Gas Evolution Reaction Lab Report

Gas Evolution Reaction Lab Report

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Gas Evolution Reaction Lab Report

Experiment 5: GAS EVOLUTION REACTION Chemistry 121

I. Background / Theory

Many metals react with acids to produce hydrogen gas. These reactions are REDOX, gas evolution reactions. The metal atoms lose electrons to become metal ions. The hydrogen ions (from the acid) gain electrons to become hydrogen atoms, and two hydrogen atoms combine to form the hydrogen molecule, H2, which is a gas at room temperature. Under standard temperature and pressure, STP, all gases act like an ideal gas, thus following the ideal gas law with respect to the relationship between moles of the gas, pressure of the gas, temperature of the gas, and volume of the gas.

Based on the relationships of these different factors, the molar volume, or the volume of one mole of a gas, for an ideal gas at STP is 22.4 L per one mole of gas, or 22.4 L/mol. From experimentation, scientists found that the molar volume of many different gases at STP is very close to 22.4 L/mol, regardless of the type of gas involved.

In this experiment, magnesium metal is reacted with aqueous hydrochloric acid (HCl), producing hydrogen gas as one of its products. This experiment seeks to determine whether your experimental data is consistent with ideal gas behavior by calculating a molar volume of the hydrogen gas produced at STP and comparing this to the ideal molar volume of 22.4 L/mol.

In order to determine the molar volume of the hydrogen gas produced at STP (L/mol) from this reaction, the volume of the gas at STP (L) must be divided by the moles of the gas at STP (mol). Both of these values need to be determined. Unfortunately, this information can not be determined directly, since the experiment is not performed at STP.

However, the moles of gas at STP can be determined indirectly by recognizing the 1:1 stoichiometric relationship between the magnesium metal, the limiting reactant, and the hydrogen gas produced, assuming that all of the magnesium reacts.

Mg (s) + 2H+ (aq) ————-> Mg2+ (aq) + H2 (g)

Thus, by converting the mass of the magnesium metal into moles of magnesium and then into moles of hydrogen gas produced, the theoretical moles of gas at STP are calculated. The mass of the magnesium ribbon, however, can also not be directly determined, since the mass of the ribbon is too small to be measured on the scales provided in the laboratory. Instead, the magnesium ribbon mass is determined from the length of the magnesium ribbon itself and a pre-determined conversion factor, the linear density, that is provided. Since this process assumes that all of the magnesium ribbon gets used during the reaction, the ideal gas law can be used directly, solving for the actual moles of hydrogen gas produced. This value is then compared to the theoretical moles of hydrogen gas to validate that the magnesium did indeed get used up during the reaction.