Chem Test 5 (Ch 8.13, 8.3, 9.1, 8.8, 6.1-6.2)

What is true about enthalpy? A. It is path-dependent B. It is equal to the heat exchanged at a constant pressure C. It is equal to the heat exchanged at a constant volume D. It is always a positive value

B

According to hybrid orbital theory, a triple bond consists of: A. 1 sigma bond and 1 pi bond B. 2 sigma bonds and 1 pi bond C. 1 sigma bond and 2 pi bonds D. 2 sigma bonds and 2 pi bonds E. 3 pi bonds

C

Consider the following bonds: Br—I, Te—Br, As—Cl, and P—S. Which of the following bonds is most polar based on periodic trends? A. Br—I B. Te—Br C. As—Cl D. P—S E. Polarity cannot be determined in the absence of quantitative values assigned to each elemen

C

What is false about a coffee-cup calorimeter? A. It is a form of constant-pressure calorimetry B. Any heat change by the system is equal but opposite to the heat change of the calorimeter C. The heat exchanged measured must have correction made for PV work before the enthalpy can be calculated D. The heat exchanged is equal to the enthalpy change E. More than one of the above is false

C

What is true of hybrid orbitals? A. They can only form from s and p orbitals B. They only appear in organic compounds C. They are both an average of the shapes and energy levels of the orbitals that form them D. They result in the same bond angles as unhybridized orbitals E. They are involved in pi bonds

C

Which of the following has the bonds correctly arranged in order of decreasing polarity (i.e. most polar bond written first)? A. Be-F > Mg-F > N-F > O-F B. O-F > N-F > Mg-F > Be-F C. Mg-F > Be-F > N-F > O-F D. Be-F > Mg-F > O-F > N-F E. Mg-F > Be-F > O-F > N-F

C

Which of the following molecules has the smallest bond angle? A. CH4 B. BCl3 C. SF6 D. NH3 E. SCl2

C

Change in internal energy

The change in internal energy can be represented as the difference in its final and initial states ΔE = E_products - E_reactants

Calorimetry and Calorimeters

• Experiments that measure the amount of heat transferred to or from a substance are calorimetry experiments • Calculating heat capacities are just one type of calorimetry experiment • Calorimeters, the device used to measure the heat transfer, vary incomplexity • Simple: coffee cup calorimeters • Complex: bomb calorimeters

The following electrostatic potential map has very little to no color difference. It shows a __________ molecule.

nonpolar

+ or - signs

q (heat) + system gains thermal energy - system loses thermal energy w (work) + work done on the system - work done by the system Delta E (change in internal energy) + energy flows into the system - energy flows out of the system

Molecular Geometries and Polarity

• A polar bond does not necessarily mean a molecule is polar. Why? • A polar molecule requires a net dipole moment • A polar bond creates a dipole, but the molecular geometry can cause it to be canceled out • Symmetrical molecular geometries that tend to be non-polar: • Linear • Trigonal Planar • Tetrahedral • Trigonal Bipyramidal • Octahedral • Square Planar • A symmetrical molecule can still be polar if the terminal atoms are different (which breaks the symmetry!)

Bond Energies

• An average bond energy is the average amount of energy (kJ/mol)that must be put into a bond to break it or dissociate it over a number of compounds that contain that bond • Remember: these are always endothermic because energy is required to break a bond and creating bonds is exothermic because energy is released • We can use these bond energies to predict the heat of reaction if we know how many of which kind of bonds are broken

Enthalpy

• Enthalpy (H) is the change in energy of a system at constant pressure, the sum of the change in internal energy and the pressure-volume product • For a constant pressure process, the change in enthalpy (DeltaH)is equal to the heat exchanged • Positive in an endothermic reaction • Negative in an exothermic reaction • The enthalpy change is often called "the heat of" • Enthalpy is one of the most used concepts in thermodynamics • Enthalpy is generally measured in kJ/mol

Consider a hypothetical molecule, MX5, which has a square pyramidal molecular geometry. How many lone pairs are on this molecule? Answer with an integer (e.g. 7).

