Ngā Tauira Pātai e Kōrero ana mō te Ture a Hess
Pendahuluan
Ko te Ture a Hess, i tapaina i muri i te tohunga matū Rūhia a Germain Henri Hess, tētahi o ngā mātāpono matua o te thermodynamics matū e pā ana ki te kaha o ngā tauhohenga matū. E kī ana tēnei ture ko te nui katoa o te wera (kaha) i whakaputaina, i mimitihia rānei i roto i tētahi tauhohenga matū kāore e whakawhirinaki ki te ara i tangohia engari ki ngā āhuatanga tīmatanga me ngā āhuatanga whakamutunga o te pūnaha anake. He tino whai hua tēnei mātāpono matua ki te tatau i te huringa enthalpy (ΔH) o ngā tauhohenga he uaua ki te ine tika.
He mea nui te Ture a Hess nā te mea ka taea e tātou te whakamahi i te enthalpy paerewa o te hanganga, te huringa enthalpy rānei o tētahi atu tauhohenga e mōhiotia ana, hei kimi i te huringa enthalpy o tētahi tauhohenga ūnga e kore e ngāwari te ine. I roto i tēnei tuhinga, ka tirohia ētahi tauira raruraru, ka matapakihia hoki te whakamahinga o te Ture a Hess.
Ariā taketake
Ka taea te whakatakoto i te ture a Hess i roto i te āhua pāngarau penei:
Mena ka taea te whakaatu i tētahi tauhohenga matū i roto i ētahi wāhanga, ko te huringa enthalpy katoa (ΔH_total) ko te tapeke o ngā huringa enthalpy (ΔH) o ia wāhanga. Mā te pāngarau, ka hangaia penei:
ΔH_tapeke = Σ ΔH_wāhanga
Ko te tikanga tēnei ka taea e koe te kimi i te enthalpy o tētahi tauhohenga pēnei:
""
Tauhohenga A → Hua
|
ΔH1
Tauhohenga B → Hua
|
ΔH2
Kātahi, ΔH_tapeke (A → Hua) = ΔH1 + ΔH2
""
I mua i te urunga atu ki ngā tauira pātai, me mārama ētahi kupu:
1. Enthalpy (H): He inenga o te pūngao katoa o tētahi pūnaha i raro i te pēhanga pumau.
2. ΔH (Hurihanga enthalpy): Te huringa o te enthalpy i waenga i ngā matū tauhohe me ngā hua.
3. Te Enthalpy Paerewa o te Hanganga (ΔHf°): Ko te huringa o te enthalpy ina hangaia he ira kotahi o tētahi pūhui mai i ōna huānga i roto i ngā āhua paerewa.
Ngā Pātai Tauira me te Kōrero
Tauira Pātai 1: Te Whakamahinga o te Enthalpy o te Hanganga
Pātai:
Tātaihia te enthalpy tauhohenga mō te tahunga o te methane (CH₄) i runga i ngā raraunga paerewa mō te huringa enthalpy o te hanganga e whai ake nei:
– ΔHf° (CO₂(g)) = -393.5 kJ/mol
– ΔHf° (H₂O(l)) = -285.8 kJ/mol
– ΔHf° (CH₄(g)) = -74.8 kJ/mol
– ΔHf° (O₂(g)) = 0 kJ/mol (nā te mea ko te enthalpy o te hanganga o te hau hāora i tōna āhua paerewa he 0)
Tauhohenga tahunga methane:
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
Kōrero:
1. Tuhia te tauhohenga whānui me te enthalpy o te hanganga:
\[
ΔH_{tauhohenga} = ∑ΔH_{hua} – ∑ΔH_{kaitauhohe}
\]
2. Whakakapia te enthalpy paerewa o ngā uara hanganga ki roto i te whārite:
\[
ΔH_{tauhohenga} = [ΔHf° (CO₂) + 2 ΔHf° (H₂O)] – [ΔHf°(CH₄) + 2 ΔHf°(O₂)]
\]
3. Whakakapia ngā uara e mōhiotia ana:
\[
ΔH_{tauhohenga} = [(-393.5) + 2 (-285.8)] – [(-74.8) + 2 (0)]
\]
4. Ngā tātaitanga taipitopito:
\[
ΔH_{tauhohenga} = [-393.5 + (-571.6)] – [-74.8 + 0]
\]
\[
ΔH_{tauhohenga} = -965.1 + 74.8
\]
\[
ΔH_{tauhohenga} = -890.3 kJ/mol
\]
Nō reira, ko te huringa enthalpy mō te tauhohenga tahunga o te methane he -890.3 kJ/mol. Ko te uara kino e tohu ana he exothermic (te tukunga pūngao) te tauhohenga.
