An experiment is performed to determine the amount of hydrogen gas released in a reaction. Hydrogen gas is released by reacting hydrochloric acid with zinc metal according to the given equation. Zn solid plus two HCl aqueous react to form ZnCl₂ aqueous plus H₂ gas. All the Zn metal was consumed, and hydrogen gas was collected by displacement of water in an inverted bottle. The following data is obtained. Why is it essential to know the water temperature in this experiment? I) So that the vapor pressure of water can be looked up. II) Because the rate of evaporation will affect the rate of reaction. III) Because the reaction will not occur below 10 degrees Celsius. A) I only, B) II only, C) I and II, D) II and III, or E) I, II and III.
In the experiment that’s described in this question, zinc metal and hydrochloric acid are reacted. In the experiment that’s described in this question, zinc metal is reacted with hydrochloric acid, which produces zinc chloride and hydrogen gas. The goal of this experiment is to determine how much hydrogen gas is released as a result of this reaction, which we’ll do by collecting the hydrogen gas that’s released as a result of the reaction over water. What this means is that as hydrogen gas is produced during this reaction, it will be taken to a different container that is initially filled with water. But as the hydrogen is produced during this reaction, some of that gas will displace some of the water in the container.
At the end of the experiment, we’ll be able to determine how much hydrogen gas we collected and therefore determine the amount of hydrogen gas that was released during the reaction. In this question, we’re being asked to explain why it’s essential to know the water temperature in this experiment. Well, in this experiment, we’re collecting hydrogen gas over water. And while currently in the diagram, I’ve shown that hydrogen gas is the only thing in our collection bottle. But at any given temperature, some amount of water molecules are able to evaporate to form water vapor. So, there would actually be some small amount of water vapor in our container as well.
So, in order to determine the amount of hydrogen gas that we collected during our experiment, which is ultimately the goal, we need to subtract out the amount of water vapor that’s also in the bottle. The way that we can do this is by first aligning the water level inside the bottle with the water level outside the bottle. Because at this point, we know that the pressure from the atmosphere pushing down on the water outside the bottle is going to be the same as the pressure from the gases, that is, the hydrogen gas and the water vapor pushing down on the inside of the bottle. In other words, at this point, the atmospheric pressure will be equal to the pressure of the hydrogen gas that we collected plus the pressure of the water vapor that’s inside the bottle.
We can measure the pressure of the atmosphere that day, and it’s listed in the table of data that was collected. The vapor pressure of water is something that we can look up. But we have to know the temperature of the water in order to look this up since the amount of water that’s going to evaporate will change depending on the temperature. With these two pieces of information, we can find the pressure of hydrogen gas that was collected and then we can solve for the amount of hydrogen gas that was released as a result of the reaction, which is ultimately the goal of this experimental setup.
What I’ve explained so far matches statement I. It’s essential to know the water temperature in this experiment so that the vapor pressure of water can be looked up. Statement II says it’s essential to know the water temperature in this experiment because the rate of evaporation will affect the rate of reaction. As we can see from this diagram, the reaction is occurring in a container that’s separate from the water. So, it wouldn’t make much sense that the rate of evaporation would affect the rate of reaction. Also, if we look at the equation that describes this reaction, water is neither a reactant nor a product. So, statement II isn’t correct.
Statement III says that it’s essential to know the water temperature in this experiment because the reaction will not occur below 10 degrees Celsius. There was nothing given in the problem to suggest that this statement is true. So, statement III is not correct. Of the statements we were given, only statement I was correct, which matches answer choice A. It was essential to know the water temperature in this experiment so that the vapor pressure of water can be looked up. So that we can accomplish our goal of determining the amount of hydrogen gas that’s released as a result of the reaction.