[DOWNLOAD] Chapter 7 Chemistry Test Answer Key
The VSEPR model assumes that electron pairs in the valence shell of a central atom will adopt an arrangement that minimizes repulsions between these electron pairs by maximizing the distance between them. The electrons in the valence shell of a central atom form either bonding pairs of electrons, located primarily between bonded atoms, or lone pairs. The electrostatic repulsion of these electrons is reduced when the various regions of high electron density assume positions as far from each other as possible.
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Figure 2. The BeF2 molecule adopts a linear structure in which the two bonds are as far apart as possible, on opposite sides of the Be atom. Two regions of electron density around a central atom in a molecule form a linear geometry; three regions form a trigonal planar geometry; four regions form a tetrahedral geometry; five regions form a trigonal bipyramidal geometry; and six regions form an octahedral geometry. Figure 3. The basic electron-pair geometries predicted by VSEPR theory maximize the space around any region of electron density bonds or lone pairs.
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Electron-pair Geometry versus Molecular Structure It is important to note that electron-pair geometry around a central atom is not the same thing as its molecular structure. The electron-pair geometries shown in Figure 3 describe all regions where electrons are located, bonds as well as lone pairs. Molecular structure describes the location of the atoms, not the electrons. We differentiate between these two situations by naming the geometry that includes all electron pairs the electron-pair geometry. The structure that includes only the placement of the atoms in the molecule is called the molecular structure. The electron-pair geometries will be the same as the molecular structures when there are no lone electron pairs around the central atom, but they will be different when there are lone pairs present on the central atom. For example, the methane molecule, CH4, which is the major component of natural gas, has four bonding pairs of electrons around the central carbon atom; the electron-pair geometry is tetrahedral, as is the molecular structure Figure 4.
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On the other hand, the ammonia molecule, NH3, also has four electron pairs associated with the nitrogen atom, and thus has a tetrahedral electron-pair geometry. One of these regions, however, is a lone pair, which is not included in the molecular structure, and this lone pair influences the shape of the molecule Figure 5. Figure 4. The molecular structure of the methane molecule, CH4, is shown with a tetrahedral arrangement of the hydrogen atoms.
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VSEPR structures like this one are often drawn using the wedge and dash notation, in which solid lines represent bonds in the plane of the page, solid wedges represent bonds coming up out of the plane, and dashed lines represent bonds going down into the plane. Figure 5. As seen in Figure 5 , small distortions from the ideal angles in Figure 3 can result from differences in repulsion between various regions of electron density. VSEPR theory predicts these distortions by establishing an order of repulsions and an order of the amount of space occupied by different kinds of electron pairs. A lone pair of electrons occupies a larger region of space than the electrons in a triple bond; in turn, electrons in a triple bond occupy more space than those in a double bond, and so on.
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This molecule has regions of high electron density that consist of two single bonds and one double bond. In the ammonia molecule, the three hydrogen atoms attached to the central nitrogen are not arranged in a flat, trigonal planar molecular structure, but rather in a three-dimensional trigonal pyramid Figure 5 with the nitrogen atom at the apex and the three hydrogen atoms forming the base. The ideal bond angles in a trigonal pyramid are based on the tetrahedral electron pair geometry.
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Again, there are slight deviations from the ideal because lone pairs occupy larger regions of space than do bonding electrons. Figure 6 illustrates the ideal molecular structures, which are predicted based on the electron-pair geometries for various combinations of lone pairs and bonding pairs. Figure 6. The molecular structures are identical to the electron-pair geometries when there are no lone pairs present first column. For a particular number of electron pairs row , the molecular structures for one or more lone pairs are determined based on modifications of the corresponding electron-pair geometry. According to VSEPR theory, the terminal atom locations Xs in Figure 6 are equivalent within the linear, trigonal planar, and tetrahedral electron-pair geometries the first three rows of the table. It does not matter which X is replaced with a lone pair because the molecules can be rotated to convert positions. For trigonal bipyramidal electron-pair geometries, however, there are two distinct X positions, as shown in Figure 7 : an axial position if we hold a model of a trigonal bipyramid by the two axial positions, we have an axis around which we can rotate the model and an equatorial position three positions form an equator around the middle of the molecule.
