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7.5 Strengths of Ionic and Covalent Bonds
There is a double-headed arrow between Lewis structures.
One of the pioneers of resonance theory used a historical analogy to describe the relationship between resonance forms and resonance hybrid.
A medieval traveler described the rhinoceros as a hybrid of a dragon and a unicorn because it had many similarities to both.
A resonance hybrid, like a rhinoceros or a dragon, is neither a dragon or a unicorn at any given time.
It is a real entity that has been shown to exist.
It has some characteristics in common with its resonance forms, but they are not real.
A double bond to carbon is needed to complete the octet on the central atom.
The double bond could be formed from any of the three oxygen atoms.
There are three forms of the carbonate ion.
The average arrangement of electrons in the carbonate ion is what we know because we can write three identical resonance structures.
Experiments show that the C-O bonds are the same.
There are many examples to practice drawing resonance structures.
The bond between the two atoms needs to be broken.
In this section, you will learn about the bond strength of covalent bonds, and then compare that to the strength of ionic bonds, which are related to the lattice energy of a compound.
The atoms are held together by bonds.
The strength of a bond is measured by the amount of energy needed to break it.
Separating a pair of atoms requires energy.
The bond dissociation energy is the amount of energy required to break a specific bond.
There are two or more bonds in Molecules with three or more atoms.
The standard enthalpy change for the endothermic reaction that breaks bonds in the molecule is equal to the sum of bond energies in the molecule.
There are four moles of C-H bonds broken per mole of the reaction, so the average C-H bond energy is 1660/4.
The four C-H bonds do not require the same amount of energy to break as the original molecule, so the remaining bonds are easier to break.
The 415 kJ/mol value is not the exact value required to break a bond.
As the number of electron pairs in the bond increases, the strength of the bond increases.
The bond length decreases as the bond strength increases.
Triple bonds are stronger and shorter than double bonds between the same two atoms, and double bonds are stronger and shorter than single bonds between the same two atoms.
A comparison of bond lengths and bond strengths for some common bonds can be found in Table 7.3.
As we move down the group, the bond strength decreases.
C-F is 439 kJ/mol, C-Cl is 330 kJ/mol, and C-Br is 275 kJ/mol.
Chapter 7 does not include chemical bonding and geometry.
This type of calculation will tell us if a reaction is exothermic or endothermic.
The symbol D represents the bond energy in kilojoules per mole, which is always a positive number.
If the bond is a single, double, or triple bond, the bond energy can be obtained from a table.
It is important that we consider bonding in all reactants and products when calculating enthalpies.
This calculation provides a rough estimate, not an exact value, for the enthalpy of reaction because D values are typically averages for one type of bond.
The bond energy of the H-H bond is 436 kJ/mol and the bond energy of the Cl-Cl bond is 243 kJ/mol.
Two moles of H-Cl bonds are formed during the reaction, releasing 2 x 432 kJ.
The excess energy is released as heat.
The answer obtained earlier for the formation of two moles of HCl agrees with the second value of -184.6 kJ.
Methanol may be an excellent alternative fuel.
Methanol can be produced from a mixture of the gases carbon monoxide, CO, and hydrogen, H2, from the high-temperature reaction of steam and carbon.
The values are calculated using two different methods.
One of the first organic chemicals deliberately synthesised by humans was ethyl alcohol.
It is the alcohol in alcoholic beverages that has many uses.
An ionic compound is stable because of the attraction between its positive and negative ions.
The strength of the attraction is measured by the lattice energy of a compound.
The equivalent but opposite convention is used by some texts, defining lattice energy as the energy released when separate ion combine to form a lattice.
If you are looking at lattice energies in another reference, be sure to check which definition is being used.
A bigger lattice energy indicates a more stable compound.
It takes 769 kJ to separate one mole of solid NaCl into two different things.
769 kJ of heat is released when one mole of Na+ and NaCl form solid NaCl.
C is a constant that depends on the type of crystal structure, Z+ and Z- are the charges on the ion, and Ro is the interionic distance.
The lattice energy is doubled when all other parameters are constant.
The lattice energy of LiF is 1023 kJ/mol, while that of MgO is 3900 kJ/ mol.
lattice energies are produced by different interatomic distances.
We can compare the lattice energy of MgF2 (2957 kJ/mol) to that of MgI2 (2327 kJ/mol) to see the effect on the smaller ionic size of F-.
The gem is made of aluminum oxide, Al2O3.
The compound Al2Se3 is used in fabrication.
The charges Z+ and Z- are the same, so the difference in lattice energy will depend on Ro.
The Se2 ion is larger than the O2 ion.
The lattice energy of Al2O3 is larger than that of Al2Se3.
Zinc oxide is a very effective sunscreen.
The Z values of both the cation and the anion in ZnO are greater, and the interionic distance of ZnO is smaller.
It's not possible to measure lattice energies directly.
The equation given in the previous section can be used to calculate lattice energy.
Figure 7.13 shows the Born-Haber cycle.
The Born-Haber cycle shows the relative energies of each step in the formation of an ionic solid.
The energy required to break the F-F bond will be accounted for in the next step.
There are two mole of Cs cations and one mole of F anions.
Solid cesium fluoride can be produced by these ion pairs.
The negative of the lattice energy is what causes the enthalpy change in this step.
The change is exothermic.
The relationship between the enthalpies of the individual steps and the formation can be shown by the law.
This is shown in Table 7.4 for CsF.
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