Answer to Question #60525 in General Chemistry for Honam

Question

Consider the following equilibrium system:
Cu(s) + 2Ag+ (aq) <-> 2Ag(s) + Cu2+ (aq)
Which of the following indicates that the system is at equilibrium?
A. [Cu2+(aq)] = [Ag+(aq)]
B. 2[Cu2+(aq)] = [Ag+(aq)]
C. the mass of Cu(s) remains constant
D. the mass of the entire system remains constant
I want to know why option D is incorrect. Thank you so much!

Answer 1

The main condition, which indicate that system is at equilibrium, is the equality of the speed of direct and reverse reactions.
The mass of the entire system remains constant not only in the equlibrium. The mass of the entire system remains constant always when reaction occur. This is one of the fundamental law of chemistry (see law of conservation of mass).
For example:
1) When reagents does not react, their mass is constant.
2) When reaction occur fully, the mass of products equal mass of reagents.
3) When system is not at equilibrium the sum of mass of all components of reaction is equal the mass of reagents before reaction.
As result, fact that the mass of the entire system remains constant doesn’t indicate that system is at equlibrium.