Question

In comparison to water, heavy water: (a) Has lower density (b) Has lower boiling point (c) Has lower dissociation constant (d) In general, reacts faster.

Short answer

(c) Heavy water has a lower dissociation constant than water.

Step-by-step solution

Understand Heavy Water

Heavy water (O or D2O) is similar to regular water, except that the hydrogen atoms are replaced with deuterium, an isotope of hydrogen with one proton and one neutron.

Analyze Density

Heavy water is denser than normal water because deuterium has greater mass than regular hydrogen. Therefore, heavy water does not have a lower density compared to water.

Consider Boiling Points

Heavy water has a higher boiling point than regular water due to stronger hydrogen bonding from the more massive deuterium atoms. Thus, it does not have a lower boiling point.

Assess Dissociation Constant

The dissociation constant of heavy water is lower than that of normal water. This is because strong hydrogen bonds in heavy water make it less likely to dissociate.

Evaluate Reaction Rates

Heavy water generally reacts slower than ordinary water. The presence of deuterium makes it less reactive due to stronger bonds.

Determine Which Statement is Correct

After analyzing the properties, the correct statement is that heavy water has a lower dissociation constant compared to ordinary water.

Key concepts

Density of Heavy Water

Heavy water, known scientifically as deuterium oxide (O or D2O), substitutes ordinary hydrogen with the isotope deuterium, which has one neutron in addition to the single proton found in regular hydrogen. This substitution makes deuterium significantly heavier than ordinary hydrogen, resulting in an increased mass for each water molecule. As a result, heavy water exhibits a higher density than regular water. To put it simply:

• Regular water (H2O) has a density of about 1.0 g/cm³.
• Heavy water (D2O) has a density of approximately 1.11 g/cm³.

This means heavy water sinks in regular water, much like a denser object sinks in a less dense liquid.
Density is an important property that influences how substances float or sink when mixed together, and understanding this helps in differentiating heavy water from its normal counterpart.

Boiling Point of Heavy Water

The boiling point of a substance is where it changes from a liquid to a gas. In the case of heavy water, the presence of deuterium creates stronger hydrogen bonds compared to regular water.
Key Points:

• Normal water boils at around 100°C.
• Heavy water boils at a slightly higher temperature of about 101.4°C.

This increase in boiling point is due to the heavier mass of deuterium, which strengthens the bonds between the molecules, thus requiring more energy (in the form of heat) to break these bonds and transition to a gaseous state. Understanding boiling point differences is crucial when studying thermal properties and the behavior of substances under varying temperatures.

Dissociation Constant

The dissociation constant is a measure of how easily a molecule breaks into ions. In water, this relates to how H2O splits into H+ and OH- ions. For heavy water, the dissociation constant is lower compared to regular water.

• This means heavy water is less willing to break apart into ions compared to H2O.
• Due to the presence of deuterium, the bond in D2O is stronger, making dissociation less likely.

The lower dissociation constant indicates that heavy water behaves differently in reactions that involve ion formation. This property is significant in fields such as chemistry and physics because it affects reaction mechanisms and dynamics.

Reaction Rate of Heavy Water

Reaction rates describe how quickly or slowly a chemical process occurs. For heavy water, reactions generally proceed at a slower pace than those involving regular water. This slower reaction rate can be attributed to the stronger hydrogen bonds formed with deuterium.

• Slower reaction rates are linked to the additional mass and resulting bond strength in D2O.
• This makes it less reactive, particularly in processes where bond breaking is a crucial step.

Understanding reaction rates is essential in predicting how heavy water interacts in biological and chemical systems. This knowledge is especially important in scenarios where temperature or reaction speed could influence outcomes, such as in nuclear reactors where heavy water is often used as a neutron moderator.