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WMU / Chemistry / CHEM 1510 / What are the four groups around an atom?

What are the four groups around an atom?

What are the four groups around an atom?

Description

School: Western Michigan University
Department: Chemistry
Course: Chem for Health Professionals 1
Professor: Sutton
Term: Fall 2018
Tags: Chemistry
Cost: 50
Name: Chemistry 1510 exam 1 study guide
Description: These are the objectives that will be on the up-coming exam.
Uploaded: 09/10/2018
12 Pages 28 Views 2 Unlocks
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Chem exam review


What are the four groups around an atom?



Chapter 4

Draw resonance structures for some ions and molecules   Sometimes lewis structures must be drawn for ions that contain  covalent bonds- polyatomic ions  

 The charge on the ion must be considered when counting and placing  valence electrons  

o If a negative charge  add one electron  

o If a positive charge  subtract one electron  

 Ex. CN- contains 10 valence electrons  4 electrons from  carbon, 5 electrons from nitrogen, and 1 electron from the  negative charge  

Drawing resonance structures  

 Resonance structures  two or more lewis structures having the same  arrangement of atoms but different arrangement of electrons  o Ex. lewis structures for HCO3 -


What are the resonance structures for some ions and molecules?



 1. Arrange the atoms  

 O

H O C O

 2. Count the electrons using group number  

C  4 valence electrons  

O  6 valence electrons * 3= 18 valence electrons  

H  1 valence electron  

Negative (-) charge  +1 valence electron  

 3. Add the bonds and lone pairs  

 4. Convert one lone pair on O into bonding pair to form a  double bond- there are two ways to do this  

 Two resonance structures differ in the location of multiple bonds and  the position of lone pairs

o Double headed arrows (↔) are used to show that two lewis  structures are resonance structures  If you want to learn more check out What is the function of the electron transport chain?

 A true structure is composed of both resonance forms and is called a  hybrid  


What is the basic features of chemical equilibrium?



 Resonance stabilizes a molecule by spreading out lone pairs and  electron pairs in multiple bonds over a larger region of space   A molecule or ion that has two or more resonance structures is called  resonance-stabilized  

Focus on environment  

 Ozone ( O3) is a resonance structure  

o Without ozone skin cancer probability is increased along with  cataracts  

Predict the shape around an atom in a molecule

 Lewis structure is used to determine shape  

 To determine shape we must first determine the number of groups  surround the atom  

o A group is either an atom or a lone pair of electrons  

 Then VSEPR theory is used to determine shape  

o The most stable arrangement keeps those groups as far away  from each other as possible  If you want to learn more check out What are the 4 stages of the estrous cycle?

 In general there are three possible arrangemnts of groups  o Linear  atom surrounded by two groups, has bond angle of 180° o Trigonal  surrounded by three groups, bond angles of 120° o Tetrahedral  surrounded by four groups, has bond angles of  109.5° We also discuss several other topics like What is the introduction of nutrition?

Two groups around an atom  

 Linear, Surrounded by two groups, bond angle of 180°

o CO2, HCN

 To determine shape – we draw the lewis structure and count the  number of groups – atoms and lone pairs  

 Ignore multiple bonds when predicting geometry, count only atoms and lone pairs  

Three groups surround an atom

 Trigonal planar, surrounded by three groups, bond angles of 120° o BF3 If you want to learn more check out What is stability vs plasticity?

 Boron is surrounded by three groups  

 To keep groups as far away from each other as possible, they are  arranged in a trigonal planar fashion  

Four groups around an atom  

 Surrounded by four groups, has bond angles of 109.5°, tetrahedral  o CH4 

 Representing a 3D shape of the tetrahedron on 2D paper  o Place two bonds in the plane of paper, one bond in front, and one behind  

 Solid line used for bonds in the plane  

 Wedge used for bond in front of plane  

 Dashed wedge used for bond behind plane  

 Trigonal pyramidal  one group around the central atom is a  nonbonded electron pair, not another atom If you want to learn more check out What does it mean when you plead nolo contendere?
If you want to learn more check out How does presentism view historical events?

