Chemistry Problems
Some plant fertilizer compounds are (NH₄)₂SO₄, Ca₃(PO₄)₂, K₂O, P₂O₅, and KCl. Which of these compounds contain both ionic and covalent bonds?
(NH₄)₂SO₄ = Made up of NH₄+ and SO₄²⁻ ions (Covalent bond between N and H in NH₄+, covalent bond between S and O in SO₄²⁻) / Has both ionic and covalent bonds Ca₃(PO₄)₂ = Made up of Ca²+ and PO₄³⁻ ions (Covalent bonds between P and O in PO₄³⁻) / Has both ionic and covalent bonds K₂O = Ionic bond / is a bond between metal and nonmetal P₂O₅ = Covalent bond / is a bond between two nonmetals KCl = Ionic bond / is a bond between metal and nonmetal
What happens to energy between two atoms placed at a short distance?
-Energy rises, because of repulsive forces -Potential energy of electron lowers because of increased attractive forces between
What is the fault of a simple model?
-Usually involve very restrictive assumptions -Can be expected to yield only qualitative observations -We must apply simple models with caution and not expect it to always be correct
What is electronegativity for identical atoms?
0-electrons in bond are shared equally, no polarity
How do nonmetals achieve electron configuration?
1) Forming covalent bond with other nonmetals 2) Taking electrons from metals to form ions = nonmetal (anions) and metals (cations)
In analyzing a sample of polluted water, a chemist measured out a 25.00 mL water sample with a pipet. At another point in the analysis, the chemist used a graduated cylinder to measure 25 mL of solution. What is the difference between the measurements 25.00 mL and 25ml?
25mL means volume is in between 24 mL and 26 mL. 25.00 mL means volume is in between 24.99 mL and 25.01 mL. Pipet measure volume with greater precision than a graduated cylinder.
What is the range of electronegativity?
4.0 (F) to 0.7 (Cs)
Compound with more than ___% ionic character are considered to be ionic compounds
50%
How many cations is 1 anion surrounded by?
6 cations Similarly, 1 cation is surrounded by 6 anions
What is a covalent bond?
A bond in which electrons (of same atom) are shared by the nuclei (such as in Hydrogen bonding)
What is an ionic compound?
A compound that conducts electric current when melted
Choose the largest ion Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺
All Group 1A elements Electronegativity decreases down a group = Size increases down a group (largest number of electrons has smallest size because cations lose an electron while anions gain electrons) Cs⁺ because least electronegativity
What is an example of a simple model that has a fault?
Aufbau Principle = determines electron configuration for elements Electron configuration for copper and chromium are incorrect when applying the model = Cu and Cr result from complex electron interactions
What is the distance at which system (atom bonds) has minimum energy
Bond Length
What is a pair of electrons that are shared?
Bond pair
Why do chemical bonds occur?
Bonds result from the tendency of a system to seek its lowest possible energy When collections of atoms are more stable (less energy) than separate atoms
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. F, Cl, Br
Br < Cl < F All are in same group, electronegativity decreases down a group
In which direction is the bond polarity? Br-Te
Br = has higher electronegativity than Te
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. C, N, O
C < N < O All are in the same period, electronegativity increases from left to right
In which direction is the bond polarity? H-Cl
Cl = has higher electronegativity than H
In which direction is the bond polarity? Cl-I
Cl = has higher electronegativity than I
What do you concentrate on when comparing sizes of ions, to other ions or to its neutral atom?
Concentrate on number of protons and electrons. A species/molecule that holds its electrons most tightly will be smallest. Anion has more electrons held by same number of protons > bigger than its neutral atom with same number of protons Cations have more protons (hold electrons more tightly) < smaller than neutral atom (more electrons held loosely) Isoelectric ions = same number of electrons, different number of protons = ion with most protons will be smallest, because protons will hold electrons more tightly
What formula is this? E = (2.31 * 10⁻¹⁹ J ∙ nm)(Q₁Q₂/r)
Coulomb's Law -Calculates energy of interaction between pair of ions E = units in Joules r = distance between ion centers in nm Q₁ and Q₂ = numerical ion charges
Arrange the ions Se²⁻, Br⁻, Rb⁺, and Sr²⁻ in order of decreasing size, based on noble gas electron configuration
Decreasing size = Increasing electronegativity All ions are part of an isoelectronic series with K electron configuration = same number of electrons = sizes depend on nuclear charge (number of protons) Se²⁻ > Br⁻ > Rb⁺ > Sr²⁻ 34 < 35 < 37 < 38 Largest ion = Smallest nuclear charge Increasing electronegativity = Decreasing nuclear charge
What is a molecule that has a center of positive charge and a center of negative charge?
