Questions 18-20 refer to the reaction 2nh3 (g) ↔ n2 (g) + 3h2 (g), and δh = 92.2 kj. what will be the effect on the concentration of h2 if the temperature decreases? a. h2 concentration will decrease. b. h2 concentration will not change. c. h2 concentration will increase. d. h2 concentration will not change, but δh will decrease e. h2 concentration will not change, but pressure will increase. what will be the effect on the concentration of nh3 if the pressure decreases? a. nh3 concentration will decrease. b. nh3 concentration will not change. c. nh3 concentration will increase. d. nh3 concentration will not change, but δh will decrease. e. nh3 concentration will not change, but entropy will increase. what will be the effect on n2 if the concentration of nh3 increases? a. n2 concentration will decrease. b. n2 concentration will not change. c. n2 concentration will increase. d. n2 concentration will not change, but δh will decrease. e. n2 concentration will not change, but pressure will increase.

 
Since the enthalpy of reaction is positive, the forward reaction is an endothermic reaction which means the energy is gained from the surrounding to happen the reaction. If the temperature decreases, according to the Le Chatelier's principle, the system tries to become equilibrium by increasing temperature. Since forward reaction is endothermic (because of the bond breaking), the backward reaction is exothermic (because of the bond making) which releases the energy to the surroundings. This makes the increase of temperature. So if the backward reaction is promoted because of the decrease of temperature, then the concentration of H₂ will decrease.

19. Answer is A.

The reactant side has 2 moles/molecules of reactants and the product side has 4 moles/molecules of products which come from 1 N₂(g) and 3 H₂(g). If the pressure is reduced in the system, according to the Le Chatelier's principle, the system tries to increase the pressure. Hence, forward reaction is promoted because of the higher number of molecules in product side. If the forward reaction is promoted, the concentration of NH₃(g) will decreased.

20. Answer is C.

If the concentration of reactant is increased in the system, according to the Le Chatelier's principle, the system tries to reduce the concentration of that reactant. So if NH₃(g) concentration is increased, then to be equilibrium, the forward reaction will be promoted. Then the concentration of N₂(g) will increase.

gravity causes stares to cluster i think so