Chem Unit 4 Review pkt
ammonium sulfate
(NH4)2SO4
The formula of an oxide of iron is FeO. State the formula of (a) the nitrate and (b) the chloride of iron with the same oxidation state
(a) Fe(NO3)2 (b) FeCl2
(a) Define the term malleable (b) Explain why gold conducts electricity and is a malleable metal
(a) Malleable means that a substance can be molded or formed into thin sheets by hammering. (b) Gold, like all metals conducts electricity as it contains delocalized electrons which are free to move. It is malleable as the close-packed layers of positive ions held in the crystal lattice can relatively easily slide over each other or be rearranged without deforming.
Pentane, C5H12, has 3 structural isomers (compounds with the same molecular formula but a different structural formula). They are pentane, 2-methybutane, and 2,2-dimethylpropane. Deduce, with an explanation, the correct order of their increasing boiling points
2,2-dimethylpropane (B.Pt. 9.5 o C) < 2-methylbutane (B.Pt. 27.9 o C) < pentane (B.Pt. 36.3 o C) The more spherical the molecule the less surface area there is to attract another molecule of the same type.
shape & angle of boron trichloride
BCl 3 contains 3 bonding electron pairs around the central boron atom so the shape will be trigonal planar with angles of 120º
Carbon dioxide is a linear molecule. It contains two carbon to oxygen double bonds at 180º to each other. Explain why the C=O bonds are polar and yet the molecule is non-polar
C=O bonds are polar due to the greater electronegativity of oxygen compared to carbon so the electrons in the C=O bond are drawn closer to the oxygen atom. The carbon dioxide molecule is non-polar as the resultant polarity of the two oppositely opposed dipoles from the two C=O bonds is zero.
calcium hydrogencarbonate
Ca(HCO3)2
Use info in sec 8 to explain why a carbon to chlorine bond is polar
Chlorine (3.2) is more electronegative than carbon (2.6). This means the shared pair of bonding electrons is closer to the chlorine atom resulting in a polar bond.
Explain why aluminum fluoride and aluminum oxide are ionic whereas aluminum chloride is covalent
Chlorine is less electronegative than oxygen or fluorine. The difference in electronegativity between aluminum and chlorine is therefore less than it is for aluminum and oxygen or fluorine and is not enough for electron transfer to occur so electrons are shared to form aluminum chloride.
Ethanol has a relative molar mass of 46 and boils at 78.5ºC. Water has a relative molar mass of 18 and boils at 100ºC under the same conditions. Both show hydrogen bonding. Explain why water has a higher boiling point
Because the oxygen atom in water has two hydrogen atoms bonded to it the polarity of the molecule is greater than ethanol where only one hydrogen atom is bonded directly to the oxygen atom. The dipole on the oxygen atom in water will effectively be 2δ - and will attract the δ + hydrogen atom of another water molecule more strongly.
Explain why there are 2 different carbon to oxygen bond lengths in a molecule of ethanoic acid
Ethanoic acid contains one C=O double bond and one C-O single bond. The double bond is shorter than the single bond.
shape & angle of hydrogen cyanide
HCN contains two negative charge centers around the central carbon atom so linear with bond angles of 180º
Explain why the carbon to carbon bond in ethyne, C2H2, is stronger and shorter than the carbon to carbon bond in ethene, C2H4
Ethyne contains a triple covalent bond whereas ethene contains a double covalent bond between the carbon atoms. The more electron pairs involved in the bonding the stronger and shorter the bond as the two nuclei are attracted more strongly together.
Fluorine and oxygen are very electronegative elements. Explain why hydrogen fluoride, HF, and water, H2O, are very polar molecules but CF4, and CO2 are nonpolar
HF is polar as the molecule only contains two atoms with different electronegativity values. H2O is polar as the molecule is bent and contains a dipole. CF4 is tetrahedral and CO2 is linear; in both cases the bond polarities cancel out to give a zero resultant dipole.
Explain why graphite, an allotrope of the non-metal carbon, si a good conductor of electricity whereas diamond which is also an allotrope of carbon is a very poor conductor of electricity
In diamond the four outer pairs of electrons around each carbon atom are strongly bonded to four other carbon atoms to give a giant tetrahedral shape so that all the electrons are localized and not free to move. In graphite three of the pairs of electrons are localized to form strong bonds with three other carbon atoms to give a planar layers of hexagonal rings. The other two electrons are delocalized and are free to move between the layers so making graphite a good conductor of electricity.