Dipolar / Dipole moment Represented by tail at positive center with arrow towards negative center
The distance between the centers of Na⁺ and Cl⁻ ions is 0.276 nm. Calculate ionic energy.
E = (2.31 * 10⁻¹⁹ J ∙ nm) [(+1)(-1) / 0.276 nm] E = -8.37 * 10⁻¹⁹ J Ion pair has lower energy than separated ions
What is the ability of an atom in a molecule to attract shared electrons to itself?
Electronegativity
Does the molecule have a dipole moment? If so, determine the direction of the bond polarity. CH₄ (tetrahedral with C at center)
Electronegativity C = 2.5 = partially negative H = 2.1 = partially positive All corners of tetrahedral are same, charges cancel No dipole moment, no polarity
Does the molecule have a dipole moment? If so, determine the direction of the bond polarity. HCl
Electronegativity Cl = 3.0 H = 2.1 HCl has a dipole moment, polarity is towards Cl⁻
Does the molecule have a dipole moment? If so, determine the direction of the bond polarity. SO₃ (planar molecule with O atoms evenly spaced around central S atom)
Electronegativity S = 2.5 = partially positive O = 3.5 = partially negative Since the corners of the trigonal planar structure are all the same element, it is symmetrical and the charges cancel out No dipole moment, no polarity
What is the electronegativity trend?
Electronegativity (the ability of an atom to attract electrons) increases across (left to right) a period in the periodic table and decreases down a group in the periodic table
Ionic character increases with ___
Electronegativity Difference No ionic bond is ever completely ionic, even a bond between atoms with maximum and minimum possible electroneagtivity
Why are there partial positive and partial negative charges in molecules such as HF and H₂O?
Electrons in bond are not shared equally HF = Fluorine (F⁻) has stronger attraction for shared electrons than Hydrogen (H⁺) H₂O = Oxygen (O²⁻) has stronger attraction for Hydrogen (H⁺)
What diagram shows charge distribution in a bond?
Electrostatic Potential Diagram Red = electron rich Blue = electron poor
What is an ionic bond?
Electrostatic forces between closely packed, oppositely charged ions
Is the formation of a gas exothermic or endothermic?
Endothermic Energy is taken in to separate the ions from the ionic compound
What is bond energy? (we can obtain information about the strength of a bonding interaction by measuring this)
Energy required to break a bond (strength of bonding interaction)
Is the formation of a solid exothermic or endothermic?
Exothermic Energy is released when ions combine to form a solid
In which direction is the bond polarity? H-F
F = has higher electronegativity than H
Models tend to become simpler as they age. As flaws are discovered in our models, we take out any extra detail.
False
True/ False Models are often correct. Molecules are based on facts and are very detailed, explain everything.
False
True/False Because SF₄ exists, OF₄ should also exist because O is in the same family as S.
False Although SF₄ can exist, OF₄ cannot exist due to the fact that S is a bigger, third-period element and can have an expanded octet in order to accommodate 4 F atoms, while O is a second-period element and cannot expand its octet to accommodate 4 F atoms.
How does this describe ionic bonding: low electrical conductivity as solids and high conductivity in solution or when molten?
For electrical conductivity, charged species must be free to move. In ionic solids, the charged ion are held rigidly in place. In solution, when the forces are disrupted (when molten), the ion move (conduct).
What is a chemical bond?
Forces that cause a group of atoms to behave as a unit.
Combustion reactions of fossil fuels provide most of the energy needs of the world. Why are combustion reactions of fossil fuels so exothermic?
Fossil fuels are carbon based. Carbon has four open electron spaces, so it can make strong single, double, or triple bonds. The more bonds in a molecule, the more energy it contains. Because carbon molecules have so many bonds, breaking those bonds release a lot of energy.
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. B, O, Ga
Ga < B < O
Predict which bond will be most polar. C-F, Si-F, Ge-F
Ge-F Germanium has the least electronegativity = more polar towards F
Where does Hydrogen fit into the electronegativity trend for other elements in the periodic table?
In 2nd row of periodic table = between Boron and Carbon 3rd row = identical to Phosphorus 4th row = Between Arsenic and Selenium 5th row = identical to Tellurium
Rank the following bonds in order of increasing ionic character: N-O, Ca-O, C-F, Br-Br, K-F
Ionic character = difference in electronegativity Br-Br
What are ion that contain the same number of electrons?