 Water has a bent shape  two groups around oxygen are lone pairs of  electrons

Total # of  

groups

# of atoms

# of lone  

pairs

Shape  

around atom

Approximate  bond angle

2

2

0

Linear

180

3

3

0

Trigonal  

planar

120

4

4

0

Tetrahedral

109.5

4

3

1

Trigonal  

pyramidal

About 109.5

4

2

2

Bent

About 109.5

Use electronegativity to determine whether a bond is polar or  nonpolar  

 Electronegativity  a measure of atoms attraction for electrons in a  bond- tells us how much a particular atoms “wants” electrons   Assigned by a value of 0 to 4  

o The higher the value the more electronegative an atom is and  the more it is attracted to the electrons in a bond  

 Electronegativity increases across a row on the periodic table and  decreases down a column of the periodic table  

 Nonmetals have a higher electronegative value compared to metals   The upper right hand corner of the periodic table (fluorine and oxygen)  is the strongest on the table and the lower left corner is the weakest on the table  

 Electronegativity values are used as a guideline to indicate whether  electrons in a bond are equally shared or unequally shared between  two atoms

o Two identical atoms are bonded together  each atom attracts  the electrons in the bond to the same extend  equally shared  electrons  nonpolar  

 Ex. Carbon-carbon bond  

o Two atoms with similar electronegativity differences are said to  be nonpolar

 Bonding atoms of different electronegativity results in unequal sharing   polar bond or polar covalent  

o Said to have a dipole  a partial separation of charge  

 Direction of polarity is indicated by an arrow- head pointing toward  more electronegative element. Tail with perpendicular line through it  drawn at less electronegative element ( ⟼∨⟻ )

 Lowercase Greek letter delta (δ) with positive or negative sign is used  to indicate unequal sharing of electron density  

o δ+  given to less electronegative atom  

o δ-  given to more electronegative atom

 Polar bond will be one in which electronegative difference between two atoms is 0.5 units or greater

 When electronegativity difference is larger than 1.9 units- electrons  essentially are transferred from the less electronegative element to the more electronegative element and bond is considered ionic

Chapter 5

Write and balance chemical equations  

 Chemical equation  expression that uses chemical formulas and other  symbols to illustrate what reactants constitute the starting materials in a reaction and what products formed  

o Reactants are written on the left  

o Products are on the right  

o Separated by a horizontal arrow, reaction arrow, the points from  reactants to products  

 Numbers written in front of any formula  coefficients

o Coefficients  show the number of molecules of a given element  or compound that react or are formed

o When no number proceeds assume there is a “1”  

 When formula contains a subscript- multiple its coefficient by the  subscript  

o Ex. 2 O2 = 4 O atoms  

 Coefficients are used because of law of conservation of matter  o States  atoms cannot be created nor destroyed in a chemical  reaction  

 Coefficients are used to balance an equation- making the number of  atoms of each element the same on both sides of the equation o CH4 + 2 O2  CO2 + 2 H2O

Reactants: Products:  

1 C atom 1 C atom

4 H atoms 4 H atoms  

4 O atoms 4 O atoms

 If hear is added, the Greek letter delta () may be written over the arrow  The physical states of the reactants and products are sometimes listed  next to formula (S), (L), (G)

 If aqueous solution is used- reactant is dissolved in water- symbol (aq)  is next to reactant  

 The number of atoms of each element is equal on both sides of the  equation  

How to balance a chemical equation  

1. Write the equation with the correct formulas  

a. Reactants on the left and products on the right of the reaction  arrow  

b. Check if equation is balanced without any coefficients  

c. The subscripts can never be changed  

2. Balance the equation with coefficients one element at a time  a. Begin with the most complex formula- one element that appears  in only one formula on both sides of the equation  

3. Check to make sure the smallest set of whole numbers is used  a. Same number of atoms on both sides

Define a mole and use avagadros number in calculations   Mole defines a quantity

o Mole  quantity that contains 6.02 * 10^23 items- usually atoms, molecules, or ions  

 Definition of mole is based on the number of atoms contains in exactly  12 g of carbon-12 isotope

 One mole is abbreviated as mol and always contains avagadros  number of particles  

o 1 mol of C atoms = 6.02 * 10^23 C atoms  

o 1 mole H2O molecules = 6.02 * 10^23 H2O molecules   We can use avagadros number as a conversion factor to relate the  number of moles of a substance to the number of atoms or molecules  it contains  

o1mol 

6.02∗1023 atoms or 6.02∗1023 atoms

1mol

o These allow us to determine how many atoms or molecules are  contained in a given number of molecules  

