Advanced Placement and Pre-AP Chemistry projects

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Advanced Placement (AP) Chemistry is a college-level course offered to students in high school. Pre-AP Chemistry is not a class but a set of strategies (covering grades K-12) to help students prepare to take AP Chemistry or a college general chemistry course.

This category has been created to allow teachers of AP and Pre-AP Chemistry to exchange ideas in an open environment. You may not post copyrighted materials unless you hold the copyright and grant others free and fair use of your material. Other participants are free to change and edit these materials. It is this "evolution" of materials, as demonstrated by Wikipedia, that leads to the best and most accurate information.

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This learning project aims to create a moral ground on which to teach. To apply morals in the classroom is the ultimate goal.

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Chemistry Labs Advanced Projects in Chemistry- Analysis of Alloys Author -Anjali Gharpure (anjali@podar.net) Chemistry Projects at the K-11 and K-12 grades require application of the theoretical concepts studied in class. Transition metals are studied under the d-block. One of the peculiar characteristics of transition metals is formation of alloys. Alloys are solid homogeneous solutions of two or more solids. In an alloy, two or more elements combine such that at least one is a metal, and the resultant material has metallic properties. Alloys are designed to have properties that are more desirable than those of pure metals. Steel is stronger than iron, while brass, an alloy of copper and zinc is more durable than copper and more attractive than zinc in appearance. Unlike pure metals, alloys do not have a single melting point. They possess a melting range. Alloys improve properties of metals, by increasing hardness, tensile strength, chemical resistance and attractive appeal by modifying color. Alloys of Zn and Copper are widely used in everyday life and known to all of us. Detection of the presence of the metals used to prepare these alloys can be achieved through semi-micro qualitative analysis. These tests are useful both for the student of chemistry as well as for a forensic laboratory chemist. Laboratory work in alloy analysis involves a three step protocol 1. Grinding the alloy sample to a fine particulate powder 2. Digesting the sample in an appropriate acid 3. Detection of the metal ions in the alloy.

Brass Zn-20-40% & Cu-60-80% Zn was detected with potassium ferrocyanide test and the Sodium hydroxide test Cu-60-80% Potassium ferrocyanide, potassium iodide and liquid ammonia tests Bronze Zn-10% Zn was detected with potassium ferrocyanide test and the Sodium hydroxide test Cu -90% Potassium ferrocyanide, potassium iodide and liquid ammonia tests

Chemical Reactions – Acknowledgements- The experiments for the analysis of the metal ions as well digesting alloys of bronze and brass were carried out by Anchit R.Giri and Shashwat Kishore of Class XII, R.N Podar CBSE Senior Secondary High School , Santacruz(W). For Copper Analysis : Liquid ammonia test - Cu(NO3)2+4NH4OH--> [Cu(NH3)4][NO3]2+4H2O-Deep blue solution

Potassium ferrocyanide test- [Cu(NH3)4]SO4+4CH3COOH--> CuSO4+ 4CH3COONH4 2CuSO4+ K4 [Fe(CN)6]-->Cu2[Fe(CN)6]+2K2SO4 - Chocolate brown precipitate

Potassium iodide test 2CuSO4+ 4KI-->Cu2I2 (white precipitate) +I2 (brown coloration)+2K2SO4

For Zn Analysis 1. Sodium Hydroxide test ZnCl2+2NaOH--> Zn(OH)2 white precipitate + 2NaCl Zn(OH)2+2NaOH--> Na2ZnO2 soluble in excess NaOH+2H2O

2. Potassium ferrocyanide test 2ZnCl2+K4 [Fe(CN)6]-->Zn2[Fe(CN)6] bluish white precipitate +4KCl

A transition metal forms an alloy with another transition metal ion with ease because of the similarity in atomic size. The lattice site in the crystal structure of one transition metal can be occupied by other transition metals giving a homogeneous solid solution termed as an alloy. Mutual substitution of one or more metals leads to the formation binary, ternary or tertiary alloys. Hardness, resistance to corrosion, tensile strength, load bearing capacity are some of the properties which can be bettered by alloy formation. Actual lab analysis impresses the composition of the alloy for the student.