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In a trigonal bipyramidal electron-pair geometry, lone pairs always occupy equatorial positions because these more spacious positions can more easily accommodate the larger lone pairs. Theoretically, we can come up with three possible arrangements for the three bonds and two lone pairs for the ClF3 molecule Figure 7. The stable structure is the one that puts the lone pairs in equatorial locations, giving a T-shaped molecular structure. Figure 7. When a central atom has two lone electron pairs and four bonding regions, we have an octahedral electron-pair geometry. Count the number of regions of electron density lone pairs and bonds around the central atom. A single, double, or triple bond counts as one region of electron density.
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Identify the electron-pair geometry based on the number of regions of electron density: linear, trigonal planar, tetrahedral, trigonal bipyramidal, or octahedral Figure 6 , first column. Use the number of lone pairs to determine the molecular structure Figure 6. If more than one arrangement of lone pairs and chemical bonds is possible, choose the one that will minimize repulsions, remembering that lone pairs occupy more space than multiple bonds, which occupy more space than single bonds. In trigonal bipyramidal arrangements, repulsion is minimized when every lone pair is in an equatorial position. In an octahedral arrangement with two lone pairs, repulsion is minimized when the lone pairs are on opposite sides of the central atom. The following examples illustrate the use of VSEPR theory to predict the molecular structure of molecules or ions that have no lone pairs of electrons. In this case, the molecular structure is identical to the electron pair geometry.
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Example 1 Predicting Electron-pair Geometry and Molecular Structure: CO2 and BCl3 Predict the electron-pair geometry and molecular structure for each of the following: a carbon dioxide, CO2, a molecule produced by the combustion of fossil fuels b boron trichloride, BCl3, an important industrial chemical Solution a We write the Lewis structure of CO2 as: This shows us two regions of high electron density around the carbon atom—each double bond counts as one region, and there are no lone pairs on the carbon atom. The electron-pair geometry and molecular structure are identical, and CO2 molecules are linear. The arrangement of three regions of high electron density gives a trigonal planar electron-pair geometry. BCl3 also has a trigonal planar molecular structure Figure 8. Figure 8. The electron-pair geometry and molecular structure of BCl3 are both trigonal planar. What are the electron-pair geometry and molecular structure of this polyatomic ion?
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Answer: The electron-pair geometry is trigonal planar and the molecular structure is trigonal planar. Due to resonance, all three C—O bonds are identical. Whether they are single, double, or an average of the two, each bond counts as one region of electron density. Example 2 Predicting Electron-pair Geometry and Molecular Structure: Ammonium Two of the top 50 chemicals produced in the United States, ammonium nitrate and ammonium sulfate, both used as fertilizers, contain the ammonium ion. We expect the four regions of high electron density to arrange themselves so that they point to the corners of a tetrahedron with the central nitrogen atom in the middle Figure 6.
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Figure 9. The ammonium ion displays a tetrahedral electron-pair geometry as well as a tetrahedral molecular structure. Check Your Learning Identify a molecule with trigonal bipyramidal molecular structure. Answer: Any molecule with five electron pairs around the central atoms including no lone pairs will be trigonal bipyramidal. PF5 is a common example.
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The next several examples illustrate the effect of lone pairs of electrons on molecular structure. Solution The Lewis structure of H2O indicates that there are four regions of high electron density around the oxygen atom: two lone pairs and two chemical bonds: We predict that these four regions are arranged in a tetrahedral fashion Figure 10 , as indicated in Figure 6. Thus, the electron-pair geometry is tetrahedral and the molecular structure is bent with an angle slightly less than In fact, the bond angle is Figure Predict the electron-pair geometry and molecular structure of this cation. Answer: electron pair geometry: tetrahedral; molecular structure: trigonal pyramidal Example 4 Predicting Electron-pair Geometry and Molecular Structure: SF4 Sulfur tetrafluoride, SF4, is extremely valuable for the preparation of fluorine-containing compounds used as herbicides i.
