AP Chemistry
Lab Write Up Guide
Zinc galvanization lab:
Overview:
In this lab you weighed a washer before and after immersing it in hydrochloric acid solution. The washer is made of steel coated in a layer of zinc. Coating steel in zinc to protect it from rusting is called galvanization. The hydrochloric acid solution consists of hydrochloric acid (HCl), a gas, dissolved in water. The acid reacts with both zinc and steel, turning the zinc metal atoms and iron metal atoms into water soluble ions. The bubbles you saw in the lab were hydrogen gas. The zinc is more reactive than the iron in the steel so the way the lab works is that you watch the washer and wait for the bubbling to change from one level of vigor to a lower level, signaling that the zinc is gone and the iron is exposed and reacting. You then dry it and weight it again and you have an idea of the mass of the zinc layer. The washer is measured and the area computed. The area of the washer multiplied by the thickness of the zinc layer is the volume of the zinc. The mass of the zinc divided by the volume is the density, a value we can look up.
Lab write up:
Title: Zinc lab or washer lab won’t cut it. We need a title that describes what we did to people not enrolled in our class but who have an AP chem level of knowledge. Here are some terms referring to our lab:
Gravimetric analysis: Gravimetric analysis describes a set of methods in analytical chemistry for the quantitative determination of an analyte based on the mass of a solid.
Oxidation: The term for the type of reaction the zinc and iron underwent during the reaction
If I were writing the lab report my title would be:
Determination of the thickness of zinc galvanization of a washer by gravimetric analysis.
Purpose:
In this lab we have a purpose instead of an abstract. The purpose was the measure the thickness of the zinc layer.
Procedure:
Describe what you did but use the passive voice.
Don’t: We weighed a washer
Do: A washer was weighed
Don’t: Add 10ml of HCl solution to the beaker containing the washer
Do: 10ml of HCl solution was added to the beaker containing the washer
Data: The data you gathered should be in a table separate from the class data which gets its own table. These tables should be neat and readable.
Calculations: Someone who reads the calculation needs to be able to follow what was done. This person should be able to understand the calculation even if they have never done this lab. You can assume they have taken algebra and geometry and know what density is.
Discussion: This section should talk about the results. Since we don’t know the real answer there is no discussion of why the value is different than the accepted value. Instead it should discuss why the standard deviation is high or low and what factors give a value to high or too low. It should also discuss any systematic errors, that is, errors that are unavoidable. A person making mistakes in weighing and measuring the washer is not an error to be discussed since it should not be happening. The main error in this lab is obviously knowing when to remove the washer. There is also an error with the reaction being faster or slower on the top and bottom of the washer since the washer rests on the bottom of the vessel. Each error discussed should include whether the error makes the answer too high or too low.
Results: Restate the thickness value for your group, the value for the whole class, and the standard deviation for the class.
Nail Lab:
Overview:
We immersed a steel nail in copper sulfate solution. The iron in the nail gives electrons to the copper ion in the aqueous solution. This turns the iron into an ion and the copper into copper metal which is a solid. By weighing the amount of copper metal produced we can get an idea of how many atoms of copper were produced. By weighing the nail before and after the reaction we can get an idea of how many iron atoms reacted. We can then determine the ratio between them.
Title: Nail lab won’t cut it. A better title would be: Quantitative analysis of the reaction between an iron nail and copper sulfate solution.
Purpose: The purpose is to determine the ratio between copper and iron.
Procedure: As always use passive voice.
Data: Should be in a neat readable table.
Calculations: There should be a section for the copper and another for the iron. It should be clear how you determined how many copper atoms were produced and how many iron atoms were consumed.
Discussion: In this lab we know that the accepted value is a 1:1 ratio. Therefore, you can calculate a % error for your value as well as the class data as well as the standard deviation of the class data. The discussion should include the concept of canceling errors which are errors that cancel each other out. One error leads to a value that is too high and another to a value that is too low. Together they make the data better. Discuss every error that could occur and whether it leads to a value that is too high or too low. Do not waste your time and mine with things like this:
“There was human error in this lab.”
“We may have messed up weighing.”
“The balance is no good so it made us have errors.”
Results: Restate the ratio and the % error for your data and the ratio, % error, and standard deviation for the class data.
Penny Lab:
Overview: Copper doesn’t react with hydrochloric acid but zinc does so the core of the new pennies reacts while the old pennies don’t.
Fill in the form.
Aluminum copper lab:
Overview: This lab is very similar to the nail lab but aluminum foil is substituted for the iron nail. The calculations are also different since we use the mole concept in this lab. If the aluminum foil did not react with the copper that is because the aluminum foil is covered with a layer of aluminum oxide which does not react. Ways of getting around that are to buff the aluminum with steel wool or to use acid to remove the layer of oxide and expose aluminum metal to the copper ions. Ideally there should be no aluminum left at the end of the reaction so that we know how much reacted (all of it) while we can measure the mass of the solid copper produced. Having leftover aluminum is a major error.
Title: Aluminum lab isn’t going to cut it. A better title involves terms like gravimetric analysis, stoichiometry, mole ratio, reaction etc.
Abstract: In this write up abstract replaces purpose. There is a difference. Abstract is basically a short summary of what was done that people can read to decide if they want to read the whole paper. A good abstract for this lab would be: The mole ratio in the reaction between aluminum foil and copper sulfate solution was determined by gravimetric analysis of a reaction with an excess of copper sulfate solution.
Procedure: Use the passive voice.
Data: A clear, neat readable table is needed.
Calculations: The reader needs to be able to follow how you took the raw data (the masses) to the mole ratio.