Advanced Chemistry Projects – Acids and Bases Contributing Author: Anjali Gharpure To make your Science Project significant choose topics from your curriculum that you have studied in theory. A science project at the K-11 or K-12 grade or for your A levels, requires demonstration of your understanding the concept taught in class supported with meaningful experimental work. It is important that your experiments are simple and non-hazardous. Irrespective of the boards for which you appear, you all learn about acids and bases. Theoretically, acids and bases are learnt according to different theories put forth by Arrhenius, Bronsted and Lowry as well as G.N.Lewis. In contrast, we also study the classical concept of acids and bases based on qualitative tests. If it tastes sour –it is an acid, but if it tastes slippery and bitter on the tongue, then it is surely a base. An acid reacts with a base to give salt and water and vice versa a base reacts with an acid also to give salt and water. According to the Arrhenius concept, an acid releases a proton in solution and a base releases hydroxyl ions in solution. The Bronsted and Lowry concept defines an acid as a proton donor but a base as a proton acceptor. The Lewis concept of an acid and base is that of an electron pair acceptor and donor with the formation of a coordinate covalent bond. The project at hand is to explain these concepts through simple experimentation. Strength versus Concentration The Lewis concept of acids and bases is incapable of explaining strengths of acids and bases. The strength of an acid or a base is directly proportional to the extent to which it produces hydronium or hydroxide ions in solution. Strong species produce stoichiometric equivalents of hydroxide ions while weak species produce less than a stoichiometric equivalent of hydroxide ions. Concentration on the other hand refers to the molarity or molar concentration in moles per litre of the solute in solution. The terms concentration and strength are not interchangeable. A 0.001molar solution of HCl is 100 % dissociated in solution and so it is a strong acid as it furnishes almost all its protons in solution. However a 2 M solution of acetic acid is a more concentrated solution than 0.001M HCl but is not stronger than HCl as the number of protons dissociated are very few. Here a meaningful experiment would be to demonstrate the difference between a weak strength acid and a strong strength acid. The simplest determination is carrying out a pH measurement. From the pH measurement the H3O+ ion concentration can be determined. pH= - log10 H3O+ from this relationship the H3O+ ion concentration for a weak as well as a strong acid can be determined. The stronger acid will have a lower pH value and thus a higher hydronium ion concentration. Ka , Kb and Acid/Base Strength Most acids, like acetic acid, are weak. They do not completely ionize in water to produce a stoichiometric equivalent of hydronium ions. This means aqueous acetic acid molecules are in equilibrium with hydronium and acetate ions.

For any system at equilibrium, the law of chemical equilibrium results in an expression with the equilibrium constant K. Including the equilibrium concentrations of aqueous species in equilibrium constant expressions, the K expression for acetic acid in water is:

where the subscript "a" indicates that an acid ionizes in water to form hydronium. Ka is the acid ionization constant or the equilibrium constant for the ionization of a weak acid in water to produce hydronium ions and the conjugate base of the acid.

An equilibrium constant expression may also be written for a weak base like ammonia in water.

The subscript "b" indicates that a base hydrolyses water to form hydroxide. Kb is the base ionization constant or the equilibrium constant for the ionization of a weak base in water to produce hydroxide and the conjugate acid of the base. Ka and Kb values indicate acid strength and base strength respectively. For example, a higher Ka value indicates higher hydronium ion concentrations or greater ionization. The equilibrium position lies further to the right when Ka is higher. A higher Kb value means a higher hydroxide ion concentration.

The strength of acetic acid is known to you. From the pH measurement the H3O+ ion concentration is known. The number of H3O+ ions in the solution will equal the number of acetate ions and so Ka = [H3O+]2/[CH3COOH] Thus an experimental determination of the Ka values for different acids and Kb values for different bases can be undertaken This project conveys through an experimental determination • the understanding of the concepts of acids and bases • the difference between strengths of acids and bases • pH+ pOH=14 • the calculation of Ka or Kb and its significance, as well as, • pH and its use to determine the hydronium ion concentration. Contact: anjali@podar.net