Isoelectric Ions
Why do some stable ionic compounds have +1 charged ions when +4, +5, or +6 charged ions have a more favorable lattice energy?
Just because an ion has a +1 charge, doesn't mean the cation can not make a stable compound. All ions can form a stable compound as long as the energy is lowered in formation.
What is the change in energy that takes place when separated gaseous ions are packed together to form an ionic solid?
Lattice Energy = indicates how strongly ions are attached -Energy released when ions form solid = exothermic = negative sign (system's point of view)
Give one example of a polar (overall dipole) compound, with a linear structure.
Linear Structure = B-A-B HCl = Cl has more electronegativity, polarity towards Cl, different atoms http://www.chemteam.info/Bonding/Molecular-Polarity.html
Give one example of a nonpolar (not overall dipole) compound, with linear structure.
Linear Structure = B-A-B H₂ = H-H (same ends = nonpolar) CO₂ = charges cancel out http://www.chemteam.info/Bonding/Molecular-Polarity.html
What does this formula calculate? Expected H-X bond energy = (H-H bond energy + X-X bond energy) / 2
Linus Pauling- American scientist Pauling's model determines values of electronegativity If X has greater electronegativity, shared electrons will be closer to X atom = polar molecule
What is a pair of electrons that is not shared?
Lone pair
How does this describe ionic bonding: relatively high melting and boiling points?
Melting and boiling only takes place when ions ion attractions are disrupted. Due to strong electrostatic forces between ions, it will take a lot of energy (high temperature) to accomplish this.
What makes up an ionic compound?
Metal and Nonmetal
What formula is this? k[(Q₁Q₂)/r]
Modified Coulomb's Law Formula for Lattice Energy k = constant that depends on electron configuration of ions Q₁Q₂ = charges of ions (negative sign when cation and anion come together = exothermic process) r = shortest distance between centers of cations and anions
Are there large collections of ions in a gas phase?
No, ions are quite far apart on average
What does Z stand for?
Nuclear charge of ion
In which direction is the bond polarity? C-O
O = has higher electronegativity than C
In which direction is the bond polarity? O-P
O = has higher electronegativity than P
Predict which bond will be most polar. O-F or O-Cl
O-F
In which direction is the bond polarity? P-H
P = has higher electronegativity than H (which is between B and C)
Predict which bond will be most polar. P-Cl or S-Cl
P-Cl P has less electronegativity than S
Hydrogen has an electronegativity value between Boron and Carbon, and identical to Phosphorus. With this in mind, rank the following bonds in order of decreasing polarity: P-H, O-H, N-H, F-H, and C-H
P-H < C-H < N-H < O-H < F-H H = P < C < N < O < F H-P will have least polarity since it will have the smallest difference in electronegativity
What is it called when atoms are unequally shared?
Polar Covalent Bond
What bonds can be formed between two atoms with different electronegativites?
Polar Covalent Bond and Ionic Bond
How does this describe ionic bonding: solubility in polar solvents
Polar molecules are attracted to ions and can break the lattice.
Order following bonds according to polarity H-H, O-H, Cl-H, S-H, and F-H
Polarity of bond increases as electronegativity increases H-H (2.1)(2.1) < S-H (2.5)(2.1) < Cl-H (3.0)(2.1) < O-H (3.5)(2.1) < F-H (4.0)(2.1) As polarity increases, covalent bond turns into polar covalent pond
Kinetic or Potential Energy Energy that results from attraction and repulsion forces among charged particles
Potential Energy
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. Na, K, Rb
Rb < K < Na All are in same group, electronegativity decreases down a group
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. S, O, F
S < O < F
Does the molecule have a dipole moment? If so, determine the direction of the bond polarity. H₂S (V - shaped with S atom at the point)
S = 2.5 = partially negative H = 2.1 = partially positive Analogous to water Dipole moment, polarity toward S
In which direction is the bond polarity? Si-S
S = has higher electronegativity than Si
Predict which bond will be most polar. S-F, S-Cl, S-Br
S-Br Br has least electronegativity
Does the molecule have a dipole moment? If so, determine the direction of the bond polarity. Cl₂
Same atoms Chlorine atoms will share the electrons equally, no dipole moment or bond polarity
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. S, Se, Cl
Se < S < Cl Se is below S (less electronegativity than S) and S is to the left of Cl (less electronegativity than Cl)
In which direction is the bond polarity? Se-S
Se = has higher electronegativity than S
Predict which bond will be most polar. C-O or Si-O
Si-O
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. Si, Ge, Sn
Sn < Ge < Si All are in same group, electronegativity increases up the group
Predict which bond will be most polar. C-H, Si-H, Sn-H
Sn-H
When scientists refer to an ionic compound, what state are they usually referring to?