(a) a simplified model of benzene, C6H6, shows the 6 carbon atoms in a ring with alternate single and double bonds between the carbon atoms. Each carbon atom is also bonded to 1 hydrogen atom. Based on this model predict the C-C-C bond angle in benzene
In benzene each carbon atom has three negative charge centers (all bonding) so the bond angles will all be approximately 120º
(b) In cyclohexane, C6H12 the 6 carbon atoms are also in a ring but are joined to each other only by single bonds. Each carbon atom is also bonded to 2 hydrogen atoms. Predict the C-C-C bond angle in cyclohexane
In cyclohexane each carbon atom has four negative charge centers (all bonding) so the bond angles will all be approximately 109.5º
Use sodium chloride as an example to explain what is meant by ionic bonding
In the crystal structure of sodium chloride each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions to give a cubic structure. The ionic bond is the sum of all the attractive forces between oppositely charged ions and the sum of all the repulsive forces between similarly charged ions.
4 types of attractive forces in order of increasing strength: dipole-dipole interactions, hydrogen bonding, London (dispersions) forces, ionic bonding
London (dispersions) forces < dipole-dipole interactions < hydrogen bonding < ionic bonding
Explain why the melting points of the alkali metals decrease as the atomic number of the metal increases
The melting point of a metal depends upon the way the atoms are packed, the size of the cations and the charge carried by each cation. The alkali metals are all packed the same way and each cation carries a charge of +1. However as the atomic number increases the size of the cation also increases so the charge density of the ions is less. This means the attraction to electrons is less so less energy is required to break the attractive forces holding the crystal structure together.
shape & angle of phosphoryl chloride
POCl3 contains 4 bonding negative charge centres so tetrahedral with angles of approximately 109.5º. In fact the Cl-P- Cl bond angle is 103º
Explain why silicon dioxide has a high melting point (above 1600ºC) whereas carbon dioxide is a gas at room temp and pressure
Silicon dioxide has a giant molecular structure with strong covalent bonds resulting in a high melting point. Carbon dioxide contains individual molecules of carbon dioxide held together by weak intermolecular forces which are easily broken so it has a low melting point.
Explain why a proton in water, H+ (aq) is often written as H3O+ (aq)
The oxygen atom in water contains two non-bonding pairs of electrons. One of these forms a dative bond with the proton (which is extremely small as it has no electrons).
Suggest a reason why diagrams of organic structures containing a phenyl ring often show the phenyl ring as a hexagon with a circle inside rather than as a hexagon with alternate double and single bonds
The phenyl group is essential a substituted benzene ring. It can be shown as two resonance structures in which the double and single C to C bonds alternate. Since all six C to C bonds are of equal length and strength a circle in the phenyl ring illustrates this better than showing alternate double and single bonds which would be of different length and strength.
One Lewis structure for the carbonate ion shows one double C=O bond and two single C-O bonds. Explain why the bond lengths of all the carbon to oxygen bonds in the carbonate ion are equal
Three resonance structures for the carbonate ion can be drawn. Each contains one C=O double bond from the carbon atom to different oxygen atoms so that the average bond order for the carbon to oxygen bond is 4/3 and all the carbon to oxygen bonds are the same length and strength.
Tin, Sn, and lead, Pb, are both in group 14. Both are metals. Suggest one reason why lead, which has a higher atomic number than tin, has a higher melting point
34. The charge on tin and lead ions is the same (+2). Tin ions have a smaller atomic radius than lead ions but their ionic radii are very similar (Sn 2+ 1.18 x 10^-10 m; Pb 2+ 1.19 x 10^-10 m) Assuming the metal atoms pack the same way one would assume that lead would have a slightly lower melting point. The fact that lead has a higher melting point may suggest that the packing arrangement in their crystal structures is different.
aluminum phosphate
AlPO4
Explain why the H-N-H angle in ammonia is smaller than the H-N-H angle in the ammonium ion.
Ammonia contains one non-bonding pair of electrons around the central nitrogen atom. This exerts a greater repulsion than the three bonding pairs so the H-N- H bond angle will be less than 109.5º. In the ammonium ion the four bonding pairs of electrons around the central nitrogen atom give the ion a regular tetrahedral shape with a bond angle of 109.5º
Explain why graphite and diamond both have very high melting points (in the region of 4000ºC) and yet diamond is a poor electrical conductor whereas graphite is a good electrical conductor
Both diamond and graphite have giant molecular structures consisting of strong covalent bonds so they have high melting points. In diamond all the C to C bonds are equal and all the electrons are held in fixed positions arranged tetrahedrally around each carbon atom. In graphite three of the C to C bonds are fixed in a hexagonal shape with bonds angels of 120 o to give a flat layer. The layers are held together by weak bonds consisting of delocalized electrons. These delocalized electrons are free to move between the layers so that graphite is a good conductor of electricity.