1

Hybridization 2-6

2 - sp 3 - sp^2 4 - sp^3 5 - sp^3d 6 - sp^3 d^2

Of the molecules below, only ________ is nonpolar. A. BF3 B. NF3 C. IF3 D. PBr3 E. BrCl3

A

The following electrostatic potential map below has a significant difference in color. It shows a __________ molecule. A. Polar B. Nonpolar C. Ionic D. Stoichiometric

A

Which of the following bonds would best be considered non-polar covalent? A. C-H B. O-H C. C-Cl D. C-O

A

The combustion of liquid hydrogen and liquid oxygen produces water vapor to propel a rocket into space. Which of the following would a chemist most likely define as part of the system? Select all that apply. A. H2(l) B. O2(l) C. H2O(g) D. The fuel tank E. The rocket F. The atmosphere around the rocket G. The Earth

ABC

What is true of hybridization and hybrid orbitals? Select all that apply. A. Non-hybrid orbitals could not form the bond angles predicted in VSEPR B. sp2 hybrid orbitals are formed from the mixing of one s and two p orbitals C. Hybrid orbitals retain the same shapes as the original orbitals D. Pi bonds do not involve hybrid orbitals E. An sp orbital is degenerate with the two remaining p orbitals

ABD

Of the following, which is a state function? Select all that apply. A. Enthalpy B. Heat C. Work D. Internal Energy E. None of the above

AD

Which of the following statements is/are true? A. If q = -17 kJ, then heat flows from the system to the surroundings B. If w = 25 kJ, the system does work on the surroundings C. If more work is done by a system than heat flowing into a system, than the internal energy of a system increases D. If an ideal gas expands against a constant external pressure, then the system is doing work on the surroundings

AD

Which of these molecules have an overall dipole moment? Select all that apply. A. H2O B. Cl2 C. CCl4 D. PCl3 E. HBr

ADE

Which of the following statements about energy is/are true? Select all that apply. A. The energy stored in bonds is a type of kinetic energy B. Thermal energy is a type of kinetic energy C. A reaction that releases heat is considered exothermic D. None of the above are true

BC

Which of the following molecules have sp3 hybridization on the central atom? Select all that apply. A. C2H2 B. CBr4 C. XeCl4 D. H2O E. SF4

BD

For which of these molecules do the electron and molecular geometry differ? Select all that apply. A. BF3 B. PCl3 C. CCl4 D. NH4 + E. CO3 2- F. XeF4

BF

Which statement is true if ΔH = - 85 J? A. The system is gaining 85 J; the surroundings are losing 85 J. The process is exothermic. B. The system is gaining 85 J; the surroundings are losing 85 J. The process is endothermic. C. The system is losing 85 J; the surroundings are gaining 85 J. The process is exothermic. D. The system is losing 85 J; the surroundings are gaining 85 J. The process is endothermic. E. Both the system and surroundings are losing 85 J. The process is exothermic.

C

If 100. g of each of the following metals is heated to 100°C and placed in a coffeecup calorimeter, which metal would cause the lowest rise in temperature? A. Sn: 0.218 J/g'C B. Ag: 0.239 J/g'C C. Au: 0.126 J/g'C D. Al: 0.921 J/g'C

C since smallest Heat Capacity

Which bond energy varies depending if it's in a certain molecular form?

C=O bonds in CO2 is 799 kJ/mol while it is 736 in everything else

Consider the Lewis structure for BrF5 below. Which of the following statement(s) is/are true? Select all that apply. A. The formal charge on all of the elements above are -1 because they are all halogens B. The molecular geometry for this molecule is octahedral C. The electron geometry for this molecule is octahedral D. The molecular geometry for this molecule is square pyramidal E. The electron geometry for this molecule is square pyramidal

CD

Which of the following reactions would not result in pressure-volume work by either the system or the surroundings? A. C3H4 (g) + 2 H2 (g) → C3H8 (g) B. 2 H2 (g) + O2 (g) → 2 H2O (l) C. 2 C3H4 (g) + 8 O2 (g) → 6 CO2 (g) + 4 H2O (g) D. Fe2O3 (s) + 3 CO (g) → 2 Fe (s) + 3 CO2 (g) E. 2 NH3 (g) → N2 (g) + 3 H2 (g) F. All of the reactions listed would result in pressure-volume work

CD

Bond formation _____________ energy; it is an ______________ process. A. requires, endothermic B. requires, exothermic C. releases, endothermic D. releases, exothermic E. releases, hybridized

D

If Vf is less than Vi and heat is absorbed, which of the following options are true for the system? A. The sign for work is negative; the total internal energy change is negative B. The sign for work is negative; the total internal energy change is positive C. The sign for work is positive; the total internal energy change is negative D. The sign for work is positive; the total internal energy change is positive E. There is not enough information to answer without knowing the exact decrease in volume and the exact amount of heat that is absorbed

D

What is false regarding the compound PCl4 - ? A. It has a trigonal bipyramidal electron geometry B. It has a seesaw molecular geometry C. The lone pair occupies an equatorial position D. The lone pair occupies an axial position E. The lone pair is more repulsive than the bonding pairs

D

A hot air balloon is filled with gas. Upon absorbing 934 J of heat, the gas expands, performing 478 J of work. What is the change in internal energy in Joules? A. 1412 J B. -456 J C. 456 J D. -1412 J