 To multiple in scientific notation -multiply the coefficients together and  add the exponents on the powers of 10  

o Ex. (3.0 * 10^5) * (2.0 * 10^2) = 6.0 * 10^7

 To divide two number in scientific notation, divide the coefficients and  subtract the exponent in the powers of 10  

o Ex. (6.0 * 10^2) / (2.0 * 10^20) = 3.0 *10^-18  

Calculate formula weight and molar mass  

 Atomic weight  average mass of an element- reported in atomic mass  units  

 Formula weight  the sum of the atomic weights of all atoms in a  compound, reported in atomic mass units (amu)  

o Used for both ionic and covalent compounds  

o “molecular weight” often used for covalent compounds because  they are composed of molecules, not ions

 Formula weight is determined by adding up atomic weights of the  formula  

 1. Write the correct formula and determine the number of atoms of  each element from the subscripts  

o FeSO4  contains 1 Fe atom, 1 S atom, and 4 O atoms

 2. Multiply the number of atoms of each element by the atomic weight  and add results  

1 Fe atom * 55.85 = 55.85 amu  

1 S atom * 32.07 amu = 32.07 amu  

4 O atoms * 16.00 amu = 64.00 amu  

Formula weight for FeSO4 = 151.92 amu  

Molar mass

 Molar mass  value of the molar mass of an element in the periodic  table is the same as the value of its atomic weight  

o Molar mass of carbon  12.01 g/mol, atomic weight  12.01 amu, 1 mol c  12.01 g  

 Value of molar mas of a compound in grams equals the value of its  formula weight in amu  

Chapter 6  

Define energy and become familiar with the units of energy   Energy  capacity to do work

o Potential energy  stored energy  

o Kinetic energy  energy of motion  

 Law of conservation of energy  total energy in the universe does not  change, energy cannot be created or destroyed

 Energy stored in chemical bonds – both ionic and covalent – is potential energy  

 Reactions that form products having lower potential energy than  reactants is favored  

 Compound with lower potential energy is more stable than a  compound with higher potential energy  

Units of energy  

 Can be measured in two units  calories (cal) or joules (J)  o Calorie  amount of energy needed to raise the temperature of 1  g of water 1° C  

o Joules and cals are related by  1 cal = 4.184 J  

 1 kcal = 1,000 cal 1 kJ = 1,000 J 1 kcal = 4.184 kJ

Describe energy changes in a reaction, and classify reactions as  endothermic or exothermic  

 When molecules come together and react, bonds are broken in the  reactants and new bonds are formed in the products  

o Breaking a bond requires energy  

 Amount of energy needed to break a bond is the same amount that is  released when that bond is formed  

o Bond breaking always requires an input of energy

o bond formation always releases energy  

 Energy absorbed or released in any reaction is called heat of reaction  or enthalpy change  

o Symbolized by ΔH  

 Heat of reaction is given a positive or negative sign depending on  whether energy is absorbed or released  

o Energy is absorbed  reaction is endothermic and ΔH is positive

o Energy is released  reaction is said to be endothermic and ΔH is  negative

 Bond dissociation energy  energy needed to break a covalent bond by equally dividing the electron between the two atoms in the bond  o Always positive numbers  

 Bond breaking is endothermic  

 Bond formation is exothermic  

o ΔH is a negative number  

 The stronger the bond the higher its bond dissociation energy  o Energies decrease down the column from HR  HCl  HBr  HI   HI is the weakest, HF is the strongest  

 Bond dissociation energies generally decrease down the column   Most reactions involve breaking and forming more than one bond  ΔH indicated the relative strength of bonds broken and formed in a  reaction  

