summer chem exam 2

formal charge on Cl in chlorate ion

0

What are the formal charges on the sulfur (S), carbon (C), and nitrogen (N) atoms, respectively, in the resonance structure that contributes most to the stability of the thiocyanate ion, SCN−?

0, 0, -1

formal charge on O in chlorate ion (a, b, c)

0, 0, -1 These formal charges can be used to predict the resonance structure that contributes most to the stability of a molecule or ion. The structure with the lowest formal charge on each atom and which places the negative charge on the most electronegative atom is generally the most stable.

How many sigma bonds are present in xanthine?

16 σ bonds A single bond always corresponds to a sigma (σ) bond, which is a pair of orbitals overlapping end to end. A double bond always corresponds to one σσ bond and one pi (π) bond. The structure shows 12 single bonds and 4 double bonds. Therefore, there are 16 σσ bonds in the molecule.

Determine the correct hybridizaton (from left to right) about each interior atom in CH3CH2OH

1st C - sp3; 2nd C - sp3; O - sp3 Draw the correct Lewis structure and count the number of electron groups on each atom; then refer to the table below to determine the correct hybridization for each atom. Both C atoms have four single bonds and therefore four electron groups. The O atom has two bonding groups (one to the C and one to the H atom) and two lone pairs, so it also has four electron groups. Four electron groups form sp3 hybrid orbital

How many resonance structures can be drawn for ozone, O3?

2

Phosphoric acid reacts with magnesium hydroxide to produce magnesium phosphate and water via the following reaction: ?H3PO4+?Mg(OH)2→?Mg3(PO4)2+?H2O Balance the equation, then enter the coefficients, in order

2,3,1,6

The figure (Figure 1) shows the reaction of element A (lavender spheres) with element B (tan spheres). Write the balanced chemical equation for this reaction in terms of A and B.

2A+3B2→2AB3

How many pi bonds are present in xanthine?

4π bonds Only one sigma (σ) bond forms between any two atoms, as it is a result of end-to-end overlap of orbitals. Additional bonds must be pi (π) bonds and require side-by-side overlap of p orbitals. A double bond always contains one σ bond and one π bond. The structure shows four double bonds; thus, there are four π bonds in the compound.

Ignoring lone-pair effects, what is the smallest bond angle in ICl5?

90 degrees

What are the expected bond angles in ICl4+?

90, 120, 180

What causes a molecule or ion to be diamagnetic?

All electrons are paired.

Determine the molecular formula of a compound that is 49.48% carbon, 5.19% hydrogen, 28.85% nitrogen, and 16.48% oxygen. The molecular weight is 194.19 g/mol.

C8H10N4O2

Determine the molecular geometry for each molecule.

CF4 - tetrahedral NF3 - trigonal pyramidal OF2 and H2S - bent Although each of these molecules has the four electron groups, the molecular geometries are not identical because of the combinations of bonding groups and lone-pair electrons. CF4 has only four bonding groups and no lone pairs, making its electron geometry and molecular geometry the same, whereas NF3 has three bonding groups and one lone pair, resulting in a trigonal pyramidal molecular geometry. Both OF2and H2S have two bonding groups and two lone pairs, resulting in a bent geometry.

Find the molecular formula of formic acid.

CH2O2 Consider a 100-gg sample and calculate the mass of each element from the given percentage Convert each of the masses to moles by using the appropriate molar mass for each element as a conversion factor Divide all the subscripts in the formula by the smallest subscript to obtain the empirical formula A molecular formula is always a whole-number multiple of the empirical formula Calculate the empirical formula mass and then the ratio between the empirical formula and the molecular formula mass

Which of the following compounds is correctly classified?

CH3CH2CH3, organic, hydrocarbon

What is a chemical bond?

Chemical bonds are the result of interactions between charged particles—electrons and protons—that compose atoms, A chemical bond is the force that holds atoms together in a compound

Why do chemical bonds form?

Chemical bonds form because they lower the potential energy of the charged particles that compose atoms.

Which element has the highest (most negative) electron affinity?

Cl Electronegativity is another periodic property. The electronegativity value for an atom describes its ability to attract electrons to itself when it is part of a chemical compound. Electronegativity increases diagonally from the lower left to the upper right of the periodic table. Highly electronegative elements (with a chi value of χ≥2.2) are usually insulating nonmetals, whereas elements with low electronegativity (with χ≤1.8) are often conducting metals.

Which statement best describes an O3 molecule?

Each atom in ozone is connected to another atom by a bond having a bond order of 1.5. Scientists previously believed that a molecule such as ozone switched rapidly back and forth between its different resonance forms. More recent experiments have shown that the molecule actually assumes the average of all the resonance forms, with the electrons delocalized over all three oxygen atoms as shown in the following diagram.