Discussion: Discuss how your results relate to the accepted value. Identify each error possible and its effect on results such as:
Spilling copper sulfate solution
Losing aluminum to buffing and use of acid to remove oxide layer
Having leftover aluminum in the reaction
Losing copper solid when trying to recover it
Results: Restate your ratio and the % error
Magnesium Lab:
In this lab there are a number of ways to lose magnesium during transfer. Also, if the product is wet, the mass measured includes water as well as magnesium hydroxide.
Copper Odyssey Lab:
Overview: In this lab we started and ended with solid copper metal. The first reaction turned the copper into copper ion with nitric acid. Note the deep blue color of copper nitrate. Next the copper was precipitated as copper hydroxide using NaOH as the source of hydroxide. This reaction process was exothermic because the precipitation reaction is exothermic as is the reaction between leftover nitric acid and hydroxide. Next the hydroxide was heated to remove water and form copper oxide. After that the oxide was reacted with HCl to make copper chloride which is soluble. Not that copper chloride is more greenish than copper nitrate. Finally we added aluminum or another metal to the copper chloride to recover copper metal. In each stage it is possible to lose copper from spilling or other errors. In other cases, such as not converting all of the copper hydroxide to copper oxide, there is no effect on the results.
Title: The best title here involves the terms % yield of copper and investigation of the reaction types etc.
Abstract: What we did here is learn about reactions and lab procedures but overall we’d like to recover copper metal. The efficiency of that recovery is called % yield.
Procedure: Use passive voice and include any changes that occurred such as using more or less of a reagent.
Data: Must include observations of each process and mass Cu recovered and % yield.
Discussion: List and discuss the errors you made as well as other possible errors and any effect on results. Also relate the observations of each reaction to what was happening. For example “A brown-orange gas was observed rising from the mixture of copper metal and nitric acid.” Tell which substance the brown gas was. Also explain why you saw boiling in a reaction you weren’t heating (if you did), why you had to heat the hydroxide to make oxide etc.
Results: % yield
Molar Volume of hydrogen:
Overview: Here we reacted magnesium metal with hydrochloric acid to make hydrogen gas. This gas was recovered and measured to give a molar volume. The measured volume is corrected through calculations to give the volume of hydrogen itself rather than the volume of the mixture of hydrogen and water vapor. The gas that is present in the eudiometer is a mixture of hydrogen and water vapor. All liquids evaporate to some degree, that is called vapor pressure. When a volume of hydrogen is recovered some water evaporates from the surface of the liquid water and turns into a gas. This gaseous water takes up space and alters the results of the lab. We can correct the measured volume of hydrogen if we know what % of the volume of gas we have is water and what % is hydrogen. We know that the total pressure of the gas in the tube is the same as the atmosphere. We know that the pressure of the water vapor is determined by the temperature of the water. If we subtract the pressure of water from the atmospheric pressure we get the pressure of hydrogen.
Title: Measuring the molar volume of hydrogen by water displacement is a great title.
Abstract: The molar volume of hydrogen produced by reaction of hydrochloric acid with magnesium ribbon was measured by collection over water. The volume was corrected for water vapor.
Procedure: Use passive voice.
Data: Readable table.
Calculations: Clearly show how you calculated the molar volume of hydrogen. I’m not going to tell you how to do it directly but factor these in:
You have a known number of moles of hydrogen from the mass of magnesium
You correct for the proportion of gas in the tube that is hydrogen vs water
You know the temperature of the gas (assume it is the same as the water)
You know the gas constant
Calculate the % error using the above.
Discussion: Discuss the errors you made as well as other possible errors and how they affect the results.
Results: Molar volume and % error.
Molar mass of butane:
Overview: This lab is similar to the hydrogen lab except instead of volume we go farther and use the mass of butane to calculate density which gives us molar mass.
Title: Measuring the molar mass of butane by water displacement is good.
Abstract: Briefly state what was done and how.
Introduction: Here you need to talk about the theory behind the lab. Make sure to include the idea of correcting the volume for the volume of water vapor.
Procedure: Passive voice
Data: Neat readable table
Calculations: The reader needs to be able to follow the calculations from the raw data to the molar mass. How you corrected for the volume of water vapor needs to be clear.
Discussion: Discuss each possible error and its effect on results. Errors can include inconsistent drying of the lighter which leads to inconsistent results. Having bubbles of air in the eudiometer is an error that will always have the same effect on the results. Do each error. Reading the eudiometer incorrectly or “human error” is not acceptable.
Results: Restate the results and % error.
Salt Lab:
Overview: This lab assumes that the salt atoms are the system and the water is the surroundings. It ignores the rest of the universe. The salt dissolves in the water. This dissolving process is really two processes. Imagine process 1 as the cations and anions of the salt being pulled apart in a vaccum. This is endothermic. Why is it endothermic? It takes force to pull positive away from negative. We can think of anything that requires force as requiring energy as a rough approximation. More correctly, we have increased the potential energy of the ions by moving them farther apart. This is the same as lifting an object and giving it potential energy. We can also think about the fact that exerting a force on the ions over a distance did work on the ions, increasing their potential energy. The second process is water crowding around the ions. Water has a negative end (oxygen) and a positive end (hydrogen). Water molecules around a cation are arrayed with the negative end towards the ion and water molecules around an anion are arrayed with their positive end towards the ion. The process where water molecules crowd around an ion is called solvation. It is exothermic because the two entities involved exert a force on each other and do work on each other as they move closer together, lowering their potential energy. The overall dissolution of the salt can be exothermic or endothermic depending upon the relative magnitude of the values of the separation of the ions and the solvation of the ions. We did all this at constant pressure so we measured the enthalpy change of the process.