Delta E = q + w 934 + (-478) C

. A student goes to the lab and performs a synthesis reaction in a large round bottom flask. The last step of the synthesis involves heating the reaction to a high temperature, and then allowing it to cool down to room temperature. To expedite the process, they decide to submerge the flask in a liquid solvent to facilitate heat transfer from the flask to the solvent. Given the information below, which of the following would be most efficient at cooling the flask? Solvent / Molar mass (g/mol) / Specific heat (kJ/kg×K) Propylene glycol 76.09 2.50 Methanol 32.04 2.51 Ethanol 46.07 2.85 A. Propylene glycol because it has the lowest specific heat B. Propylene glycol because it has the highest molar mass C. Either propylene glycol or methanol because they both have similar specific heats that are both the lowest in the table D. Methanol because it has a very low specific heat and the lowest molar mass E. Ethanol because it has the highest specific heat

E

# of electron groups

Electron Geometry: 2 - linear 3 - Trigonal planar 4 - Tetrahedral 5 - Trigonal bipyramidal 6 - octahedral

Exothermic vs Endothermic

Exothermic: q(rxn)<0 System -(heat)-> Surroundings Endothermic: q(rxn) > 0 Surroundings --(heat)-> System

True or False: the heat exchanged in a bomb calorimeter is equal to theenthalpy change.A. TrueB. False

F

Heat (q)

Heat is the transfer of thermal energy between a system and surroundings due to a temperature difference • The amount of heat (q) needed to change the temperature of a substance depends on • How much the temperature changes • The quantity of the substance• The nature of the substance • The SI unit for heat is the Joule • 1 calorie (cal) is the quantity needed to change the temperature of one gram of water by one degrees Celsius • Kilocalorie (Calorie) is more widely used • 1 cal = 4.184 J

Which of the following statements below are true regarding the molecule below which is illustrated incorrectly? Select all that apply. Hint: the molecular geometry shown below is not a standard geometry you have learned. Electron geo = trigonal bipyramidal Molecular geo = seesaw

LP should be in equatorial position

Bond Polarity

No or very little difference in EN (< 0.4-0.5) = (nonpolar) covalent • Moderate difference in EN (between ~0.4 to ~1.8) = polar covalent• Large difference in EN (> ~1.8-2.0) = ionic • You can estimate this difference from the position of elements on the periodic table (probably what we'll ask you to do on an exam) or calculate it with Pauling electronegativity values • Even these calculations can be misleading on the boundaries • Know that N, O, F, and Cl generally make polar or ionic bonds • Period 2 has an especially high electronegativity • C-H is nonpolar • Nonmetal + metal is still probably ionic

Pressure-Volume Work

Recall that we defined work as the force acting over a distance • Pressure-volume work specifically looks at the volume change against an external pressure • The equation for this can be written as: w = -PΔV P = Pext or Patm

Identifying reactions that do or do not result in pressure-volume work

To do this, we look for a change in moles of gas only • Which of the options below result in pressure-volume work? A A) CO2 (s) → CO2 (g) 0 mol (g) -> 1 mol (g) B) H 2 (g) + Cl 2 (g) → 2 HCl (g) 2 mol (g) -> 2 mol (g) C) H 2O (s) → H 2O (l) 0 mol (g) -> 0 mol (g) D) 25 X (s) + 5 Z (g) → 5 X5Z (g) 5 mol (g) -> 5 mol (g)

Bonding

We can think of bonding in many different ways • Overlapping of electron clouds • Sharing of electrons in empty orbitals • The orbitals as we've conceived of them present two issues: • Not all atoms have enough empty orbitals to make the number of bonds that we know they can form • The shape of the s and p orbitals doesn't match VSEPR bond angles

Heat and Work

• Heat: flow of energy due to a temperature difference• Work: the force acting over a distance • Internal energy: sum of the kinetic and potential energies of all the particles that compose the system • State function: a property independent of its pathway, and only depends on its present state • Internal energy is a state function • Heat and work are not state functions

Hybridization

• How do we explain what we observe experimentally? Hybrid orbitals • S, p, and d orbitals can "mix," forming new orbitals with characteristics of all the component orbitals • They have a new shape - a combination of the component orbital shapes • The new hybrid orbitals are degenerate - an average of the energy levels of the component orbitals • They get a new orbital designation that is a combination of the component orbitals

Experimental Determination of Specific Heat

• If we have two substances, the heat transferred from substance 1 to substance 2 will be equal in magnitude, opposite in sign: q(sub2) = -q(sub1) • We can substitute in our specific heat capacity equation:݉ m2c2(T_F - T_I)_2 = -(m1c1(T_F - T_I)_1) T(F2) = T(F1) • Different substances will have different heat capacities depending on chemical composition, molar mass, intramolecular forces, etc.

