$3.Anot$: `2Cu+2OH^{-}->Cu_2O+H_2O+2e`

$Catot:$ `2H_2O+2e->H_2+2OH^-`

`4.`

$a) Catot$:`2Cu^{2+}+2e+H_2O->Cu_2O+2H^{+}`

b) Để pH có giá trị max thì `K_{sp}>=[Cu^{2+}][OH^-]^2`

`=>[OH^-]=\sqrt{(2.10^{-19})/(0,1)}~~1,41.10^{-9}(M)`

`=> pH=14+log[OH^-]~~5,15`

`c) `

`Cu_2O+H_2O+2e->2Cu+2OH^{-} K_1=(2.(-0,36))/(0,059)`

`Cu^{2+}+2e->Cu.     K_2=(2.0,337)/(0,059)`

`H_2O->H^{+}+OH^{-}.    K_w=10^{-14}`

Ta có: `K=K_1^{-1}.K_2^{2}.K_w^{2}~~10^{7,051}`

`=> (2.E_{Cu^{2+}//Cu_2O}^0)/(0,059)=7,051`

`=> E_{Cu^{2+}//Cu_2O}^0=0,208(V)`

Để pH min thì `E_{Cu^{2+}//Cu}=E_{Cu^{2+}//Cu_2O}`

`=> 0,337+(0,059)/(2).log[Cu^{2+}]=0,208+(0,059)/(2).log(([Cu^{2+}]^2)/[H^+]^2)`

`=> [H^+]^2~~([Cu^{2+}])/(23598,335)`

`=> [H^+]~~0,0020585 => pH~~2,686`