Standard enthalpy change of formation
The definition says,
"This is the enthalpy change measured at 100 kPa and a specified temperature, usually 298 K, when one mole of a substance is formed from its elements in their standard states."
I have two questions/confusions about it-
1. Do both the elements (reactants) and the product formed (from those elements) have to be in their standard states ?
2. Since the definition mentions one mole of a substance, does that mean one mole of ONLY ONE product has to form ?
If so, then why is the thermal decomposition of Calcium Carbonate considered to have standard enthalpy change of formation even though two products (Calcium Oxide and Carbon Dioxide) are formed ?
"This is the enthalpy change measured at 100 kPa and a specified temperature, usually 298 K, when one mole of a substance is formed from its elements in their standard states."
I have two questions/confusions about it-
1. Do both the elements (reactants) and the product formed (from those elements) have to be in their standard states ?
2. Since the definition mentions one mole of a substance, does that mean one mole of ONLY ONE product has to form ?
If so, then why is the thermal decomposition of Calcium Carbonate considered to have standard enthalpy change of formation even though two products (Calcium Oxide and Carbon Dioxide) are formed ?
(edited 5 years ago)
Original post by Missradioactive
The definition says,
"This is the enthalpy change measured at 100 kPa and a specified temperature, usually 298 K, when one mole of a substance is formed from its elements in their standard states."
"This is the enthalpy change measured at 100 kPa and a specified temperature, usually 298 K, when one mole of a substance is formed from its elements in their standard states."
Edited : my bad I didn't check properly
According to https://en.m.wikipedia.org/wiki/Standard_enthalpy_of_formation its both reactants and products...
(edited 5 years ago)
Original post by Missradioactive
"This is the enthalpy change measured at 100 kPa and a specified temperature, usually 298 K, when one mole of a substance is formed from its elements in their standard states."
2. Since the definition mentions one mole of a substance, does that mean one mole of ONLY ONE product has to form ?
If so, then why is the thermal decomposition of Calcium Carbonate considered to have standard enthalpy change of formation even though two products (Calcium Oxide and Carbon Dioxide) are formed ?
"This is the enthalpy change measured at 100 kPa and a specified temperature, usually 298 K, when one mole of a substance is formed from its elements in their standard states."
2. Since the definition mentions one mole of a substance, does that mean one mole of ONLY ONE product has to form ?
If so, then why is the thermal decomposition of Calcium Carbonate considered to have standard enthalpy change of formation even though two products (Calcium Oxide and Carbon Dioxide) are formed ?
I don't think decomposition of CaCO3 is not standard enthalpy change of formation. Primarily because the definition states only one substance being formed, not two of 'em
So for instance, formation of carbon dioxide reaction equation will be totally different from formation of calcium oxide equation...
For CO2
C (s) + O2 (g) ----> CO2(g)
For CaO
Ca(s) + 1/2O2 (g) -----> CaO (s)
(edited 5 years ago)
Original post by Missradioactive
for not being here sooner There's something wrong with the quote response timing
(edited 5 years ago)
Enthalpy of formation is the energy change associated with the formation of 1 mol of a substance from its constituent elements. All substances (reactants and products) must be in their standard states. If it is the standard enthalpy of formation, they are in their standard states under standard conditions.
Decomposition of CaCO3 cannot be enthalpy of formation as you are neither forming 1 mol of substance (you are forming two substances) nor do you form it (them) from their constituent elements.
Decomposition of CaCO3 cannot be enthalpy of formation as you are neither forming 1 mol of substance (you are forming two substances) nor do you form it (them) from their constituent elements.
Oops sorry I was late, but I think the other posters have summed it up pretty nicely. To spot a standard enthalpy of formation equation you have to first see that there is only one mole of a 'product', which is formed from its elements in the reactants side of the equation, all in standard states.
Ca (s) + C (s) + 3/2 O2 (g) --> CaCO3 (s)
I think this would be the equation for the standard enthalpy of formation of calcium carbonate.
Ca (s) + C (s) + 3/2 O2 (g) --> CaCO3 (s)
I think this would be the equation for the standard enthalpy of formation of calcium carbonate.
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