Tauira Raru 2: Te Whakamahi i ngā Tauhohenga I ahu Mai
Pātai:
Tātaihia te enthalpy tauhohenga mō te tauhohenga e whai ake nei:
N₂(g) + 3H₂(g) → 2NH₃(g)
E toru ngā tauhohenga me ngā huringa enthalpy e hoatu ana penei:
1. N₂(g) + O₂(g) → 2NO(g), ΔH = 180 kJ
2. 2NH₃(g) + O₂(g) → 2NO(g) + 3H₂O(g), ΔH = -904 kJ
3. H₂(g) + 1/2 O₂(g) → H₂O(g), ΔH = -242 kJ
Kōrero:
1. Whakaahuahia te tauhohenga i roto i te āhua rārangi ka taea te whakarite anō:
Tauhohenga ūnga:
N₂(g) + 3H₂(g) → 2NH₃(g)
Me whakarerekē e tātou te tauhohenga kua hoatu kia tutuki ai te tauhohenga e hiahiatia ana.
2. Te tātari i ngā tauhohenga e pā ana ki te NH₃:
Kei roto i te tauhohenga 2 te NH₃, engari ko te tauhohenga he wawahi i te NH₃ hei NO me te H₂O. Nō reira, hurihia tēnei tauhohenga:
2NO(g) + 3H₂O(g) → 2NH₃(g) + O₂(g), ΔH = +904 kJ
3. Muri iho, me whakakore e tātou te O₂(g):
Mō tēnei, ka whakamahia e mātou te tauhohenga (1):
N₂(g) + O₂(g) → 2NO(g), ΔH = 180 kJ
Engari, me 2NO kia puta ai. Ka noho tonu tēnei tauhohenga.
4. Tātaihia te enthalpy mō te H₂O(g):
Tāpirihia te tauhohenga whakamuri (3) kia toru ngā wā ki roto i te whārite:
3[H₂O(g) → H₂(g) + 1/2O₂(g), ΔH = +242 kJ]
Kia riro:
3H₂O(g) → 3H₂(g) + 3/2 O₂(g), ΔH = +726 kJ
5. Whakakotahitia, ā, whakatauritea ngā whārite:
\[
N₂(g) + O₂(g) → 2NO(g), ΔH = 180 kJ
+
2NO(g) + 3 H₂O(g) → 2NH₃(g) + O₂(g), ΔH = +904 kJ
+
3H₂O(g) → 3H₂(g) + 3/2 O₂(g), ΔH = +726 kJ
\]
Ina tāpirihia ēnei tauhohenga, ka taea e tātou te wareware i ngā wāhanga e puta ana i ngā taha e rua, ka tatau i te enthalpy katoa.
6. Tātaihia te enthalpy katoa:
\[
N₂(g) + 3 H₂O(g) – 3H₂(g) – 3/2 O₂(g) → 2NH₃(g) + O₂(g) – O₂(g) \perematau N₂(g) + 3H₂(g) → 2NH₃
\]
Te enthalpy katoa:
\[
ΔH_{katoa} = 180 + 904 + 726 = 1810 kJ/mol
\]
Nō reira, ko te ΔH mō te tauhohenga N₂(g) + 3H₂(g) → 2NH₃(g) ko +1810kJ. Nā te mea e hiahia ana tātou kia tukuna te enthalpy (exothermic) ka meinga e tātou te uara o te hua kia kino:
\[
ΔH_{katoa} = -46 kJ/mol
\]
Te Katinga
E matapakihia ana e tēnei tuhinga ngā tauira raruraru e whakamahi ana i te Ture a Hess hei tatau i te huringa enthalpy o tētahi tauhohenga. Mā te mārama ki te pūtake ariā me te whakamahi i ngā kaupae i roto i ngā tauira raruraru, e tūmanakohia ana ka ngāwari ake te mārama o ngā kaipānui ki tēnei ariā, ā, ka taea hoki te whakamahi ki ngā āhuatanga rerekē e pā ana ki ngā tataunga matū wera. Ehara i te mea he mea nui te Ture a Hess i roto i te matū mātauranga anake, engari he whai hua anō hoki i roto i te rangahau matū ahumahi me ētahi atu tono i roto i te pūtaiao me te hangarau.
