A student mixes 5 mL of 2.00 x 10^-3 M Fe(NO3)3 with 5 mL 2.00 x 10 ^-3 M KSCN. They find that in the equilibrium mixture concentration of FeSCN^2+ is 1.40 x 10^-4 M.
Find each of the following: the initial concentration in solution for Fe^3+ and SCN, the equilibrium constant for the reaction, and explain what happened to the K and the NO3 ions in the solution.
To find the initial concentration in solution for Fe^3+ and SCN, we need to understand the stoichiometry of the reaction.
The balanced chemical equation for the reaction between Fe^3+ and SCN is as follows:
Fe^3+ + 3SCN- ↔ Fe(SCN)3
From the balanced equation, we can see that 1 mole of Fe^3+ reacts with 3 moles of SCN- to form 1 mole of Fe(SCN)3. This means that the initial concentration of Fe^3+ and SCN- in the mixture before any reaction occurs is equal to the concentration of Fe(SCN)3 formed during the reaction.
Given that the concentration of Fe(SCN)2+ in the equilibrium mixture is 1.40 x 10^-4 M, we can conclude that the initial concentration of Fe^3+ and SCN- is also 1.40 x 10^-4 M each.
Now, let's move on to calculating the equilibrium constant (K) for the reaction. The equilibrium constant expression for the reaction is given by:
K = ([Fe(SCN)2+]) / ([Fe^3+][SCN-]^3)
The given concentration of Fe(SCN)2+ is 1.40 x 10^-4 M. The initial concentration of Fe^3+ and SCN- is also 1.40 x 10^-4 M each. Plugging in these values, we get:
K = (1.40 x 10^-4 M) / ((1.40 x 10^-4 M)(1.40 x 10^-4 M)^3)
Simplifying this expression we get:
K = 1 / (1.40 x 10^-4 M^2)
To find what happened to the K and NO3 ions in the solution, we need to look at the reactants in the original equation. In this case, the KSCN dissociates into K+ and SCN-, while Fe(NO3)3 dissociates into Fe^3+ and NO3- ions.
During the reaction, the Fe^3+ ions and SCN- ions react to form Fe(SCN)2+ ions. The K+ ions and NO3- ions do not participate in the reaction and therefore remain in the solution. This means that the K+ ions and NO3- ions are spectator ions and do not affect the equilibrium constant or the final concentration of Fe(SCN)2+.
In summary, the initial concentration in solution for Fe^3+ and SCN- is 1.40 x 10^-4 M each. The equilibrium constant for the reaction is 1 / (1.40 x 10^-4 M^2). The K+ and NO3- ions do not participate in the reaction and therefore do not affect the equilibrium constant or the final concentration of Fe(SCN)2+.
initial concn Fe(NO3)3 = 2.00E-3 x (5/10) = ?
initial concn KSCN = 2.00E-3 x (5/10) = ?
.........SCN^- + Fe^3+ ==> Fe(SCN)^2+
I.......1E-3....1E-3..........0
C.........-x.....-x...........x
E.......1E-3-x.,.1E-3-x.......x
The problem tells you that x = 1.4E-4M
Evaluate 1E-3 -x, substitute into the Kc expression, and solve for Kc.
Where do NO3^- and K^+ go. They become spectator ions in the solution; ie., one associated with KNO3 and the other two with FeSCN(NO3)2