 ΔH is negative  more energy is released forming bonds than is needed to break bonds  

o Bonds formed in products are stronger

o Exothermic  

o Energy is released, products lower in energy  

 ΔH is positive  more energy needed to break bonds  o Bonds broken in reactants are stronger  

o Endothermic  

o Energy is absorbed  products higher in energy than reactants  

Endothermic reaction 

Exothermic reaction

Heat is absorbed 

Heat is released

ΔH is positive 

ΔH is negative

Bonds broken in reactants are  

stronger  

Bonds broken in products are  stronger

Products higher in energy 

Reactants higher in energy

Energy diagrams  

 In order for two molecules to react, they must collide  

o Kinetic energy is used to break bonds  

 Collisions must have the proper orientation and enough energy  for the reaction to occur  

o No reaction will occur if orientation is not correct  

 Many collisions are ineffective  

 Energy of reacting molecules determines whether a particular  collision will lead to a reaction  

 Reactions occur when the reactants possess sufficient energy  o Collisions with sufficient energy and proper orientation lead to a reaction

 Energy diagram  schematic representation of the energy  changes in a reaction, which plots energy on the vertical axis and the progress of the reaction- reaction coordinate- on horizontal  axis  

o Reactants written on the left side and products on the right with smooth curve how energy changed with time connects the  

 Transition state  unstable intermediate located at the top of the  energy hill in an energy diagram  

o Located at the top of the energy hill  

o Dashed lines are used for any bond is that formed or  

broken  

 Different in energy between the reactants and the transition  state is called energy of activation (Ea)

o Minimum amount of energy needed for a reaction to occur  o Energy barrier

 Magnitude of energy of activation determines the reaction rate  how fast the reaction occurs  

o Energy of activation is high  reaction is slow  

o Energy of activation is low  reaction is fast  

 The difference in energy between the reactants and products is  ΔH

o Products are lower in energy than reactants  bonds in  products are stronger  ΔH is negative  exothermic  

 Ea  difference between reactants and transition state  

 ΔH difference between reactants and products  

 A large Ea does not tell anything about the relative energies of  the reactants and products  

o Size of Ea determines rate of reaction

o Sign of ΔH determine the products or reactants are lower in energy  

Predict the effect of concentration, temperature, and the presence  of a catalyst on the rate of reaction

 Rate of reaction depends on the number of collisions and the  effectiveness of each collision  

 Increasing concentration of reactants increases number of collisions  reaction rate increases

 Increasing temp increases reaction rate  

o Increasing temp increases kinetic energy  increases collisions   Reaction rate doubles for each 10 degrees C temp is raised  o Rate is halved for each 10 degrees C temp is lowered  

 Catalyst  substance that speeds up the rate of reaction  o Recovered unchanged

o Not appear in product  

 Catalysts accelerate a reaction by lowering energy of activation  o No effects on energies of reactants and products  

 Metals are often used as catalysts  

 Hydrogenation  addition of hydrogen to an alkene  

Describe the basic features of chemical equilibrium and write an  expression for an equilibrium constant

 Reactions that go to completion is when reactant are completely  converted to products  

 Reactions can sometimes be reversible  reactants form products then  the products come together to reform reactants  

 Reversible reaction  can occur in either direction to products or form  products to reactants  

o Two full headed arrows are used to indicate  

o Forward reactions proceeds left to right  

o Reverse reaction proceeds from right to left  

 When the rate of forwards reaction equals the rate of the reverse  reaction the net concentrations of all species do not change and the  system is at equilibrium  

o Equilibrium  forward reaction rate = reverse reaction rate  o Net concentrations of reactants and products do not change   Equilibrium constant (K)  characteristic value for a reaction at a given  temperature and equal to the ratio of the concentration of the products multiplied together to the concentrations of the reactants multiplied  together  each term is raised to a power equal to its coefficient in the  balanced chemical equation  

o Concentration  the number of moles in a given volume  o Brackets [] are used to symbolize concentration  reported in  mol/L  

 aA + bB ⇄ cC + dD

o equilibrium constant (K) = [C]c[ D]d 

[ A]a[B]b 

Use Le Chateliers principle to predict what happens when  equilibrium is disturbed  

 Le Chateliers principle  if a chemical system is disturbed or stressed,  the system will react in the direction that counteracts the disturbance  or relieves stress  

Concentration  

 Added reactant will drive equilibrium to the right  

 Added product will drive the equilibrium to the left  

 The K value will always stay the same

 If water is continuously removed – all of the starting material can be  converted to product  

Temperature  

 We must know if the reaction is endothermic or exothermic  o This is for temperature only  

