A level chemistry help

11

Hi,
I'm super confused about something in the thermodynamics topic... When 'bond enthalpy' or 'lattice enthalpy' is mentioned, are values supposed to go up or down in a hess cycle/ is it making or breaking the bond :frown:

an explanation would be super useful 😊

Reply 1

ab044

3

Bond enthalpy is defined as the amount of energy needed to break bonds so arrows going from the molecule to the atoms should have positive values.
Lattice energy is defined as the energy released when an ionic lattice is formed so arrows going from the ions to the compound should have negative values.

Reply 2

mitostudent

OP

11

Original post by ab044

Bond enthalpy is defined as the amount of energy needed to break bonds so arrows going from the molecule to the atoms should have positive values.
Lattice energy is defined as the energy released when an ionic lattice is formed so arrows going from the ions to the compound should have negative values.

Hi, Thanks for responding! Does this mean 'lattice enthalpy' means enthalpy of lattice formation?

Reply 3

ab044

3

Original post by mitostudent

Hi, Thanks for responding! Does this mean 'lattice enthalpy' means enthalpy of lattice formation?

Yep that's generally how it's defined

Reply 4

Kallisto

Entertainment Forum Helper, Life & Style Forum Helper

22

Original post by mitostudent

Hi, Thanks for responding! Does this mean 'lattice enthalpy' means enthalpy of lattice formation?

It refers to the amount of energy that is required to change the formation of a molecule or atom in a crystalline ionic compound, as in the lattice formation in kinds of salts for instance. In other words: to separate the atomic or molecular compounds to become (gaseous) ions.

(edited 11 months ago)

Reply 5

UtterlyUseless69

12

Original post by mitostudent

Hi,
I'm super confused about something in the thermodynamics topic... When 'bond enthalpy' or 'lattice enthalpy' is mentioned, are values supposed to go up or down in a hess cycle/ is it making or breaking the bond :frown:

an explanation would be super useful 😊

Bond enthalpy goes up, since it’s endothermic (+ve - think of it as rather akin to the energy input required to break a chemical bond).

Lattice enthalpy is generally defined as being the formation enthalpy change by most exam boards (e.g the enthalpy change when gaseous ions combining to form 1.00 mol of an ionic lattice) and so generally points down as it will be exothermic (-ve).

(edited 11 months ago)

Reply 6

charco

Study Forum Helper

18

Original post by UtterlyUseless69

Bond enthalpy goes up, since it’s endothermic (+ve - think of it as rather akin to the energy input required to break a chemical bond).
Lattice enthalpy is generally defined as being the formation enthalpy change by most exam boards (e.g the enthalpy change when gaseous ions combining to form 1.00 mol of an ionic lattice) and so generally points down as it will be exothermic (-ve).

Beware with Lattice enthalpy - there are two opposite definitions.
Lattice enthalpy of formation (exothermic)
Lattice enthalpy of dissociation (endothermic)
It depends on your specification which is used.
In most A' level specs it is the former
In IB (for example) it is the latter.

Reply 7

UtterlyUseless69

12

Original post by charco

Beware with Lattice enthalpy - there are two opposite definitions.
Lattice enthalpy of formation (exothermic)
Lattice enthalpy of dissociation (endothermic)
It depends on your specification which is used.
In most A' level specs it is the former
In IB (for example) it is the latter.

True - hence, I said “generally”. If the OP does AQA chemistry as the sheet they have asked about on another thread suggests, they can use either definition, but the formation enthalpy tends to be more common at A level if memory serves.

(edited 11 months ago)

Reply 8

NathanL

14

Hey! I remember being super confused about this too. So, bond enthalpy is about breaking bonds, so you’ll see positive values there. For lattice enthalpy, it’s usually about forming bonds, so that’s where you get negative values. I found drawing out the Hess cycle really helped me visualize this.

Reply 9

Kallisto

Entertainment Forum Helper, Life & Style Forum Helper

22

Original post by NathanL

Hey! I remember being super confused about this too. So, bond enthalpy is about breaking bonds, so you’ll see positive values there. For lattice enthalpy, it’s usually about forming bonds, so that’s where you get negative values. I found drawing out the Hess cycle really helped me visualize this.

Look on charo's comment in #7. It makes a difference whether it is endothermic or exothermic energy to form or dissociate the lattice. According to this kind of enthalpy, the sign is either positive (endothermic) or negative (exothermic).

Reply 10

NathanL

14

Original post by Kallisto

Look on charo's comment in #7. It makes a difference whether it is endothermic or exothermic energy to form or dissociate the lattice. According to this kind of enthalpy, the sign is either positive (endothermic) or negative (exothermic).

I don't know what you're seeing, but I'll take your word, thanks for correcting me

Reply 11

Mariam127

2

Original post by mitostudent

Hi,
I'm super confused about something in the thermodynamics topic... When 'bond enthalpy' or 'lattice enthalpy' is mentioned, are values supposed to go up or down in a hess cycle/ is it making or breaking the bond :frown:

an explanation would be super useful 😊

Hi there!
I completed my A levels and am going to study computer science. I am offering A-level tutoring for CHEMISTRY and biology.
I also have lots of Anki flashcards I am more than happy to share with my students.
Kindly get in touch if you want more info!

A level chemistry help

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