Uranium compounds

Key takeaways

Uranium compounds belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Uranium compounds to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Uranium compounds from memory before moving on to harder problems.

Reference excerpt

Uranium compounds are compounds formed by the element uranium (U). Although uranium is a radioactive actinide, its compounds are well studied due to its long half-life and its applications. It usually forms in the +4 and +6 oxidation states, although it can also form in other oxidation states.

Oxidation states and oxides

Oxides

Calcined uranium yellowcake, as produced in many large mills, contains a distribution of uranium oxidation species in various forms ranging from most oxidized to least oxidized. Particles with short residence times in a calciner will generally be less oxidized than those with long retention times or particles recovered in the stack scrubber. Uranium content is usually referenced to U3O8, which dates to the days of the Manhattan Project when U3O8 was used as an analytical chemistry reporting standard. Phase relationships in the uranium-oxygen system are complex. The most important oxidation states of uranium are uranium(IV) and uranium(VI), and their two corresponding oxides are, respectively, uranium dioxide (UO2) and uranium trioxide (UO3). Other uranium oxides such as uranium monoxide (UO), diuranium pentoxide (U2O5), and uranium peroxide (UO4·2H2O) also exist. The most common forms of uranium oxide are triuranium octoxide (U3O8) and UO2. Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octoxide is (depending on conditions) the most stable compound of uranium and is the form most commonly found in nature. Uranium dioxide is the form in which uranium is most commonly used as a nuclear reactor fuel. At ambient temperatures, UO2 will gradually convert to U3O8. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.

Aqueous chemistry

Salts of many oxidation states of uranium are water-soluble and may be studied in aqueous solutions. The most common ionic forms are U3+ (brown-red), U4+ (green), UO+2 (unstable), and UO2+2 (yellow), for U(III), U(IV), U(V), and U(VI), respectively. A few solid and semi-metallic compounds such as UO and US exist for the formal oxidation state uranium(II), but no simple ions are known to exist in solution for that state. Ions of U3+ liberate hydrogen from water and are therefore considered to be highly unstable. The UO2+2 ion represents the uranium(VI) state and is known to form compounds such as uranyl carbonate, uranyl chloride and uranyl sulfate. UO2+2 also forms complexes with various organic chelating agents, the most commonly encountered of which is uranyl acetate. Unlike the uranyl salts of uranium and polyatomic ion uranium-oxide cationic forms, the uranates, salts containing a polyatomic uranium-oxide anion, are generally not water-soluble.

Carbonates The interactions of carbonate anions with uranium(VI) cause the Pourbaix diagram to change greatly when the medium is changed from water to a carbonate containing solution. While the vast majority of carbonates are insoluble in water (students are often taught that all carbonates other than those of alkali metals are insoluble in water), uranium carbonates are often soluble in water. This is because a U(VI) cation is able to bind two terminal oxides and three or more carbonates to form anionic complexes.

Effects of pH When the pH is increased of a uranium(VI) solution in a non-complexing aqueous medium, the uranium is converted to a hydrated uranium oxide hydroxide and at high pHs it becomes an anionic hydroxide complex. When carbonate is added, uranium is converted to a series of carbonate complexes if the pH is increased. One effect of these reactions is increased solubility of uranium in the pH range 6 to 8, a fact that has a direct bearing on the long term stability of spent uranium dioxide nuclear fuels.

Hydrides, carbides and nitrides Uranium metal heated to 250 to 300 °C (482 to 572 °F) reacts with hydrogen to form uranium hydride. Even higher temperatures will reversibly remove the hydrogen. This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium carbide, nitride, and halide compounds. Two crystal modifications of uranium hydride exist: an α form that is obtained at low temperatures and a β form that is created when the formation temperature is above 250 °C. Uranium carbides and uranium nitrides are both relatively inert semimetallic compounds that are minimally soluble in acids, react with water, and can ignite in air to form U3O8. Carbides of uranium include uranium monocarbide (UC), uranium dicarbide (UC2), and diuranium tricarbide (U2C3). Both UC and UC2 are formed by adding carbon to molten uranium or by exposing the metal to carbon monoxide at high temperatures. Stable below 1800 °C, U2C3 is prepared by subjecting a heated mixture of UC and UC2 to mechanical stress. Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (UN2), and diuranium trinitride (U2N3).

Halides

All uranium fluorides are created using uranium tetrafluoride (UF4); UF4 itself is prepared by hydrofluorination of uranium dioxide. Reduction of UF4 with hydrogen at 1000 °C produces uranium trifluoride (UF3). Under the right conditions of temperature and pressure, the reaction of solid UF4 with gaseous uranium hexafluoride (UF6) can form the intermediate fluorides of U2F9, U4F17, and UF5. At room temperatures, UF6 has a high vapor pressure, making it useful in the gaseous diffusion process to separate the rare uranium-235 from the common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Uranium compounds

Start with the simplest possible case. Write down what Uranium compounds claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Uranium compounds before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Uranium compounds ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Uranium compounds

In research: Uranium compounds appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Uranium compounds in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Uranium compounds is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical compounds by element, Uranium, Uranium compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Uranium compounds outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Uranium compounds in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Uranium compounds means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Uranium compounds out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Uranium compounds in simple terms?

Uranium compounds are compounds formed by the element uranium (U). Although uranium is a radioactive actinide, its compounds are well studied due to its long half-life and its applications.

Why does Uranium compounds matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Uranium compounds?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Uranium compounds.