Making Hydrogen Gas From a Decomposition Reaction
Summary
A very simple reaction of acid and metal to produce Hydrogen gas and metal salt will provide the volume of gas to evaluate. The acid in the lab is HCl and the metal is magnesium. Your job is to determine the amount of gas produced at room conditions and then using the combined gas law predict how much gas you would have produced at S.T.P..
CollapseLearning Goals
Determine the mass of magnesium
Determine the moles of magnesium used
Determine the partial pressure due to hydrogen
Write a balanced equation
Determine the moles of magnesium used
Determine the partial pressure due to hydrogen
Write a balanced equation
Context for Use
This lab is designed for high school chemistry (grades 11-12). Students use about 1 1/2 hrs. for pre-lab discussion, lab, and post-lab questions.
Stuidents should be able to read, name and write chemical equations. They should also be able to balance chemical equations.
Stuidents should be able to read, name and write chemical equations. They should also be able to balance chemical equations.
Description and Teaching Materials
Instructions:
1. Obtain a piece of magnesium (Mg) ribbon approximately 1.0 cm long. (This is variable due to differences in Mg and air pressure.) **Measure the length carefully; your teacher will give you the conversion from length to grams. Taking too much will cause too much gas produced and therefore no results in the lab! Less is better in this lab, however you must know how much magnesium you start with. Record!
2. Fold the magnesium ribbon so that it can be encased
in a small spiral case made of fine copper wire. Let
enough copper wire to serve as a handle.
3. Obtain a gas collection tube and one or two holed
stopper that fits the opening of the tube.
4. Tilt the gas collection tube slightly and pour in 3 M
hydrochloric acid (HCl) to about the 15 ml mark. Again more is not
better. The acid will be in excess in this lab so you don't need more!
5. With the tube still tilted, slowly fill the rest of the volume with tap water. While pouring, make sure the lip area is rinsed so any acid will be rinsed into to the tube. Add the water slowly, since we want the acid (which is denser) to stay at the bottom!
6. Holding the copper wire by the end, place the wire caged magnesium into the collection tube while keeping the cage below the lines of gradation. With the wire tail in the mouth of the tube, place the rubber stopper into the mouth of the tube, thus securing the wire cage.
7. Holding your finger over the hole(s) in the stopper, invert the tube and
place the stopper end of the tube into a beaker of water. The acid will
slowly make its way to the magnesium. The gas produced will push the
excess water (and acid) into the beaker.
8. After the reaction stops, wait about 5 minutes to allow the tube and
its contents to come to room temperature. Bubbles clinging to the sides
of the tube can be dislodged by tapping the tube gently.
9. Cover the hole(s) in the stopper with your finger and transfer to a large cylinder filled
with water. Raise and lower the tube until the level of water in the tube is the same as outside the tube. The pressure inside the tube is now the same as the pressure outside!
Record the volume of gas in the tube when the pressure is equalized!
10. Clean up by pouring liquids down the sink. Return the copper wire to be used again!
11. Record the room temperature and pressure.
1. Obtain a piece of magnesium (Mg) ribbon approximately 1.0 cm long. (This is variable due to differences in Mg and air pressure.) **Measure the length carefully; your teacher will give you the conversion from length to grams. Taking too much will cause too much gas produced and therefore no results in the lab! Less is better in this lab, however you must know how much magnesium you start with. Record!
2. Fold the magnesium ribbon so that it can be encased
in a small spiral case made of fine copper wire. Let
enough copper wire to serve as a handle.
3. Obtain a gas collection tube and one or two holed
stopper that fits the opening of the tube.
4. Tilt the gas collection tube slightly and pour in 3 M
hydrochloric acid (HCl) to about the 15 ml mark. Again more is not
better. The acid will be in excess in this lab so you don't need more!
5. With the tube still tilted, slowly fill the rest of the volume with tap water. While pouring, make sure the lip area is rinsed so any acid will be rinsed into to the tube. Add the water slowly, since we want the acid (which is denser) to stay at the bottom!
6. Holding the copper wire by the end, place the wire caged magnesium into the collection tube while keeping the cage below the lines of gradation. With the wire tail in the mouth of the tube, place the rubber stopper into the mouth of the tube, thus securing the wire cage.
7. Holding your finger over the hole(s) in the stopper, invert the tube and
place the stopper end of the tube into a beaker of water. The acid will
slowly make its way to the magnesium. The gas produced will push the
excess water (and acid) into the beaker.
8. After the reaction stops, wait about 5 minutes to allow the tube and
its contents to come to room temperature. Bubbles clinging to the sides
of the tube can be dislodged by tapping the tube gently.
9. Cover the hole(s) in the stopper with your finger and transfer to a large cylinder filled
with water. Raise and lower the tube until the level of water in the tube is the same as outside the tube. The pressure inside the tube is now the same as the pressure outside!
Record the volume of gas in the tube when the pressure is equalized!
10. Clean up by pouring liquids down the sink. Return the copper wire to be used again!
11. Record the room temperature and pressure.
Teaching Notes and Tips
This is a very simple lab, but be sure to review "Dalton's Law of Partial Pressures".
Assessment
1. Determine the mass of the magnesium used in the reaction. (length x conversion factor)
2. Determine the moles of magnesium used.
Anytime gas is collected over water, there is always a certain amount of water vapor that will also be in the collection tube. This amount of evaporated water adds to the pressure. "Dalton's Law of Partial Pressures" says that the total pressure is the sum of all the gases present. In this lab, we only want to know about Hydrogen gas. We must make an adjustment to eliminate the pressure due to evaporated water. This pressure is called vapor pressure. To calculate the pressure due to hydrogen, we must subtract vapor pressure from the total pressure. Phydrogen = Ptotal - Pvapor. Vapor pressure at various temperatures have been calculated for you in Table 1.
3. Determine the partial pressure due to hydrogen by subtracting the vapor pressure from total pressure.
4. The answer for question 3 is the pressure
due to hydrogen gas at the temperature and
pressure in the classroom. Using the combined
gas law calculate the volume of gas that you
would have obtained at S.T.P..
5. Write a balanced equation for the reaction.
6. Calculate the amount of hydrogen you would have produced if you used 1.00 moles of
magnesium. Calculate the percent yield you had in this lab.
2. Determine the moles of magnesium used.
Anytime gas is collected over water, there is always a certain amount of water vapor that will also be in the collection tube. This amount of evaporated water adds to the pressure. "Dalton's Law of Partial Pressures" says that the total pressure is the sum of all the gases present. In this lab, we only want to know about Hydrogen gas. We must make an adjustment to eliminate the pressure due to evaporated water. This pressure is called vapor pressure. To calculate the pressure due to hydrogen, we must subtract vapor pressure from the total pressure. Phydrogen = Ptotal - Pvapor. Vapor pressure at various temperatures have been calculated for you in Table 1.
3. Determine the partial pressure due to hydrogen by subtracting the vapor pressure from total pressure.
4. The answer for question 3 is the pressure
due to hydrogen gas at the temperature and
pressure in the classroom. Using the combined
gas law calculate the volume of gas that you
would have obtained at S.T.P..
5. Write a balanced equation for the reaction.
6. Calculate the amount of hydrogen you would have produced if you used 1.00 moles of
magnesium. Calculate the percent yield you had in this lab.
Standards
9-12.I.B.3 apply mathmatics