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Predict the electron-pair geometry and molecular structure of a SF4 molecule. Solution The Lewis structure of SF4 indicates five regions of electron density around the sulfur atom: one lone pair and four bonding pairs: We expect these five regions to adopt a trigonal bipyramidal electron-pair geometry. To minimize lone pair repulsions, the lone pair occupies one of the equatorial positions. The molecular structure Figure 11 is that of a seesaw Figure 6. Check Your Learning Predict the electron pair geometry and molecular structure for molecules of XeF2. Answer: The electron-pair geometry is trigonal bipyramidal. The molecular structure is linear. Example 5 Predicting Electron-pair Geometry and Molecular Structure: XeF4 Of all the noble gases, xenon is the most reactive, frequently reacting with elements such as oxygen and fluorine. Predict the electron-pair geometry and molecular structure of the XeF4 molecule.
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Solution The Lewis structure of XeF4 indicates six regions of high electron density around the xenon atom: two lone pairs and four bonds: These six regions adopt an octahedral arrangement Figure 6 , which is the electron-pair geometry. To minimize repulsions, the lone pairs should be on opposite sides of the central atom Figure The five atoms are all in the same plane and have a square planar molecular structure. Check Your Learning In a certain molecule, the central atom has three lone pairs and two bonds.
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Check Your Understanding Reading 7. An element is a pure substance that cannot be broken down by physical or chemical means. A compound is a pure substance composed of two or more elements. A compound can be broken down by chemical means. An element is shown with a chemical symbol. A compound is shown with a chemical formula. Water is a compound that you drink every day. Benzene contains carbon and hydrogen. Benzene and ethane both contain the elements carbon and hydrogen. However, benzene contains six atoms of carbon while ethane only contains two atoms. Both benzene and ethane contain six hydrogen atoms. Answers may vary. The subscript 6 comes after the chemical symbol H in benzene.
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The subscript indicates how many atoms of an element are present. A chemical symbol indicates the identity of an element. A chemical formula indicates the identity of elements and number of atoms within a compound. Elements form compounds when atoms form chemical bonds between one another. Most elements combine easily with one another to form compounds.
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Reading 7. Products are the substances that are created in a chemical reaction. Reactants are the substances. Answers are: Eggs, flower, baking soda, milk, and chocolate chips are the reactants. The cookie is the product. The product of the chemical reaction the cookie has a brown color. Color change is evidence of a chemical change. When the temperature changes during a chemical reaction chemical bonds between atoms are broken and rearranged. The formation of bubbles can indicate a physical change when a liquid is boiled. Water makes the liquid medium in which the reaction occurs so the solid product settles out eventually as a precipitate.
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Connection - Spiderman's Favorite Compound Amino acids are the building blocks of proteins and spider silk is a protein. Many spiders eat their own webs in order to recycle the amino acids and build new proteins. Spider farms would be difficult to maintain since spiders tend to eat each other! Also, the silk produced by spiders is often as thin as a human hair and not very useful in that form. This can be done using a small amount of each solid 1 g placed in disposable heat resistant test tube.
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Use tongs over an alcohol burner or Bunsen burner and use special care not to overheat the substances. Young students can be trusted to do these tests only after demonstrating their maturity with flame and willingness to adhere to all teacher and safety instructions.
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As their name implies they contain a carboxylic acid functional group and an amine functional group. The alpha designation is used to indicate that these two functional groups are separated from one another by one carbon group. In addition to the amine and the carboxylic acid, the alpha carbon is also attached to a hydrogen and one additional group that can vary in size and length. In the diagram below, this group is designated as an R-group. Within living organisms there are 20 amino acids used as protein building blocks. They differ from one another only at the R-group postion. The basic structure of an amino acid is shown below: Figure Thus, as the amino acids are linked together to form a specific protein, they are placed within a very specific order that is dictated by the genetic information contained within the RNA.
