The following describes the reaction between a copper penny and nitric acid in an open beaker:
Cu (s) + 4 HNO3 (aq) --> Cu(NO3)2 (aq) +2 H2O (l) + 2 NO2 (g)
The mass of the beaker and contents was monitored until the reaction ended. The results: (224.6g, 0s) and (215.8g, 118.6s)
What chemical is represented by the change in mass? Explain why it cannot be the mass change of copper.
the gaseous product (NO2) represents the change in mass
... it does not stay in solution in the beaker
the copper stays in solution
The loss in mass is due to the NO2 gas escaping into the atmosphere. It can be the Cu because all of the copper solid left but ended up as Cu(NO3)2. I have no idea what the numbers mean, especially 0s and 118.6s.
oops. typo. It CAN'T be the Cu ----
the "s" data must be elapsed time for the reaction.
The chemical represented by the change in mass in this reaction is the formation of copper(II) nitrate (Cu(NO3)2), which is soluble in water (aq).
To understand why the change in mass cannot be attributed to the copper, we need to examine the balanced chemical equation:
Cu (s) + 4 HNO3 (aq) → Cu(NO3)2 (aq) + 2 H2O (l) + 2 NO2 (g)
According to the equation, the copper atoms (Cu) in the solid state will react with the nitric acid (HNO3) to form copper(II) nitrate (Cu(NO3)2) in aqueous solution. However, the equation clearly shows that the copper atoms are still present in the final product as copper(II) ions (Cu2+). This means that the copper itself does not disappear or get consumed during the reaction; it simply changes form from a solid to an aqueous ion.
In an open beaker experiment like this, it is common for volatile gases to form and escape into the atmosphere, and this can cause a change in the overall mass of the system. In this case, the production of nitrogen dioxide (NO2) gas is indicated in the chemical equation. As the gas is released during the reaction, it escapes from the beaker into the surrounding environment, resulting in a decrease in the overall mass of the system. Therefore, the change in mass observed can be attributed to the loss of the volatile gas, NO2, rather than a change in the mass of the copper or the resulting copper(II) nitrate.
To confirm this, we can calculate the mass of the nitrogen dioxide gas produced using the difference in the initial and final masses.
Initial mass: 224.6 g
Final mass: 215.8 g
Mass change = Initial mass - Final mass
= 224.6 g - 215.8 g
= 8.8 g
Since the nitrogen dioxide (NO2) gas has a molar mass of approximately 46 g/mol, we can calculate the moles of NO2 produced by dividing the mass change by the molar mass of NO2:
Moles of NO2 = Mass change / Molar mass of NO2
= 8.8 g / 46 g/mol ≈ 0.191 moles
Therefore, the observed change in mass is due to the escape of approximately 0.191 moles of nitrogen dioxide gas (NO2) from the open beaker during the reaction.