Rust and Other Oxides Prelab

Transcription

1 Rust and Other Oxides Prelab Name Total /10 SHOW ALL WORK NO WORK = NO CREDIT 1. What is the purpose of this experiment? 2. Define an oxide and how it is formed. 3. In this laboratory experiment, two oxides of iron can form. These two oxides are iron(ii) oxide and iron(iii) oxide. Write the balanced molecular equations for the reaction of iron metal reacting with molecular oxygen to produce iron(ii) oxide and iron(iii) oxide. NOTE: You should write TWO separate chemical equation. 4. You are going to heat arsenic in the presence of oxygen to form an oxide. An empty crucible weighs g and after adding some arsenic, the mass of the crucible and arsenic is g. After heating the arsenic and completely converting it to the oxide, the mass of the crucible and oxide is g. Determine the empirical formula of the arsenic oxide. 1

2 Rust and Other Oxides In this experiment, you will be able to determine the empirical formula for an iron oxide from results you will obtain by heating iron metal in the presence of oxygen in the air. Special mention goes to Brian Stahl, John Witkowski, John Curtin, Nevin Gundez, Kelly Lyons and Tom Pomorski for developing this laboratory experiment so you can have a better understanding of the formation of oxides. Introduction Whether it is alone or in air, molecular oxygen, O 2, is a very reactive substance when it is heated and many elements will react with it. When an element reacts with molecular oxygen, it chemically combines with the oxygen and forms an oxide. The element you will be working with in this laboratory experiment is iron. Iron comprises approximate 5% of the earths crust and contributes to 35% of the earth s mass. Iron forms many alloys with other metals such as nickel, manganese, magnesium and copper. It also forms alloys with nonmetals such as carbon and silicon in the production of steel. Most of the iron in the earths crust exists as iron oxides and this is the kind of oxides you will be studying in this laboratory experiment. Iron can form one of two iron oxides. The first is iron(ii) oxide where the charge on iron is +2 and iron(iii) oxide where the charge on the iron is +3. In this experiment, iron metal in the form of a powder will be heated and react with oxygen to form one or both of the iron oxides. You can determine the amount of oxygen that is present in the oxide by taking the mass of the iron oxide present and the original mass of the iron powder that you weighed when you started the experiment. For an example calculation, we will have the reaction of phosphorous with molecular oxygen to form some phosphorous oxide. In this example, we will have g of phosphorous burn in the presence of oxygen and produce g of the oxide. The first step is to determine the mass of the oxygen in the oxide. We obtain this value by subtracting the mass of the phosphorous in the beginning from the mass of the oxide g g = g of oxygen The next step is to convert both the mass of the phosphorous and oxygen to moles g P x 1mole P = x 10-3 moles of P g P g O x 1 mole O = x 10-2 moles of O 16.00g O 2

3 Next, you want to divide each value you have obtained by the smallest number of moles x 10-3 = x 10-2 = x x 10-3 Since you want your empirical formula to have whole positive numbers, you must multiply these numbers by two. This gives you the empirical formula for the phosphorous oxide produced as P 2 O 5. Procedure 1. Obtain a stainless steel crucible (no lid). Brush the crucible with a paper towel to get rid of any soot. DO NOT WET THE CRUCIBLE!!!!!!!!!! 2. Place the EMPTY crucible in a clay triangle on an iron ring which is attached to a ring stand. Adjust the height of the iron ring so that the bottom of the crucible will be in the hottest part of the flame from your bunsen burner. If you are unsure of your setup, have your instructor check it for you. 3. Heat the crucible strongly for about five minutes. Make sure that the hottest part of the flame just touches the bottom of the crucible. The bottom of your crucible should have a red-hot glow during this time. After five minutes have passed, move the burner from under the crucible and allow the crucible to cool for three to five minutes. You should not feel any heat when you place the back of your hand or the inside of your wrist about a half inch from the bottom of the crucible when it is cooled. Do not touch the crucible when you do this. 4. Take the cooled crucible and transfer it to the pan of a balance using crucible tongs and wire gauze to place under the crucible. If you have to wait in line for a balance, DO NOT put your crucible on the lab bench. Rest the crucible on your wire gauze. 5. Obtain and record the mass of your crucible. Remember to use the same balance each time you obtain a mass for your crucible. 6. Repeat steps 3, 4 and 5 until two CONSECUTIVE masses differ by no more than g. When you have achieved this, record this final mass and move on to step seven. 7. Weigh out approximately g of iron powder and place in the crucible. Weigh the crucible with the iron powder in it. The difference between the empty crucible and the mass of the crucible and iron will determine the amount of iron in the crucible. DO NOT WRITE THE MASS OF THE IRON THAT YOU WEIGHED BEFORE PLACING IT IN THE CRUCIBLE. 8. Set up your crucible the same way as in step 2. Heat the crucible strongly again for approximately 15 minutes. 3

4 9. After 15 minutes has passed, allow the crucible to cool as before in step When the crucible is cooled, obtain the mass of the crucible and record it. 11. Heat the crucible strongly again for around three minutes. Cool as before and obtain the mass of the crucible and record it. 12. If your two masses do not differ by more than g then you are finished and you can dispose of the contents properly and clean the crucible (just wipe with a dry paper towel). If the difference is greater than g then repeat step 11 until two CONSECUTIVE masses meet the requirements. 4

5 Results 1st Heating 2nd Heating 3rd Heating 4th Heating (if necessary) (if necessary) Mass of empty crucible Last reading of mass of empty crucible Mass of crucible and Fe Mass of Fe Mass of crucible and oxide Last reading of mass of crucible and oxide Mass of Oxide Mass of Oxygen in Oxide Moles of Fe in Oxide Moles of Oxygen in Oxide Empirical Formula of Oxide (according to your data) Calculations Questions 1. Write the balanced molecular equations for the reaction of iron with molecular oxygen to form both iron(ii) and iron(iii) oxide. (You should have two equations written). 2. Calculate the mass (in grams) of iron(ii) oxide and iron(iii) oxide that can be produced from the amount of iron you weighed out in step 7. (You should have two values, one for iron(ii) oxide and one for iron(iii) oxide). 5