Solid state (ions packed together in a way that minimizes repulsion forces and maximizes attraction forces)
Give one example of a nonpolar (not overall dipole) compound, with tetrahedral molecular structure.
Tetrahedral Molecular Bonds = 109.5° apart CCl₄ = C⁴⁺ and 4Cl⁻ cancel each other CH₄ = same tetrahedron corners http://www.chemteam.info/Bonding/Molecular-Polarity.html
Give one example of a polar (overall diploe) compound, with tetrahedral molecular structure.
Tetrahedral Molecular Bonds = 109.5° apart NH₃ = triangle corners different CH₃Cl = C⁴⁺, 3H⁻, and Cl⁻ cancel each other CH₃F = different tetrahedron corners, polarity likely towards F H₂O = O has greater electronegativity than H, two O lone pairs http://www.chemteam.info/Bonding/Molecular-Polarity.html
What is the purpose of a binary ionic compound?
The ions form so that valence electron configuration of nonmetal = electron configuration of next noble gas atom metal = valence orbitals empty, still noble gas configuration
In which direction is the bond polarity? Br-Br
There is no dipole moment, it will form a polar covalent bond. Neither will have a charge.
Choose the largest ion Ba²⁺, Cs⁺, I⁻, Te²⁻
This is an isoelectronic series of ions, all have Xe electron configuration Ion with smallest charge (number of protons) = biggest Te²⁻ > I⁻ > Cs⁺ > Ba²⁺ 52 < 53 < 55 < 57
Predict which bond will be most polar. Ti-Cl, Si-Cl, Ge-Cl
Ti-Cl Ti has least electronegativity
Predict the order of increasing electronegativity, using the periodic table, not the actual electronegativity value. Tl, S, Ge
Tl < Ge < S Electronegativity decreases down the table
Predict which bond will be most polar. Al-Br, Ga-Br, In-Br, Tl-Br
Tl-Br
What is the purpose of a covalent bond?
To share electrons in a way that completes the valence electron configuration of both atoms (both nonmetals obtain noble gas electron configuration)
Give one example of a nonpolar (not overall dipole) compound, with trigonal planar structure.
Trigonal Planar Bonds = 120° apart BCl₃ = all triangle corners have Cl (same element = nonpolar), B³⁺ and 3Cl⁻ SO₃ = same triangle corners, S²⁻ and 3O²⁻ http://www.chemteam.info/Bonding/Molecular-Polarity.html
Give one example of a polar (overall dipole) compound, with trigonal planar structure.
Trigonal Planar Bonds = 120° apart SO₂ = triangle corners not same, undefined polarity with S lone pair, and O double and single bonds BCl₂Br = Cl more electronegative, electronegativity towards Cl, triangle corners are different http://www.chemteam.info/Bonding/Molecular-Polarity.html
True/ False Models are human inventions, always based on an incomplete understanding of how nature works. A model does not equal reality.
True
True/ False We often learn much more when a model is wrong than when it is right.
True
True/False Ions adopt noble gas configuration in ionic compounds
True
True/False It is impossible to satisfy the octet rule for all atoms in XeF₂
True In order for all electron-filling needs to be satisfied, Xe will from 2 single bonds with F. Xe will have 3 lone pairs and F will have 3 lone pairs. Xe has the correct number of electrons, but it does not follow the octet rule.
True/False The bond in NO⁺ should be stronger than the bond in NO⁻
True NO⁺ bonds have triple bonding, NO⁻ bonds have double bonding.
What can we do to determine the charge distribution in a molecule?
We can study its behavior in an electric field
How are ionic substances formed?
When atom that loses electrons relatively reacts easily with an atom that has a high affinity for electrons
When is zero point of energy between two atoms?
When atoms have infinite separation, atoms are too far apart to have any interaction
How does this describe ionic bonding: brittleness
When we bend a piece of material, ions slide across each other. When ions slide, they may come into contact with same charge, creating a repulsion force, causing ionic solid to snap.
Is energy a state function?
YES
Is it possible for a polar molecule to not have a dipole moment?
Yes Individual particles are arranged in such a way that charges cancel each other out
What is the formula for the percent ionic character of a bond?
[(measured dipole moment of X-Y) / (calculated dipole moment of X-Y)] ∙ 100%
Ion size ______ as Z increases (positively)
decreases
What does this sign indicate? δ
fractional charge (lowercase delta)