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The primary sequence of a protein is linked together using dehydration synthesis that combine the carboxylic acid of the upstream amino acid with the amine functional group of the downstream amino acid to form an amide linkage. Within protein structures, this amide linkage is known as the peptide bond. Subsequent amino acids will be added onto the carboxylic acid terminal of the growing protein. Thus, proteins are always synthesized in a directional manner starting with the amine and ending with the carboxylic acid tail. New amino acids are always added onto the carboxylic acid tail, never onto the amine of the first amino acid in the chain. In addition, because the R-groups can be quite bulky, they usually alternate on either side of the growing protein chain in the trans conformation. The cis conformation is only preferred with one specific amino acid known as proline. Figure The addition of two amino acids to form a peptide requires dehydration synthesis. Proteins are very large molecules containing many amino acid residues linked together in very specific order.
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Proteins range in size from 50 amino acids in length to the largest known protein containing 33, amino acids. Macromolecules with fewer than 50 amino acids are known as peptides. The identity and function of a peptide or a protein is determined by the primary sequence of amino acids within its structure. There are a total of 20 alpha amino acids that are commonly incorporated into protein structures Figure Due to the large pool of amino acids that can be incorporated at each position within the protein, there are billions of different possible protein combinations that can be used to create novel protein structures! For example, think about a tripeptide made from this amino acid pool. At each position there are 20 different options that can be incorporated.
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Thus, the total number of resulting tripeptides possible would be 20 X 20 X 20 or , which equals 8, different tripeptide options! Now think about how many options there would be for a small peptide containing 40 amino acids. There would be options, or a mind boggling 1. Each of these options would vary in the overall protein shape, as the nature of the amino acid side chains helps to determine the interaction of the protein with the other residues in the protein itself and with its surrounding environment. Thus, it is useful to learn a little bit about the general characteristics of the amino acid side chains. The different amino acid side chains can be grouped into different classes based on their chemical properties Figure For example, some amino acid side chains only contain carbon and hydrogen and are thus, very nonpolar and hydrophobic. Others contain electronegative functional groups with oxygen or nitrogen and can form hydrogen bonds forming more polar interactions.
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Still others contain carboxylic acid functional groups and can act as acids or they contain amines and can act as bases, forming fully charged molecules. The character of the amino acids throughout the protein help the protein to fold and form its 3-dimentional structure. It is this 3-D shape that is required for the functional activity of the protein ie. For proteins found inside the watery environments of the cell, hydrophobic amino acids will often be found on the inside of the protein structure, whereas water-loving hydrophilic amino acids will be on the surface where they can hydrogen bond and interact with the water molecules.
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Proline is unique because it has the only R-group that forms a cyclic structure with the amine functional group in the main chain. This cyclization is what causes proline to adopt the cis conformation rather than the trans conformation within the backbone. This shift is structure will often mean that prolines are positions where bends or directional changes occur within the protein. Methionine is unique, in that it serves as the starting amino acid for almost all of the many thousands of proteins known in nature.
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Cysteines contain thiol functional groups and thus, can be oxidized with other cysteine residues to form disulfide bonds within the protein structure Figure Disulfide bridges add additional stability to the 3-D structure and are often required for correct protein folding and function Figure Disulfide bonds are formed between two cysteine residues within a peptide or protein sequence or between different peptide or protein chains.
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In the example above the two peptide chains that form the hormone insulin are depicted. Disulfide bridges between the two chains are required for the proper function of this hormone to regulate blood glucose levels. Protein Shape and Function The primary structure of each protein leads to the unique folding pattern that is characteristic for that specific protein. Recall that this is the linear order of the amino acids as they are linked together in the protein chain Figure These specific motifs or patterns are called secondary structure.
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Common secondary structural features include alpha helix and beta-pleated sheet Figure Within these structures, intramolecular interactions, especially hydrogen bonding between the backbone amine and carbonyl functional groups are critical to maintain 3-dimensional shape. Every helical turn in an alpha helix has 3. The R groups are attached to the carbons and extend above and below the folds of the pleat.
