U3 Lesson 5 Check for Understanding
IMFs Practice
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Evaluate the following statements as true or false, and explain your answer.
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Covalent bonds and IMFs have approximately the same strength. IMFs are much weaker than covalent bonds, so this is false.
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Energy is used to break covalent bonds when a substance changes phase (melts or boils). True, according to our notes, from solid to liquid, some IMFs are broken, and from liquid to gas, the remainder of them are broken, which requires energy. this is called the endothermic process.
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In each pair below:
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State the strongest IMF (LDF, Dip-Dip, H-bond) that must be broken to boil the substance
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Identify (circle or highlight if you print this page) the substance with the higher boiling point. (Hint: determine the polarity of the molecule to identify the IMF)
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NH3 vs. Ar
LDF
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CH4 vs. CH3Cl nonpolar - polar
nm nm - nm nm nm
LDF - dipole dipole
dipole dipole is stronger
higher boiling point would be CH3Cl, because it has a stronger bond.
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Rank the following substances in order of weakest to strongest IMF. Explain your reasoning in each case.(Hint: determine the polarity of the molecule to identify the IMF)
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H2O, CH4, H2S
nm nm - nm nm - nm nm
polar - nonpolar - polar
H bonding - LDF - dipole dipole
by weakest to strongest:
CH4, H2S, H2O
CH4 has london dispersion forces, which are the weakest of the intermolecular forces, so it is the weakest of the 3. H2S has dipole dipole forces, which are stronger than london dispersion forces, but weaker than H2O’s H bonding.
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HBr, CF4, Ar nm nm - nm nm - nm polar - nonpolar - nonpolar dipole dipole - LDF - LDF by weakest to strongest: Ar, CF4, HBr Ar has london dispersion forces, because it is a noble gas, and so it is the weakest. CF4 Also has london dispersion forces, so it is weaker than HBr, which has dipole dipole forces.
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Rank the following substances in order from lowest to highest melting point. Explain your reasoning in each case.(Hint: determine the polarity of the covalent compounds (molecules) to identify the IMF)(BEWARE: there could also be ionic compounds and metals present that have INTRA MOLECULAR FORCES)
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H2O, S8, Cr
nm nm - nm nm - metallic
polar - nonpolar
H bonding - LDF - metallic
Cr, H2O, S8
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Na, Cl2, NaCl
m - nm nm - nm m
- nonpolar
metallic - LDF - metallic
cl2, NaCl, Na
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The diagrams below represent the structures of iodine, sodium and sodium iodide.
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Identify which of the structures (A, B and C) correspond to iodine (I2), sodium (Na) & sodium
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State the type of chemical bond (i.e. intramolecular force) found in each structure.
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State the force of attraction in each substance that must be broken for the substance to melt.
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Rank them in order of increasing melting point.
 A |  B |  C | |
|---|---|---|---|
| a. Structure (I2, Na or NaI) | NaI | Na | I2 |
| b. Intramolecular Force | ionic | metallic | Covalent ( LDF ) |
| c. force of attraction to break (IMF) | highest Ionic | second highest Metallic | lowest LDF |
| d. Rank MP (1 low - 3 high) | 3 | 2 | 1 |
work: 