Question

The physical properties of \(\mathrm{D}_{2} \mathrm{O}\) differ from those of \(\mathrm{H}_{2} \mathrm{O}\) because (a) \(\mathrm{D}\) has a different electron configuration than O. (b) \(\mathrm{D}\) is radioactive. (c) \(\mathrm{D}\) forms stronger bonds with O than \(\mathrm{H}\) does.(\boldsymbol{d} ) D is much more massive than H .

Short answer

The physical properties of D2O differ from those of H2O because (c) D forms stronger bonds with O than H does. This is due to the heavier mass of deuterium, resulting in different vibrational frequencies of the bonds with oxygen, which in turn leads to the different physical properties observed between D2O and H2O.

Step-by-step solution

List the properties of D and H atoms

The exercise mentions four possibilities for the differences in physical properties between D2O and H2O. We will analyze each option one by one. (a) D has a different electron configuration than O: Both D and H have the same electron configuration, as D is effectively a hydrogen isotope. (b) D is radioactive: D is not radioactive. (c) D forms stronger bonds with O than H does: This statement is accurate, due to the higher mass of deuterium, which affects the bond strength with O. (d) D is much more massive than H: Deuterium is indeed heavier than hydrogen, but we need to assess whether it might directly lead to different properties of the compounds with oxygen.

Evaluate the impact of these differences on the properties of D2O and H2O

Now that we have listed the properties of D and H, let's evaluate how their differences affect the physical properties of their compounds with oxygen. (a) Since both D and H have the same electron configuration, it doesn't affect their interaction with O, thus this statement is not correct. (b) Radioactivity doesn't play a role in this context, as D is not radioactive. (c) Since deuterium is heavier than hydrogen, the vibrational frequencies of their bonds with oxygen are different. As a result, the \(\ce{O-D}\) bonds in D2O are stronger than \(\ce{O-H}\) bonds in H2O. That makes option (c) correct and is the key factor that causes different physical properties in D2O and H2O. (d) While it's true that D is heavier than H, the direct impact of that fact is reflected in option (c) being correct. So, listing the mass difference is not sufficient to explain the differences in physical properties between D2O and H2O.

Establish the correct explanation

After evaluating each option, the most accurate explanation for the differences in physical properties between D2O and H2O is: (c) D forms stronger bonds with O than H does. This is because the heavier mass of deuterium results in different vibrational frequencies of the bonds with oxygen, leading to the different physical properties observed between D2O and H2O.

Key concepts

Deuterium Isotope

Deuterium, symbolized as D or ²H, is a stable isotope of hydrogen that plays a fascinating role in altering the physical properties of water when it replaces the more common hydrogen isotope, ¹H. Deuterium possesses one proton and one neutron, doubling the mass of a regular hydrogen atom which has no neutrons. The presence of the additional neutron does not influence the electron configuration, as both deuterium and hydrogen have only one electron.

Despite being chemically similar, the increased mass of deuterium introduces intriguing effects when it forms compounds, such as heavy water (D₂O). This isotope effect is crucial in understanding why D₂O has different physical characteristics than standard water (H₂O).

Hydrogen-Oxygen Bond Strength

The strength of the bond between hydrogen and oxygen in water molecules significantly influences the compound's physical properties. In heavy water (D₂O), the bonds between deuterium (D) and oxygen (O) are indeed stronger compared to the hydrogen-oxygen bonds in regular water (H₂O). This enhanced strength is directly attributable to the greater mass of the deuterium atom.

The bond strength affects various physical aspects such as boiling and freezing points. Because of the stronger O-D bonds, D₂O has a higher boiling point and a higher freezing point than H₂O. This difference exemplifies how isotopes can significantly modify the physical characteristics of a given compound.

Isotope Effects on Physical Properties

When we talk about isotope effects on physical properties, we are referring to the changes that occur in a substance due to the presence of different isotopes. Since deuterium and hydrogen have vastly different masses, D₂O and H₂O exhibit different physical properties. The heavier mass of deuterium means that it forms stronger hydrogen-oxygen bonds, which translates into changes in the physical behavior of the resulting water molecule.

These changes can include alterations in melting and boiling points, density, viscosity, and even refractive index. For instance, heavy water is more viscous and denser than regular water, a fact that has practical implications in nuclear reactors, where D₂O is used as a neutron moderator due to its ability to slow down neutrons more efficiently than H₂O.

Vibrational Frequencies of Molecular Bonds

Molecular bonds are not static; they constantly vibrate, and these vibrations occur at certain frequencies which are determined by the masses of the atoms and the strength of the bond between them. In the context of heavy water, the O-D bonds have lower vibrational frequencies compared to the O-H bonds in regular water because of the increased mass of deuterium.

These variations in vibrational frequencies are significant because they affect the absorption and emission of infrared radiation, a property that can be measured to identify different compounds. The distinct vibrational frequencies of D₂O make it possible to distinguish it from regular water using spectroscopic techniques, highlighting the profound impact of isotopes on the behavior of molecules.