Tauira o ngā Pātai Kōrero mō te Waikawa Tote
Ko te waikura tote he āhuatanga matū e uru ana ki te tauhohenga o te tote ki te wai, ka puta he waikawa, he kawa rānei hei hua taha. He kaupapa nui tēnei tukanga i roto i ngā akoranga matū nā te mea mā te mārama ki te waikura tote ka taea te whakarato i te māramatanga taketake ki ngā āhuatanga o ngā otinga me te āwhina ki te whakatau i te pH o ngā otinga. I roto i tēnei tuhinga, ka matapakihia e mātou ētahi tauira raruraru me ō rātou otinga e pā ana ki te waikura tote.
Ngā Ariā Taketake o te Wairewa Tote
Ka puta te waikura tote ina pirau te tote i roto i te wai, ā, ka tauhohe ōna katote ki ngā ngota wai hei whakaputa waikawa, hei kawa rānei. Kāore e waikura katoa ngā tote i roto i te wai; ko ētahi he kūpapa, arā, kāore e whakarerekē i te pH o te wairewa. Heoi, ka taea e ētahi tote te whakaputa waikura waikawa, waikura rānei, i runga i te pūtake o te waikawa me te kawa i hanga ai te tote.
Hei mārama ki tēnei tikanga, he mea nui kia mōhio koe ki ēnei:
– Ko ngā tote o ngā waikawa kaha me ngā kawakore kaha (pērā i te NaCl) kāore e waimeha, ā, ka noho kūpapa tonu te otinga.
– Ko ngā tote o ngā waikawa ngoikore me ngā kawakore kaha (pērā i te CH3COONa) ka puta he otinga kawakore.
– Ko ngā tote o ngā waikawa kaha me ngā kawakore ngoikore (pērā i te NH4Cl) ka puta he wairewa waikawa.
– Ka taea e ngā tote o ngā waikawa ngoikore me ngā kawakore ngoikore te tauhohe ki te wai, ā, ka taea te whakatau i te pH whakamutunga i runga i te kaha whanaunga o te waikawa me te kawakore.
Ngā Pātai Tauira me te Kōrero
Pātai 1: Te Whakapūngao o te Pāhare pākawa waro (CH3COOK)
Pātai:
He waikawa, he kawakore, he kūpapa rānei te otinga pāhare pākawa waro (CH3COOK)? Tuhia te tauhohenga waikura e puta ana, ā, whakatauhia te pH o te otinga.
Kōrero:
Ko te pāhare pākawa waro (CH3COOK) he tote o te waikawa ngoikore (waikawa waro, CH3COOH) me te turanga kaha (pāhare pākawa waro, KOH). Mai i tēnei, ka taea e tātou te whakatau ko te otinga CH3COOK he turanga pea nā te mea ka taea e te katote waro (CH3COO⁻) te waikura i roto i te wai.
Ko te tauhohenga penei:
\[ CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^- \]
Ka tangohia e ngā iona acetate (CH3COO⁻) ngā porotona (H⁺) mai i te wai (H2O) hei hanga i te waikawa acetic (CH3COOH) me ngā iona hauwai (OH⁻). Nā te noho mai o ngā iona hauwai (OH⁻) i roto i te wairewa ka waiho hei matūriki.
Hei whakatau i te pH, me whai i te kukū o ngā iona OH⁻. Hei tauira, he 0,1 M te wairewa CH3COOK kei a tātou, ā, ko te pūmau iona o te waikawa acetic (Ka) he 1,8 × 10⁻⁵.
Ko ngā mahi i mahia ko:
1. Te whakamahi i te pūmau turanga (Kb) mō te katote CH3COO⁻:
\[ K_b = \frac{K_w}{K_a} = \frac{10^{-14}}{1,8 \times 10^{-5}} = 5,56 \times 10^{-10} \]
2. Wehewehea ngā iona CH3COO⁻ i roto i te otinga:
\[ K_b = \frac{[CH_3COOH][OH^-]}{[CH_3COO^-]} \]
Kia 'x' te taumata o te wehenga:
\[ 5,56 \times 10^{-10} = \frac{x^2}{0,1-x} \]
Mā te kore e aro ki te \(x\) iti i roto i te taupū, ka taea e tātou te whakangawari:
\[ x^2 \tata ki te 5,56 \times 10^{-11} \Rightarrow x = \sqrt{5,56 \times 10^{-11}} = 7,45 \times 10^{-6} \]
Te kukū o te OH⁻:
\[ [OH^-] = 7,45 \times 10^{-6} \]
pOH o te otinga:
\[ pOH = -\log [OH^-] = -\log (7,45 \times 10^{-6}) \approx 5,13 \]
Mā te whakamahi i te whanaungatanga i waenga i te pH me te pOH:
\[ pH = 14 – pOH = 14 – 5,13 = 8,87 \]
Nō reira, he waiwai te otinga CH3COOK, ā, ko te pH o te 8,87 pea.