 Temp is increased  reaction that absorbed heat is favored  o Endothermic

 Temp is decreased  reaction that releases heat is favored  o Exothermic  

 Endothermic reactions absorb heat – increasing temp – increases rate  of forward reaction  

o Equilibrium shifts right  

 Exothermic reactions release heat  increasing temp increases rate of  reverse reaction  

o Equilibrium shifts left  

Pressure  

 Pressure increases  equilibrium shifts in the direction that decreases  number of moles in order to decrease pressure  

 Pressure decreases  equilibrium shifts in direction that increases  number of moles in order to increase pressure  

Change

Effect on equilibrium

Concentration

Adding reactant

Equilibrium favors products

Removing reactant

Equilibrium favors reactants

Adding product

Equilibrium favors reactants

Removing product

Equilibrium favors products

Temperature

Increasing temperature

Endothermic  equilibrium favors  products  

Exothermic  equilibrium favors  reactants

Decreasing temperature

Endothermic  equilibrium favors  reactants  

Exothermic  equilibrium favors  products

Pressure

Increasing pressure

Equilibrium favors side with fewer  molecules

Decreasing pressure

Equilibrium favors side with more  molecules

Use Le Chateliers principle and reaction rates to explain the  regulation of body temp  

 Normal body temp is 37 degrees C

 Temp decreases  reactions are slow, less heat is generated, it  becomes harder and harder to maintain adequate body temp   When temperature increases  body must rid heat  

o Blood vessels near skin surface dilate to release more heat   When temp decreases  body must generate more heat itself  o Blood vessels constrict to hold in more heat  

 The body works to counteract a change in the environment when  homeostasis is disturbed  

Chapter 8  

Describe the fundamental properties of a solution and determine  whether a mixture is a solution, colloid, or suspension   Solution  homogeneous mixture of two or more substances   Pure substances  elements, covalent compounds and ionic  compounds  

 Mixture  matter compose of two or more substances  

o Can be heterogeneous or homogenous  

 Heterogeneous  does not have a uniform composition   Homogeneous  uniform composition

 Solution  homogeneous mixture that contains small particles o Any phase can be a solution  

 Solute  substance present in a lesser amount in solution o Thing being dissolved

 Solvent  substance in larger amount of solution  

o What is doing the dissolving  

 Aqueous solution  solution with water  

 One component of a solution cannot be filtered away from the other  component  

 Colloids and suspensions are mixture that contain larger particles than  solution  

o Colloid  homogeneous mixture with larger particles  

 Opaque appearance  

 Particles that are 1 nm- um  

o Suspension  heterogeneous mixture that contains large  particles suspended in a liquid  

 Contains particles greater than 1 um in diameter  

Classify a substance as an electrolyte or nonelectrolyte   Electrolyte  substance that conducts an electric current in water   Nonelectrolyte  substance does not conduct electric current in water   Electrolytes can be classified as strong or weak  

o Strong  dissociates completely to form ions when dissolved in  water  

 ions

o Weak  dissolves in water to yield largely uncharged molecules   Molecules mostly  

 Equivalent  number of moles of charge that a mole of ions contributes to a solution  

o K+ has 1 equivalent

 Number of equivalents per mole of an ion is equal to the charge on the  ion

Predict whether a substance is soluble in water or a nonpolar  solvent  

 Solubility  amount of solute that dissolved in a given amount of  solvent  

o Unsaturated  solution that has fewer than the maximum nuber  of grams of solute  

o Saturated  solution that has the maximum number of grams of  solute that can be dissolved  

 Strength of interactions between compound and solvent decides if a  compound is soluble in a given solvent  

 Like dissolves like  

o Ionic and polar covalent compounds are soluble in water – polar  solute  

 Can hydrogen bond

o Nonpolar compounds soluble in nonpolar solvents  

 Ion-dipole interaction  attraction of an ion to a dipole in another  molecule  

 Solvation  process of surrounding particles of a solute with solvent  molecules  

 Water solubility for uncharged molecules occur with small polar  molecules or those with many O or N atoms that can hydrogen bond to water  

 Dissolving a solute is a physical process  

 Solvation releases more energy than required  exothermic   Separation requires more energy  endothermic (heat is absorbed)

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