Gas Evolution reaction lab report

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.

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PV = nRT

Experiment 5: GAS EVOLUTION REACTION Chemistry 121

The determination of the volume of the gas at STP involves a more complicated calculation process. Since the experimental conditions are not at STP, the combined gas law is used to determine the volume of gas at STP.

This is done by assigning condition one as the experimental values and condition two as the STP values.

The volume of hydrogen gas at STP is then determined by rearranging the equation and solving for VSTP. The temperature and pressure at STP are standards and therefore are already known, 273.15 K and 1.00 atm, respectively.

Since the reaction is arranged so that the gas is collected over water in a sealed, graduated tube, the volume, VExp, can be read directly from the tube, as is done for a buret or a pipet, once the pressure is accounted for as described below.

Since the magnesium is placed in the tube so that the gas will bubble straight up displacing the water while filling the tube, the pressure of the gas above the water is actually a mixture of two gases, the hydrogen gas of interest and water vapor. As the hydrogen gas moves through the liquid water, some of the water molecules at the liquid-gas interface will acquire enough energy to escape into the gas phase. Although there is not a lot of water vapor present in the gas phase, the amount is large enough to affect the pressure and must therefore be corrected.

Dalton’s law of partial pressures is used to correct for this. By making the internal gas pressure (pressure inside the tube) equal to the external gas pressure (pressure of the atmospheric gases around the tube), the experimental pressure of the hydrogen gas can be determined. The inside and outside pressures are adjusted to be equal by raising or lowering the tube in water inside a larger graduated cylinder located in the sink so that the inside water level is equal to the outside level.

The external pressure is atmospheric pressure, or the room air pressure, which is read on the barometer. The internal pressure is the sum of the partial pressures of the experimental hydrogen gas and the partial pressure of the water vapor.

The vapor pressure of water can be read from a table posted on the wall of the lab, listing the vapor pressure of water at various temperatures. The pressure of the experimental hydrogen gas is then calculated by rearranging the above equation.

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P1V1 T1

= P2V2 T2

PExternal = PInternal

Patmospheric = PH2 + PH2O

PExpVExp TExp

= PSTPVSTP TSTP

PH2 = Patmospheric − PH2O