Question

Fluorine reacts with hydrogen (H) and deuterium (D) to form hydrogen fluoride (HF) and deuterium fluoride (DF), where deuterium \({ }_{1}^{2} \mathrm{H}\) ) is an isotope of hydrogen. Would a given amount of fluorine react with different masses of the two hydrogen isotopes? Does this violate the law of definite proportion? Explain.

Short answer

Fluorine reacts with different masses of H and D. This does not violate the law of definite proportion as HF and DF are distinct compounds.

Step-by-step solution

Understanding the Law of Definite Proportion

The law of definite proportion states that a chemical compound always contains the same elements in exactly the same proportion by mass, regardless of the size or the source of the sample. This means that the ratio of the masses of the elements in a compound is constant.

Identifying Elements in the Reaction

Fluorine reacts with two isotopes: hydrogen (H) and deuterium (D). Both reactions form similar compounds: hydrogen fluoride (HF) and deuterium fluoride (DF), respectively.

Comparing Atomic Masses of Isotopes

Hydrogen ( _{1}^{1}H ) has an atomic mass of approximately 1 amu, while deuterium ( _{1}^{2}H ) has an atomic mass of approximately 2 amu. Therefore, there is a difference in the mass of the isotopes.

Calculating Mass Ratios in Compounds

In HF, the mass ratio of H to F is 1:19 (since the atomic mass of fluorine is about 19 amu). In DF, the mass ratio of D to F is 2:19 because deuterium has twice the mass of hydrogen. Thus, the mass ratio varies between HF and DF.

Evaluating the Law of Definite Proportion

Because the mass ratios of the hydrogen isotopes (H and D) to fluorine differ (1:19 vs. 2:19), each isotope reacts with fluorine in a different mass proportion, which seems to contradict the law of definite proportions. However, since HF and DF are different compounds (with different isotopes), this situation does not actually violate the law.

Key concepts

Isotopes

Isotopes are variants of the same chemical element that have the same number of protons but different numbers of neutrons. This means that while they share the same atomic number, their atomic masses differ. For example, hydrogen typically has one proton and no neutrons ( _{1}^{1}H (protium)), while its isotope, deuterium ( _{1}^{2}H), has one neutron in addition to the proton.

The presence of additional neutrons affects the atomic mass, making deuterium heavier than regular hydrogen. Isotopes play an essential role in chemical reactions because they can form different compounds with distinct properties, such as hydrogen fluoride (HF) with hydrogen and deuterium fluoride (DF) with deuterium.

This difference in atomic mass leads to varying mass ratios when these isotopes combine with other elements in chemical reactions.

Chemical Compounds

Chemical compounds are substances formed when two or more elements chemically bond together in fixed proportions by mass. This bond results in a compound with properties different from those of the individual elements.

For instance, when fluorine bonds with hydrogen, the compound formed is hydrogen fluoride (HF). When fluorine bonds with deuterium, it forms deuterium fluoride (DF). Despite both being fluorides, these are distinct compounds due to the differences in the isotopes involved.

Chemical compounds are subject to the Law of Definite Proportions, which states that a given compound will always contain the exact ratio of elements by mass, regardless of the amount or source of the compound. This principle ensures that every molecule of hydrogen fluoride, for example, will have the same hydrogen to fluorine mass ratio.

Atomic Mass

Atomic mass is a fundamental concept that refers to the weighted average mass of an atom's nucleus, which is primarily due to the combined mass of its protons and neutrons. It is expressed in units called atomic mass units (amu).

For hydrogen ( _{1}^{1}H), the atomic mass is roughly 1 amu, as it mainly consists of a single proton. In comparison, deuterium ( _{1}^{2}H) has an atomic mass of approximately 2 amu due to the presence of an additional neutron.

This difference in atomic mass affects how isotopes like hydrogen and deuterium participate in chemical reactions and combine into compounds. For example, the atomic mass difference results in hydrogen fluoride (HF) having a different mass ratio compared to deuterium fluoride (DF), highlighting the importance of atomic mass in chemical reactions and compound formation.

Mass Ratio

The mass ratio in a chemical compound is the ratio of the masses of its constituent elements. This ratio is significant because it dictates the proportions in which elements combine to form compounds according to the Law of Definite Proportions.

In hydrogen fluoride (HF), the mass ratio of hydrogen to fluorine is 1:19, given that hydrogen has a mass of 1 amu and fluorine has a mass of 19 amu. Conversely, in deuterium fluoride (DF), the mass ratio is 2:19, since deuterium has a mass of 2 amu.

These differing mass ratios are a direct consequence of using different hydrogen isotopes in forming HF and DF. Despite this difference, it does not violate the Law of Definite Proportions because HF and DF are distinct compounds, each with its own specific mass ratio that remains constant.