Greenhouse effect A greenhouse heats up because the transparent roof material admits visible radiation from the sun. When that radiation is adsorbed by material in the greenhouse, some of the energy is re-irradiated but in a lower energy, longer wavelength form as infrared radiation. The roof is not transparent to infrared, and so this radiant (heat) energy is trapped in the greenhouse.

A similar thing happens to the planets Venus and Earth. Venus, closer to the sun than the Earth gets more sun irradiation and would be expected to be warmer. One calculation based on the balance of energy absorbed and energy re-irradiated predicts Venus temperature to be about 100 ^oC and Earth's temperature to be about -15 ^oC. Both planets are warmer than predicted (Venus at about 450 ^oC and Earth at about 15^oC), and this discrepancy is accounted for by a "greenhouse effect" in which gases in the atmosphere absorb infrared radiation and prevent it from escaping.

Note the model in Figure 3.2 which shows various processes that contribute to the overall balance of energy on the Earth.

Gases which absorb the IR radiation are called greenhouse gases. Some important green houses gases are:

CO₂ (0.03%, or near 300 ppm on Earth, 96% on Venus)

H₂O vapor (variable concentration)

CH₄ (1.7 ppm) methane, also called "marsh gas"

Others (reference Table 3.1, are N₂O, O₃, and freon 11 and freon 12, though these are probably less significant in overall energy balance).

H₂O has an opposite effect as well in cloud formation. Clouds cut down on the absorption of visible radiation, and have a cooling effect. The amount of water vapor and clouds, and the balance between them, is a very complex relationship to unravel, and this complexity probably contributes to some of the controversy over computer models that predict the future.

Temperature and CO₂ concentrations have fluctuated over thousands of years (Figure 3.1) and their changes parallel each other. CO₂ concentrations have been increasing in recent years (Figure 3.3) and the increase is attributed to increased use of fossil fuels and other human activity, as well as deforestation which slows down the removal of CO₂ by photosynthesis. The Carbon cycle (Figure 3.8) is very complex and involves a massive amount of CO₂. Also in recent years, global temperatures have been slowly increasing (Figure 3.4). Many scientists want to link this increase to the CO₂ increase and the effect of CO₂ as a greenhouse gas.

Here are some web sites to visit to see various studies on CO₂ and temperature changes which provide some of the background for this conclusion.

To understand these things a little better, you need to add two more chemical skills to your repertoire. Understanding molecular geometry, and understanding molecular mass and molar mass.

Molecular Geometry

Structural formulas only tell us the order in which atoms are connected in a structure. A geometrical formula is required to get a sense of the 3-dimensional distribution of the atoms in space. Fortunately, there is a very simple model, equivalent to the Lewis dot structure model for predicting structural formulas, that can give us insight into this question. It is called the valence shell electron pair repulsion (or VSEPR) model.

The VSEPR model simply states that pairs of valence electrons around a central atom tend to distribute themselves in space in a way to get furthest away from each other. For example:

This is referred to as the Electron Pair Geometry.

The Molecular Geometry will depend on whether all pairs are bonded to an atom, or whether there are lone pairs not bonded to an atom. Lets compare the three compounds methane (CH₄), ammonia (NH₃) and water (H₂O). All have four electron pairs around the central atom, but varying numbers of lone pairs. Note that in the molecular geometry, the electron pairs do not show up.

The Chime plug-in for your web browser gives you the ability to rotate and otherwise manipulate 3-D molecular structures. For some practice on these simple molecular structures, check the molecular geometry link on the course page. Practice trying to represent these 3-D structures in a 2-D drawing.

Molecular mass and molar mass

The Law of definite composition defines a compound as a pure substance composed of two or more elements in definite proportion by weight. In other words, no matter where you obtain it, pure water will always contain a fixed amount of oxygen (88.81%) and a fixed amount of hydrogen (11.19%). This observation is explained by Dalton’s atomic theory which says that water is composed of identical molecules, each of which contains the same number of hydrogen and oxygen atoms. Since each molecule contains a definite weight proportion of hydrogen and oxygen, a bulk sample of molecules will contain the same proportion. (We now know the formula for water to be H₂O. Dalton didn’t know that, assuming it was HO).

The relative weights of elements in compounds should therefore reflect the relative weights of the atoms of those elements. For example:

 H₂O: 88.81% O/11.19% H = 7.9 g O / 1.0 g H

CH₄: 74.87% C/25.13% H = 2.98 g C / 1.0 g H

CO: 42.88% C/57.12% O = 0.75 g C / 1.0 g O = 3.0 g C / 4.0 g O.

Show how these data give relative masses of H, C, and O atoms as follows: mass H: mass C: mass O = 1:12:16

Using such data on molecular composition, one can set up a scale of relative weights without needing to know the actual weight of an atom. The first such atomic weight scale used hydrogen as a reference, since it is the lightest atom. Later, oxygen, assigned a value of 16, was used as a reference because more elements form compounds of oxygen than they do of hydrogen. As measurements became more exact, and the presence of isotopes was recognized, with the relative weight varying if isotopic composition varied, finally a new standard was adopted. The ¹²C isotope of carbon is assigned a relative weight of 12 exactly, and all weights are given relative to this form of carbon. You will note that the atomic weight of carbon is listed as 12.011. That is because it contains a little bit of ¹³C and ¹⁴C in its natural state. The units for these relative weights are often given as atomic mass units (amu).

The first periodic tables listed elements by increasing relative atomic masses. If you look carefully at the atomic masses of all the elements, you will note that a couple would be out of order. Ultimately it was recognized that the atomic number was the important feature identifying an element—the atomic number specifying the positive charge on the nucleus and the number of protons in the nucleus.

For some practice in calculating relative masses from formulas, go to the interactive drill page.