Question

Lecture 3. Intermolecular interactions

• Describe different types of van der Waals forces
• What properties of water are differing it from other common liquids?
• What is the cause of unique properties of water?
• What are water clusters and why are they called “flickering clusters”?
• What is water temperature anomaly?
• What is hydration of the first order and what is hydration of the second order?
• What are the changes of entropy in the process of conformational changes of organic macromolecules?
• What are the ways of classification of chemical reactions?
• What sort of changes are related with monomolecular reactions?
• Under what conditions can a bimolecular reaction become a first order reaction?
• What is activation energy of a chemical reaction?

Answer 1

*The three types of van der Waals forces

1) dispersion (weak),

2) dipole-dipole (medium),

3) hydrogen (strong). Ion-dipole bonds (ionic species to covalent molecules) are formed between ions and polar molecules.

The main characteristics of van der Waals forces are:

• They are weaker than normal covalent and ionic bonds.
• Van der Waals forces are additive and cannot be saturated.
• They have no directional characteristic.
• They are all short-range forces and hence only interactions between the nearest particles need to be considered (instead of all the particles). Van der Waals attraction is greater if the molecules are closer.
• Van der Waals forces are independent of temperature except dipole – dipole interactions

  What makes water different from other liquids?

  Water is not attracted to everything. Because water molecules are polar, they are more attracted to molecules that are also polar or that have a charge (like an ion). ... Because of density differences between water and oil, this means that they form two separate liquid layers

• , water has the highest surface tension for all liquids. Water's high surface tension is due to the hydrogen bonding in water molecules. Water also has an exceptionally high heat of vaporization

  What are the properties of water that make it unique?

  The main properties of water are its polarity, cohesion, adhesion, surface tension, high specific heat, and evaporative cooling.

• Polarity. A water molecule is slightly charged on both ends. ...
• Cohesion. Hydrogen bonds hold water molecules together, as seen in the picture above. ...
• Adhesion. ...
• High Specific Heat
• What are water clusters and why are they called “flickering clusters”?
• In chemistry a water cluster is a discrete hydrogen bonded assembly or cluster of molecules of water.^[1] These clusters have been found experimentally or predicted in silico in various forms of water; in ice, in crystal lattices and in bulk liquid water
• FLICKERING CLUSTERS:
• Flickering clusters are short-lived groups of water molecules that are interlinked by hydrogen bonds in liquid water. These clusters are representative of the fact that Hydrogen Bonds are easily broken and reformed
• What is water temperature anomaly?

  A temperature anomaly is the difference from an average, or baseline, temperature. ... A positive anomaly indicates the observed temperature was warmer than the baseline, while a negative anomaly indicates the observed temperature was cooler than the baseline. An anomaly is a departure from average conditions.

• Sea surface temperature is the temperature of the top millimeter of the ocean's surface. An anomaly is a departure from average conditions. These maps compare temperatures in a given month to the long-term average temperature of that month from 1985 through 1997.

• What is hydration of the first order and what is hydration of the second order?

  The water causes the hardening of concrete through a process called hydration. Hydration is a chemical reaction in which the major compounds in cement form chemical bonds with water molecules and become hydrates or hydration products.

  For the hydration of alkenes, the general chemical equation of the reaction is the following:

  RRCH=CH₂ in H₂O/acid → RRCH(-OH)-CH₃

  In the first step, the alkene acts as a nucleophile and attacks the proton, following Markovnikov's rule. In the second step an H₂O molecule bonds to the other, more highly substituted carbon. The oxygen atom at this point has three bonds and carries a positive charge (i.e., the molecule is an oxonium). Another water molecule comes along and takes up the extra proton. This reaction tends to yield many undesirable side products, (for example diethyl ether in the process of creating Ethanol) and in its simple form described here is not considered very useful for the production of alcohol.

  Two approaches are taken. Traditionally the alkene is treated with sulfuric acid to give alkyl sulfate esters. In the case of ethanol production, this step can be written:

  H₂SO₄ + C₂H₄ → C₂H₅-O-SO₃H

  Subsequently, this sulfate ester is hydrolyzed to regenerate sulfuric acid and release ethanol:

  C₂H₅-O-SO₃H + H₂O → H₂SO₄ + C₂H₅OH

  This two step route is called the "indirect process".

• What are the changes of entropy in the process of conformational changes of organic macromolecules?

• Conformational entropy is the entropy associated with the number of conformations of a molecule. The concept is most commonly applied to biological macromolecules such as proteins and RNA,The main contributions to entropy are the conformational entropy of the polypeptide chain itself and ordering of water molecules around hydrophobic side chains of the protein. To elucidate the role of conformational entropy upon thermal unfolding in more detail, conformational dynamics in the time regime of picoseconds was investigated with neutron spectroscopy. Confined internal structural fluctuations were analyzed for α-amylase in the folded and the unfolded state as a function of temperature. A strong difference in structural fluctuations between the folded and the unfolded state was observed at 30°C, which increased even more with rising temperatures. A simple analytical model was used to quantify the differences of the conformational space explored by the observed protein dynamics for the folded and unfolded state. Conformational entropy changes, calculated on the basis of the applied model, show a significant increase upon heating. In contrast to indirect estimates, which proposed a temperature independent conformational entropy change, the measurements presented here, demonstrated that the conformational entropy change increases with rising temperature and therefore contributes to thermal unfolding.

• What are the ways of classification of chemical reactions?
  Most chemical reactions can be classified into one or more of five basic types: acid–base reactions, exchange reactions, condensation reactions (and the reverse, cleavage reactions), and oxidation–reduction reactions. The general forms of these five kinds of reactions are summarized in Table

  Name of Reaction	General Form	Examples
  Oxidation–Reduction (redox)	oxidant + reductant → reduced oxidant + oxidized reductant	C7H16(l) + 11O2(g) → 7CO2(g) + 8H2O(g)
  Acid–Base	acid + base → salt	NaOH(aq) + HNO3(aq) → NaNO3(aq) +H2O(l)
  Exchange: Single Replacement	AB + C → AC + B	ZnCl2(aq)+ Mg(s) → MgCl2(aq)+ Zn(s)
  Exchange: Double Replacement	AB + CD → AD + CB	BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
  Combination (Synthesis)	A + B → AB	CO2(g) + H2O(l) → H2CO3(aq) N2(g) + 2O2(g)→ 2NO2(g)
  Decomposition	AB → A + B	CaCO3(s) → CaO(s) + CO2(g)

  What sort of changes are related with monomolecular reaction

• Monomolecular Reactions

  chemical reactions in which a single molecule undergoes transformation in the elementary event. Monomolecular reactions include the numerous decomposition reactions of complex molecules and isomerizations. For example, the decomposition of ethyl chloride

  C₂H₅Cl→C₂H₄ + HCl

  and the isomerization of methyl isonitrile to acetonitrile

  CH₃NC→CH₃CN

  are monomolecular reactions.

• Under what conditions can a bimolecular reaction become a first order reaction?
• bimolecular reaction can be kinetically first order in behaviour provided one of the reactants is taken in such a large excess that its concentration may hardly change. Such a reactant will not contribute to the order. Thus, a bimolecular reaction will be of first order

  Example:

  Hydrolysis of ethyl acetate (0.01 mol) with 10 mol of water:

  

  So in this reaction, the concentration of water is taken in excess and the bimolecular reaction behaves as first order reaction.

• What is activation energy of a chemical reaction?

• It is defined as the least possible amount of energy (minimum) which is required to start a reaction or the amount of energy available in a chemical system for a reaction to take place