The formula of silver nitrate is AgNO₃ and the formula of sodium sulphide is Na₂S.
AgNO₃ + Na₂S ----> ???
We have to predict the products of this reaction, this is a double replacement reaction or a precipitation where the ions of the reactants will be exchanged.
We have four ions:
silver ion: Ag⁺ nitrate ion: NO₃⁻ sodium ion: Na⁺ sulphide ion: S²⁻
If we have to combine the ions in a different way we can form:
silver sulphide: Ag₂S sodium nitrate: NaNO₃
The unbalanced reaction will be:
AgNO₃ + Na₂S ----> Ag₂S + NaNO₃
We have to write the states of these compounds and balance it. These all are aqueous solutions except from Ag₂S that insoluble, so we can write it as a solid.
AgNO₃ (aq) + Na₂S (aq) ----> Ag₂S (s) + NaNO₃ (aq)
Finally we have to balance it. We have to determine the number of atoms of each element that we have on both sides of the equation:
AgNO₃ (aq) + Na₂S (aq) ----> Ag₂S (s) + NaNO₃ (aq)
Ag: 1 Ag: 2
N: 1 N: 1
O: 3 O: 3
Na: 2 Na: 1
S: 1 S: 1
We have two atoms of Ag on the right side of the equation and just one atom of Ag on the left. We can change the coefficient for AgNO₃ and write a 2 in front of it.
2 AgNO₃ (aq) + Na₂S (aq) ----> Ag₂S (s) + NaNO₃ (aq)
Ag: 2 Ag: 2
N: 2 N: 1
O: 6 O: 3
Na: 2 Na: 1
S: 1 S: 1
Now the Ag is balanced. We have two atoms of N on the left side and just one on the right side. We can change the coefficient for NaNO₃ and write a 2 there.
2 AgNO₃ (aq) + Na₂S (aq) ----> Ag₂S (s) + 2 NaNO₃ (aq)
Ag: 2 Ag: 2
N: 2 N: 2
O: 6 O: 6
Na: 2 Na: 2
S: 1 S: 1
And finally the equation is balanced.
Answer: 2 AgNO₃ (aq) + Na₂S (aq) ----> Ag₂S (s) + 2 NaNO₃ (aq)