Pātai 2: Te Whakapūngao o te Haukinia Pūhaumāota (NH4Cl)
Pātai:
He waikawa, he kawakore, he kūpapa rānei te otinga ammonium chloride (NH4Cl)? Tuhia te tauhohenga waikura e puta ana, ā, whakatauhia te pH o te otinga.
Kōrero:
He tote te haukiniuma konupora (NH4Cl) o te waikawa kaha (HCl) me te turanga ngoikore (NH3). Mai i tēnei, ka taea e tātou te whakatau he waikawa te otinga nā te mea ka taea e te katote haukiniuma (NH4⁺) te waikura i roto i te wai.
Ko te tauhohenga penei:
\[ NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+ \]
Ka tukuna e ngā iona haukinia (NH4⁺) ngā porotona (H⁺) ki ngā ngota wai (H2O) hei hanga i te haukinia (NH3) me ngā iona hauwainiua (H3O⁺). Nā te noho mai o ngā iona hauwainiua (H3O⁺) i roto i tēnei wairewa ka waikawa te wairewa.
Hei tauira, he 0,1 M te nui o te NH4Cl kei a tātou. Ko te pūmau ionization turanga (Kb) mō te NH3 he 1,8 × 10⁻⁵.
Ko ngā mahi i mahia ko:
1. Te whakamahi i te pūmau waikawa (Ka) mō te katote NH4⁺:
\[ K_a = \frac{K_w}{K_b} = \frac{10^{-14}}{1,8 \times 10^{-5}} = 5,56 \times 10^{-10} \]
2. Wehewehea ngā iona NH4⁺ i roto i te otinga:
\[ K_a = \frac{[NH_3][H_3O^+]}{[NH_4^+]} \]
Kia 'x' te taumata o te wehenga:
\[ 5,56 \times 10^{-10} = \frac{x^2}{0,1-x} \]
Mā te kore e aro ki te \(x\) iti i roto i te taupū, ka taea e tātou te whakangawari:
\[ x^2 \tata ki te 5,56 \times 10^{-11} \Rightarrow x = \sqrt{5,56 \times 10^{-11}} = 7,45 \times 10^{-6} \]
Te kukū o te H3O⁺:
\[ [H_3O^+] = 7,45 \whakareatia ki te 10^{-6} \]
pH o te otinga:
pH = -\log [H_3O^+] = -\log (7,45 \times 10^{-6}) \approx 5,13 \]
Nō reira, he waikawa te otinga NH4Cl, ā, ko te pH o te 5,13 pea.
I ngā tauira e rua i runga ake nei, ka kite tātou me pēhea te tautuhi i te momo waikura hei āwhina i te whakatau i te āhua o tētahi otinga. Mā te whakamahi i te pūmau taurite ionization (Ka, Kb rānei) me ētahi whakaaro māmā noa iho, ka taea e tātou te whakaoti i ēnei raruraru me te ngāwari noa iho.
Ehara i te mea ko te whakangungu i ngā pūkenga tātari i roto i ngā tātaitanga pāngarau anake tēnei matapakinga mō ngā raruraru waikura tote, engari ka whakahōhonu ake hoki i te māramatanga ki ngā ariā taketake o te matū otinga. Ko te mārama ki te āhua o te takoha a ngā iona o ngā pūtake rerekē i ngā porotona, i ngā kaiwhiwhi rānei ki ngā ngota wai, me te pānga o tēnei ki te pH o te otinga, he mea nui ki te matatau ki ngā kaupapa matatau maha i roto i te matū.
