Why is chemical equilibrium dynamic?
It's dynamic because something is still happening. Namely, if you have a process at equilibrium, the concentrations of the reagents and products may not change. However, this isn't because there is no reaction taking place - it's because the forward and reverse reactions take place at the same rate.
How does a reaction reach dynamic equilibrium?
I just answered this question elsewhere on the site, so I'll just link to the answer for convenience's sake
What are equilibrium constants?
Equilibrium constants (Keq) are a way of the quotient of a reaction which has reached equilibrium.
The equilibrium expression allows a calculation to be performed which tells us the levels of reactants vs. products.
A large Keq means that the reaction REALLY favors the formation of products (forward reaction).
A small Keq means that the reaction REALLY favors the formation of reactants (reverse reaction).
Does equilibrium constant change with concentration?
Equilibrium constants only change with temperature.
When learning about the derivation of equilibrium constants, we learn that it is derived from the reaction rates such that the constant may be calculated from the concentrations of all reactants and products and that it is distinct from a reaction quotient.
aA+bB⇔cC+dD
Keq=[C]c[D]d[A]a[B]b
Q=[C]c[D]d[A]a[B]b
Even though the variables appear to be the same, the calculation for Keq is done with the concentrations when the reaction is at equilibrium. We know that reaction rates change with temperature, such as the famous experiment of the color change of NO2 gas in a sealed glass tube to the colorless N2O4 gas as it is heated and cooled. This is because even though the actual equilibrium point changes with more or less reactants and products, the quotient remains the same.
If you diluted the NO2 in the above example with additional N2O4, the equilibrium constant would remain a constant and the concentrations of the gases would change according to Le Chatelier's Principle.