Describe the structures of both diamond and silicon and suggest why silicon has a lower melting point than diamond
Both diamond and silicon have a giant tetrahedral structure but C-C bonds are stronger than Si-Si bonds so are easier to break
CHCl3 has a boiling point of 61.2ºC. Propanone, CH3COCH3, has a boiling point of 56.2ºC. Explain why a mixture of CHCl3 and propanone can have a higher boiling point (measured under the same atmospheric conditions) than either of the 2 pure components
Both trichloromethane and propanone are polar molecules and there will be dipole-dipole interactions between the molecules in the pure liquids. When they are mixed the δ + hydrogen atom in trichloromethane can form a type of hydrogen bond with the δ - oxygen atom in propanone resulting in a stronger intermolecular force - more energy will be required to break this attraction and hence the higher boiling point.
chromium (III) hydroxide
Cr(OH)3
caesium iodide
CsI
copper(I) nitrate
CuNO3
sodium carbonate
Na2CO3
shape & angle of phosphine
PH3 contains 3 bonding & 1 non-bonding electron pairs so trigonal planar with bond angles of approximately 107º
Explain why the bond between 2 oxygen atoms in oxygen gas is a double bond whereas the bond between 2 nitrogen atoms in nitrogen gas is a triple bond
Oxygen has the electron configuration 1s 2 2s 2 2p 4. When two oxygen atoms combine to form oxygen gas each atom needs to share two more electrons so the bond between the atoms is made from two shared pairs of electrons so that each atom attains the electron configuration a noble gas. Nitrogen has the electron configuration 1s 2 2s 2 2p 3. Each nitrogen atom needs to share three electrons with the other nitrogen atom to form a triple bond (three shared pairs of electrons) between the two atoms so that both achieve a noble gas electron configuration.
The oxygen to oxygen bonds in oxygen, O2(g) and ozone, O3(g) are both broken down by UV light in the atmosphere. Suggest why higher energy ultraviolet light is required to break the oxygen to oxygen bond in oxygen gas compared to the oxygen to oxygen bond in ozone gas
Ozone can be drawn as two resonance hybrids in which the bond order for the O to O bond is 3/2. This means that the O to O bond in ozone is weaker than the O=O double bond in oxygen so less energy is required to break it
The following 5 compounds have identical or very simliar molar masses. Deduce the increasing order of their boiling points (lowest first). ethanol: CH3CH2OH, propane: C3H8, ethanal: CH3CHO, methanoic acid: HCOOH, methoxymethane: CH3OCH3
Propane (London dispersion forces, B.Pt. -42.2 o C) < methoxymethane (weak dipole-dipole, B.Pt. -24.8 o C) < ethanal (dipole-dipole, B.Pt. 20.8 o C) < ethanol (hydrogen bonding, B.Pt. 78.5 o C) < methanoic acid (strong hydrogen bonding, B.Pt. 101 o C).
Explain why the oxide of sodium, Na2O, is ionic whereas the oxide of chlorine, Cl2O is covalent
Sodium is an electropositive metal on the left of the periodic table and forms an ionic bond with the highly electronegative non-metallic element oxygen which is on the right of the periodic table. Oxygen and chlorine are both non-metals on the right of the periodic table with similar electronegativity values so share electrons to form covalent bonds.
Explain why sulfur forms ionic compounds containing the S2- ion
Sulfur has the electron configuration 1s 2 2s 2 2p 6 3s 2 3p 4. By gaining two electrons to form the S2- ion it obtains a full outer energy level of electrons with an electron configuration similar to a noble gas (argon). It forms ionic compounds with positive ions.
Explain why sulfur dioxide molecules, SO2, have a bent shape whereas carbon dioxide molecules, CO2, are linear
The sulfur atom in SO2 contains three negative charge centers arranged to give a trigonal planar shape. The two bonding negative charge centers to the oxygen atoms give the molecule its bent shape with an angle of approximately 120º. In carbon dioxide there are only two negative charge centers (both bonding) around the central carbon atom so the molecule is linear.
Explain why water in a beaker heats up quickly in a microwave oven whereas when the same volume of CCl4 is placed in the beaker and the microwave switched on for the same length of time there is no increase in the temp. of the CCl4
Water is a polar molecule as it has a bent shape and polar O-H bonds. These are given energy by the oscillating microwaves as they continually try to line up with the field and so heat up. Tetrachloromethane is made up of non-polar molecule as the resultant polarity of the four symmetrical C-Cl bonds is zero. The molecules are thus unaffected by the microwave radiation.
