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3 Unit How do we determine structure? Module 3: Inferring Charge Distribution Chemistry XXI Central goal: To infer electron charge distribution in a molecule based on both differences of electronegativity between bonded atoms and molecular geometry. The Challenge Modeling How do I explain it? The next step in the path to prediction of the properties of molecular compound involves the analysis of the actual distribution of valence electrons among different atoms. Composition Connectivity Geometry Charge Distribution Chemistry XXI How can we identify regions of high or low electron density in a molecule? How can we use charge distribution to predict how a molecule will interact with light or other molecules? High electron density Low electron density Explaining Attractions How would you explain this behavior? Let s think Chemistry XXI Bond Polarity The uneven charge distribution results from different types of atoms in a bond attracting electrons with different strength. For example, in the HCl molecule: Electrons in the bond spend more time closer to the Cl atom. Chemistry XXI H Cl We say that Cl is more electronegative than H, and that the bond is polar. Electronegativity Electronegativity, , is a measure of the ability of an atom in a bond to attract electrons to itself. Chemistry XXI General Trend: increases up a group and down a period in the periodic table Dipole Moment Differences in electronegativities between atoms in a bond lead to an unequal distribution of charge. (Bond polarity) Dipole moment The existence of + and partial charges creates a dipole moment in the bond. Chemistry XXI The dipole moment, , of the bond is represented by an arrow going from the + to the partial charge. The larger the difference of electronegativities , the larger and the more polar the bond. Let s Think Arrange the following molecules in order of increasing bond polarity. Electronegativity Chemistry XXI H 2.1 C 2.5 N 3.0 O 3.5 F 4.0 Cl 3.0 Molecular Polarity Determining how charge is distributed in a molecule is of central importance to make predictions about how it will interact with light and other molecules. However, we need to explore both bond polarity AND overall molecular polarity. For example, the CO2 molecule: Chemistry XXI + But the charge is symmetrically distributed and the individual dipole moments cancel out. has polar bonds Molecular Polarity Bond dipole moments add like vectors. The net dipole moment of the CO2 molecule is zero. We say that the molecule is non polar, although it has polar bonds. What about the H2O molecule? Chemistry XXI O H + H The water molecule has a net dipole moment and, thus, it is polar. Symmetry In many cases, molecular polarity can be inferred by analyzing the symmetry of the charge distribution. For a molecule to be non polar, the charge distribution has to be fully symmetrical. CH4 Methane Benzene C6H6 Chemistry XXI Acetylene C2H2 Dimethyl ether C2H6O ? Let s Think Are these molecules polar or non polar? Molecule Polarity Molecule Polarity F2 O3 Chemistry XXI HBr CCl4 NH3 CHCl3 Let s apply! Chemistry XXI Assess what you know Light-Matter Interactions Let s apply what we have learned to analyze an interesting case that illustrates the power of the ideas we have discussed in explaining and predicting the properties of relevant systems. Chemistry XXI Global warming is associated with the presence of gases in the atmosphere that can absorb IR radiation. Why does this happen? Global Warming A Natural Phenomenon Earth s atmosphere normally traps 84% of the solar energy re-emitted by the planet s surface. This phenomenon is called greenhouse effect and helps to keep an average temperature of 15 oC or 59 oF. Chemistry XXI Without this effect, Earth would 60 oF cooler. The temperature in Tucson right now would be around 10 oF. Greenhouse Effect Step 2 3 1 Some of the energy is absorbed by gases in the Earth receives energy from the sun (radiation), Earth absorbs the solar the form of atmosphere andit in the form of visibleand infrared re-emitted (in energy light. most of releases it as infrared radiation. radiation) back to the Earth s surface Visible light Chemistry XXI Infrared light Let s apply! Greenhouse Gases Abundance The following substances are considered greenhouse gases because they absorb and reemit IR radiation: H2O CO2 CH4 N2O O3 CF2Cl2 Chemistry XXI These other atmospheric components are not greenhouse gases: N2 O2 Ar What may be the reason for the different properties in terms of IR absorption? Hint: Analyze bond and molecular polarity. Energy Transfer Chemistry XXI Let s Explore Let s explore the contributions of different gases to the Greenhouse Effect: Go to: http://www.chem.arizona.edu/chemt/C21/sim Greenhouse Gases Analyze the IR spectrum of the different individual gases. Then, explore their relative contributions to the GH effect by analyzing the scale IR spectra. Arrange gases from lower to higher contribution to the Greenhouse Effect. What factors may determine these contributions? Chemistry XXI Chemistry XXI Contributors Absorption Patterns Contributions to GH Chemistry XXI H2O 36-70% CO2 9-26% O3 3-7% CH4 4-9% Let s apply! Global Warming Global warming refers to the measured increase in the average temperature of the Earth's nearsurface air and oceans since the mid 20th century, and its projected continuation. Chemistry XXI What specific trends do you observe? How do you explain them? Relative Effects w a r m i n g c o o l i n g Chemistry XXI CO2 Correlation Chemistry XXI Recent rapid increase mostly associated with burning of carbon-based fuels. World by Carbon Emissions Species Loses Severe storms Drought Chemistry XXI Flooding Changing Forests Lon term effects

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