Law of Conservation of Energy

• Law of Conservation of Energy - the total energy of the universe remains constant; energy is neither created nor destroyed q_system + q_surroundings = 0 • Heat gained by the system is lost by the surroundings q_system = - q_surroundings • We can use this to determine the heat capacities of substances by experiment

Electron pairs in bonds and lone pairs repel each other

• Lone pair-lone pair > Lone pair-bond pair > bond pair-bond pair • The first step is to draw the Lewis structure of the compound • From there, we take note of the lone pairs and bonds present and assign the electron geometry • Based on the electron geometry, we can then assign the molecular geometry • Geometries are assigned to the central atom; if there are more complex molecules, then we will often choose a central atom • In any given molecule, you will need to be able to assign electron and molecular geometries, and their angles

Systems

• Open system: freely exchanges energy and matter with its surroundings • Closed system: can exchange energy, but not matter with its surroundings • Isolated system: cannot exchange energy or matter with its surroundings (doesn't 100% exist)

Polar Covalent Bonds

• Polar covalent bonds involve the unequal sharing of electrons • They result in partial charges • We can use electrostatic potential maps to visualize the electron distribution

The Nature of Energy

• Potential energy: energy due to position or composition • Ball sitting on top of a hill, compressed spring, etc. (energy in bonds) • Kinetic energy: energy due to to motion • Ball rolling down a hill, a spring that has been released, etc. (breaking bonds or forming bonds) • Law of Conservation of Energy: energy cannot be created or destroyed, but it can be transferred • Universe = system + surroundings • System: part of the universe chosen to study • Surroundings: rest of the universe

Bomb Calorimeter

• Same relationship used: -q(calorimeter) = q(rxn) • Big difference is the calorimeter itself has a heat capacity, Ccal, which is the heat required to change the temperature of the calorimeter by one degree Celsius q(calorimeter) = Ccal DeltaT

Energy

• The main type of energy that concerns chemists is chemical energy, the potential energy stored in chemical bonds and intramolecular attractions • In a chemical reaction, the chemical energy changes. Some of this change appears as the heat of reaction (q rxn) • The amount of heat exchanged between system and surroundings when a chemical reaction occurs at constant temperature • One of the common types of reaction studied by calorimetry is combustion • Heat of combustion

Heat capacity

• The quantity of heat required to change the temperature of a system by one degree is the heat capacity (C) C = q/DeltaT DeltaT = T(F)-T(I) • We usually use molar or specific heat capacity! The heat capacity per mole or gram of a substance at a constant pressure q = nC_(p,m) Delta T q = mC_pDeltaT • q will be positive if heat is gained and negative if heat is lost

Coffee Cup Calorimeter

• The simplest type of calorimeter • 2 Styrofoam cups • Reaction mixture • Thermometer • Stirrer • Using the Law of Conservation of Energy, we can say: q(calorimeter) = -q(rxn)

Heats of Reaction

• To predict the heat of a reaction using our table of average bond energies: • We can add up the energy of all the bonds broken • Add up the energy of all the bonds formed (and flip the sign) • Add the two together to get the total energy Sum (BE Bonds Broken) + Sum -(BE Bonds Formed) Sum (BE Bonds Broken) - Sum (BE Bonds Formed) Sum (BE Reactants) - Sum (BE Products)

Predicting Endothermic vs Exothermic

• Usually heats of formations are better estimates than average bond energies • There are exceptions, like catalytic reactions where intermediates exist for such a short time that they can't be properly studied • They do give us an easy way to quickly predict whether a reaction will be endothermic or exothermic • If the reactants have weak bonds and the products have strong bonds, it's likely exothermic • If the reactants have strong bonds and the products have weak bonds, it's likely endothermic

Delta E equation

• We can also say that the flow of heat or work can change the internal energy of a system ΔE = q + w

Molecular Structure: VSEPR

• We will build off Lewis structures to see a three-dimensional model of compounds • There are two main geometries we will look at... • Electron geometry: the geometry of all atoms and electrons in a compound • Molecular geometry: the geometry of all the atoms in a compound • The valence shell electron pair repulsion (VSEPR) theory will help us assign these geometries

Sigma and Pi bonds

• Why does the number of electron groups and not number of electron pairs determine hybridization: Two different types of bonds: Sigma and Pi Bonds • Sigma bonds result from the head-to-head overlap of orbitals • The first type of bonds made in an electron group • Involve s or hybrid orbitals • Pi bonds result from the side-to-side overlap of p orbitals • The bonds that are made after sigma bonds are formed, thus they only appear in double or triple bonds • Do not involve hybrid orbitals!