthalpy of vaporisation.

Yes the same logic applies for all states.

Reply 6

SANTR

Original post by charco

Enthalpy of condensation is the energy change when 1 mole of a vapour is changed to 1 mole of a liquid. It is the opposite of the enthalpy of vaporisation.

Yes the same logic applies for all states.

Ah I see, thanks for clarifying.
Regarding my initial question, is it that the intermolecular forces between the water molecules in steam are weaker compared to that in liquid water or are the intermolecular forces e.g. Hydrogen bonds non-existent altogether, that makes it have a lower enthalpy change?

(edited 8 years ago)

Reply 7

charco

Study Forum Helper

Original post by SANTR

Ah I see, thanks for clarifying.
Regarding my initial question, it that the intermolecular forces between the water molecules in steam are weaker compared to that in liquid water or are the intermolecular forces e.g. Hydrogen bonds non-existent altogether, that makes it have a lower enthalpy change?

In vapour state the particles are considered to be so far apart as to have negligible intermolecular forces. This is one of the assumptions of the ideal gas laws.