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The pleated segments align parallel or antiparallel to each other, and hydrogen bonds form between the partially positive nitrogen atom in the amino group and the partially negative oxygen atom in the carbonyl group of the peptide backbone. The alpha helix and beta-pleated sheet are common structural motifs found in most proteins. They are held together by hydrogen bonding between the amine and the carbonyl oxygen within the amino acid backbone. The most highly traveled and culturally significant of these was called the Silk Road. The reason that the Silk road was so culturally significant was because of the great distance that it covered.
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Essentially the entire ancient world was connected by one trade route. The silk road had an astounding effect on the creation of many societies. It was able to bring economic wealth into areas along the route, and new ideas traveled the distance and influence many things including art. An example of this is Buddhist art that was found in India. The painting has many western influences that can be identified in it, such as realistic musculature of the people being painted. Also, the trade of gun powder to the West helped influence warfare, and in turn shaped the modern world. The real reason the Silk Road was started though was for the product that it takes its name from: Silk. The Silk showed that the rulers had power and wealth because the silk was not easy to come by, and therefore was definitely not cheap.
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Silk was first developed in China, and is made by harvesting the silk from the cocoons of the mulberry silkworm. The silk itself is called a natural protein fiber because it is composed of a pattern of amino acids in a secondary protein structure. The secondary structure of silk is the beta pleated sheet.
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The primary structure of silk contains the amino acids of glycine, alanine, serine, in specific repeating pattern. These amino acids are used as side chains and affect things such as elasticity and strength. The beta pleated sheet of silk is connected by hydrogen bonds. The hydrogen bonds in the silk form beta pleated sheets rather than alpha helixes because of where the bonds occur. The hydrogen bonds go from the amide hydrogens on one protein chain to the corresponding carbonyl oxygen across the way on the other protein chain. This is in contrast to the alpha helix because in that structure the bonds go from the amide to the carbonyl oxygen, but they are not adjacent. The carbonyl oxygen is on the amino acid that is four residues before. The formation of this secondary structure in the silk protein allows it to have very strong tensile strength.
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Silk also helped to form one of the greatest trading routes in history, allowing for the exchange of ideas, products and cultures while advancing the societies that were involved. Silk contains both anti-parallel and parallel arrangements of beta sheets. In consequence very bulky side chains make the structure unstable. This explains why silk is composed almost entirely of glycine, alanine, and serine, the three amino acids with the smallest side chains.
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The complete 3-dimensional shape of the entire protein or sum of all the secondary structures is known as the tertiary structure of the protein and is a unique and defining feature for that protein Figure Primarily, the interactions among R groups creates the complex three-dimensional tertiary structure of a protein. The nature of the R groups found in the amino acids involved can counteract the formation of the hydrogen bonds described for standard secondary structures. For example, R groups with like charges are repelled by each other and those with unlike charges are attracted to each other ionic bonds. When protein folding takes place, the hydrophobic R groups of nonpolar amino acids lay in the interior of the protein, whereas the hydrophilic R groups lay on the outside. The former types of interactions are also known as hydrophobic interactions.
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Interaction between cysteine side chains forms disulfide linkages in the presence of oxygen, the only covalent bond forming during protein folding. The tertiary structure of proteins is determined by a variety of chemical interactions. These include hydrophobic interactions, ionic bonding, hydrogen bonding and disulfide linkages. All of these interactions, weak and strong, determine the final three-dimensional shape of the protein. When a protein loses its three-dimensional shape, it is usually no longer be functional. In nature, some proteins are formed from several polypeptides, also known as subunits, and the interaction of these subunits forms the quaternary structure. Weak interactions between the subunits help to stabilize the overall structure. For example, insulin a globular protein has a combination of hydrogen bonds and disulfide bonds that cause it to be mostly clumped into a ball shape. Insulin starts out as a single polypeptide and loses some internal sequences during cellular processing that form two chains held together by disulfide linkages as shown in figure Three of these structures are then grouped further forming an inactive hexamer Figure The hexamer form of insulin is a way for the body to store insulin in a stable and inactive conformation so that it is available for release and reactivation in the monomer form.
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