Which of following changes that affect the composition of our atmosphere involve physical changes?

Fog forms from water vapor when the temperature drops below the dew point. Freezing rain develops when a warm air mass overrides a cold air mass.

Give some examples of some typical bond lengths

H − H bond length = 74 pm. F − F bond length = 143 pm. Cl − Cl bond length = 199 pm. Br − Br bond length = 228 pm. I − I bond length = 266 pm. C ≡ C bond length = 120 pm. C = C bond length = 134 pm. C − C bond length = 154 pm.

Place the following in order of decreasing bond length: H-F, H-I, H-Br

H-I > H-Br > H-F

What is hybridization? Why is hybridization necessary in valence bond theory?

Hybridization is a mathematical procedure in which the standard atomic orbitals are combined to form new atomic orbitals called hybrid orbitals. Hybrid orbitals are localized on individual atoms, and they have shapes and energies different from those of standard atomic orbitals. Hybrid orbitals are necessary in valence bond theory because they correspond more closely to the actual distribution of electrons in chemically bonded atoms.

How do you determine how many dots to put around the Lewis symbol of an element?

In a Lewis structure, dots surrounding the symbol of an element denote the valence electrons of an element. The number of the valence electrons for the main-group elements equals the group number they are in.

How many molecules of ethanol (C2H5OH) (the alcohol in alcoholic beverages) are present in 145 mL of ethanol? The density of ethanol is 0.789 g/cm3

N = 1.50×1024

Arrange these elements in order of decreasing electronegativity: P, Na, N, Al

N, P, Al, Na Electronegativity increases from left to right across a row in the periodic table and decreases from top to bottom down a column.

In which cases do you expect deviations from the idealized bond angle?

NF3, OF2, H2S NF3 has three bonding groups and one lone pair whereas OF2 and H2S have two bonding groups and two lone pairs. Lone pairs and double bonds take up more room than single bonds and they repel the bonding groups to a greater extent, resulting smaller bond angles. For example, NH3 has bond angles of 107. Thus, only CF4 will have the idealized bond angles of 109.5

A Venus probe detects an element with a first ionization energy of 2088 kJ/mol. Given the table of known ionization energies provided in the introduction, what is the most likely identity of this element?

Ne The electron affinity of atoms is also periodic. The electron affinity of an atom is the energy that is lost or gained when an electron is added to a neutral atom of the element in a gaseous state. A more negative electron affinity is considered a higher electron affinity value. It tends to become more negative going from left to right across a row in the table and less negative going down a column. Electron affinities are not as smoothly periodic as some of the other properties, with some major exceptions opposing the general trends. For example, the group 2 and group 18 elements have positive electron affinities, and each of the row 2 elements B through F are less negative than the elements just below them in the column. In general, however, electron affinities tend to be more negative as you move toward the upper right-hand corner of the periodic table.

Which of following changes that affect the composition of our atmosphere involve chemical reactions?

Oxygen gas changes to ozone during thunderstorms. When coal, oil, and natural gas are decomposed in landfills they produce methane gas. Carbon dioxide is produced by the combustion of gasoline in an automobile engine.

Place the following elements in order of decreasing atomic radius: chlorine (Cl), lead (Pb), aluminum (Al), and fluorine (F).

Pb, Al, Cl, F Ionization energy is another periodic property of the elements. The ionization energy of an atom is the energy required to remove an electron when the element is in a gaseous state. Ionization energies tend to increase from left to right across the row, but they decrease going down a column. The highest ionization energies are thus found in the upper right corner of the periodic table.

Identify whether each molecule given below is polar or nonpolar.

Polar: CHCl3, BrF3, OF2 Nonpolar: CS2 BrF3 and OF2 have an unsymmetric distribution of bonding pairs and lone pairs around the central atom and both are polar. CHCl3 has an unsymmetrical distribution of atoms (electronegativities) around the central atom and is polar. CS2 has a symmetric distribution of bonding pairs around the central atom and is nonpolar.

Which factors influence bond lengths?

Smaller atoms have shorter bond lengths, double bonds are shorter and stronger than single bonds, triple bonds are shorter and stronger than double bonds

Use the ionic bonding model to determine which has the higher melting point, RbCl or SrO. Explain your answer.

SrO has a higher melting point because the ions have larger charges, You would expect SrO to have the higher melting point because, in our bonding model, the strontium and oxygen ions are held together in a crystalline lattice by charges of 2+ for strontium and 2− for oxygen. In contrast, the RbCl lattice is held together by charges of 1+1+ for rubidium and 1−1− for chlorine. According to Coulomb's law, as long as the spacing between the cation and the anion in the two compounds does not differ that much, the higher charges in SrO result in lower potential energy (more stability), and therefore a higher melting point. The experimentally measured melting points of these compounds are 718 ∘C∘C for RbCl and 2430 ∘C∘C for SrO, in accordance with the model.