Advanced Chemistry Project - Study of Bromination Reactions in Phenol, Aniline and cinnamic Acid Contributing Author : Anjali Gharpure (anjali@podar.net) The theoretical study of electrophilic substitution reactions is best studied by actually carrying out laboratory reactions and making a comparative study of the yields of the products under the available conditions. At the K-11-12 level, the instruction of electrophilic substitution reactions is superficial, until it is impressed upon by lab work. An electrophile is an electron loving substituent. An electrophile is attracted to electrons and participates in a chemical reaction by accepting an electron pair. The benzene ring acts as a source of electrons. In an electrophilic substitution reaction the hydrogen atoms on the benzene ring are replaced by the attacking reagent which is deficient in electrons. Bromination is an electrophilic substitution reaction. Benzene, C6H6, is a planar molecule containing a ring of six carbon atoms each with a hydrogen atom attached. There are delocalised electrons above and below the plane of the ring. The presence of the delocalised electrons makes benzene particularly stable. Benzene resists addition reactions because that would involve breaking the delocalisation and losing that stability. Two effects work in electrophilic substitution reactions. One is induction, which is an effect that occurs through the sigma- bond system. Electron withdrawing groups will "pull" electron density away from the ring making the electrons of the ring less available for attack by an electrophile. Electron donating groups do the opposite. Induction also plays a role in some of the directional effects. The other effect is resonance, which occurs through the pi system of bonds in the molecule. Resonance plays an important role in the stability of intermediates of the reactions. Sometimes these two effects are opposite to one another. One effect may be stronger and exert greater influence. The ease with which a compound suddenly becomes receptive to bromination due to presence of electron donating groups like –OH in phenol and –NH2 in aniline is studied. Phenols are potentially very reactive towards electrophilic aromatic substitution. In aqueous solutions, phenol undergoes ionization to give the phenoxide ion. The negative charge on the oxygen of the phenoxide ion donates electrons into the benzene ring to a large extent. The strong activation often means that milder reaction conditions than those used for benzene and also a trisubstituted reaction product. The reaction is carried out in the laboratory without a strong Lewis acid like Friedel Crafts catalyst. If bromine water is added to a solution of phenol in water, the bromine water is decolorized and a white precipitate is formed which smells of antiseptic. The precipitate is 2,4,6-tribromophenol. Ar-OH+3Br2 Ar-OH Br3 +3HBr The NH2 group in aniline strongly activates the aromatic ring through the delocalization of the lone pair of electrons of the N-atom over the entire aromatic ring. Aromatic amines undergo the reaction readily and it is impossible to stop the reaction at the monosubstitution stage. Aniline too on bromination gives a tribromosubstituted derivative. Ar –NH2+ 3Br2--> 3HBr+ Ar-NH2-Br3 Cinnamic acid is an alkenoic acid. It undergoes an electrophilic addition reaction. The yield of the product is fairly high as the reaction mechanism does not involve large change in bond energies. The double bond present in cinnamic acid consists of one sigma and one pi-bond. Pi- electrons form an electron cloud which lies above and below the plane of the sigma bonded carbon atoms. The pi electron cloud is thus more exposed and less tightly held by the two carbon atoms. The pi electrons attract electrophiles and undergoes electrophilic addition reactions. Bromine molecule is non-polar but when it comes in the vicinity of a double bond, the pi electrons of the double bond begin to repel the bromine molecule. The bromine molecule gets polarized and the positive end of the bromine molecule is attracted towards the pi-electrons forming a carbocation. This is the slow and rate determining step of the reaction. The carbocation is higly reactive and undergoes a nucleophilic attack by the bromide ion giving a dibromo addition product across the double bond. One gm of cinnamic acid was placed in a conical flask and dissolved in10mL of hot water. The hot solution was added to 10 mL of a strong solution of 20% bromine. The derivative was filtered and purified in hot alcohol. The melting point of the di- bromo derivative of cinnamic acid was determined to be 195oC Ar –CH=CHCOOH+3Br2--> Ar-CHBr-CHBr-COOH Acknowledgements-The bromo derivatives were patiently prepared by Manasvi Lalvani, Shreya Krishnan and Sanjana Siddhra, Grade 12, of R.N. Podar Sr. Secondary CBSE High School, Santacruz (W).

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