Consider the Lewis structure for diatomic oxygen. This structure predicts the following?

Stability of the diatomic molecule, double bond, number of lone pairs

Neither the VSEPR model nor the hybridization model is able to account for the experimental observation that the F−Ba−F bond angle in gaseous BaF2 is 108 rather than the predicted 180. Suggest some possible explanations for this observation.

The 5d orbitals are very close in energy to the 6s orbitals in the Ba atom. Thus, these may contribute to the bonding scheme rather than the 6p orbitals

Which choice best describes the polarity of ICl5?

The molecule is polar and has polar bonds.

Draw the Lewis structure for NO2 - including any valid resonance structures. Which of the following statements is TRUE?

The nitrite ion contains two N-O bonds that are equivalent to 1 and 1/2 bonds.

How important is the resonance structure shown here to the overall structure of carbon dioxide?

The resonance structure does not provide a significant contribution to the resonance hybrid as it has a +1 formal charge on a very electronegative oxygen. The most electronegative O atom carries a positive formal charge. Therefore, this resonance structure does not contribute to the resonance hybrid.

What causes a molecule or ion to be paramagnetic?

There is at least one unpaired electron.

What is the formula for titanium(IV) oxide?

TiO2. The sum of the charges of all of the ions in the compound must sum to 0. The charge on Ti is 4+ (as indicated by the Roman numeral) and the charge on O is 2− (and there are two of them), so the charges in this formula sum to 0.

Explain how to name molecular inorganic compounds.

To name a binary molecular inorganic compound, list the full name of the first element with a prefix to indicate the number of atoms in the compound. After that, write the base name of the second element with a prefix, followed by the suffix -ide. Generally, the name of the element with the smallest group number is written first. If the two elements lie in the same group, write the element with the greatest row number first. If there is only one atom of the first element in the formula, prefix mono- is normally omitted.

Determine the idealized bond angle for each molecule.

all 109.5 CF4 NF3 OF2 H2S The idealized bond angles for tetrahedral electronic geometry are 109.5∘; however, the actual bond angles for molecules may differ depending on how strongly each electron group repels other groups.

Determine the electron geometry for each molecule.

all tetrahedral CF4 NF3 OF2 H2S Each of these molecules has four electron groups. CF4 has four bonding groups, NF3 has three bonding groups and one lone pair, and both OF2 and H2S have two bonding groups and two lone pairs. Therefore, the electron geometry is the same for each.

A probe sent to the planet Mercury has measured the electronegativities of several elements it has detected on the planet. Which element would you expect to identify as a metal?

element 2 The element with an electronegativity value of 1.61 is aluminium (Al), which is indeed a metal. Arsenic ( χ=2.18 ) is a semimetal, and chlorine (χ=3.16 ), xenon (χ=2.60 ), and nitrogen (χ=3.04 ) are nonmetals.

In a laboratory experiment, a fermenting aqueous solution of glucose and yeast produces carbon dioxide gas and ethanol. The solution was heated by burning natural gas in a Bunsen burner to distill the ethanol that formed in the flask. During the distillation, the ethanol evaporated and then condensed in the receiving flask. The flame of the burner was kept too close to the bottom of the flask and some of the glucose decomposed into a black carbon deposit on the inside of the flask. During this experiment the following changes occurred. Which of these changes involved a physical change and not a chemical change?

evaporation and condensation of ethanol

Which are physical properties?

gas at room temperature bluish color pungent odor A physical property is any property that does not involve a change in composition.

What is the electron geometry of ICl5?

octahedral

Insert the lone pairs in the molecule. What kinds of orbitals do the lone pairs occupy?

sp2 and sp3 The lone pairs on the N atoms in position 2 and position 3 occupy sp2 orbitals; the lone pairs on the N atoms in positions 1 and 4 occupy sp3 orbitals. You can determine the hybridization by counting the number of electron groups around the atom.

What shape would you expect for XeF4?

square planar

What is the molecular geometry of ICl5?

square pyramidal

What is the electron geometry of PF3?

tetrahedral

A covalent bond is best described as

the sharing of electrons between atoms.

What is the molecular geometry of SO32−?

trigonal pyramidal

Which of the following describes a scenario in which the molecular geometry would have the smallest bond angle(s)?

two bonding domains (or bonding pairs) and two nonbonding domains (or lone pairs)

Which are chemical properties?

very reactive decomposes on exposure to ultraviolet light A chemical property does involve a change in composition.