Ionic precipitation

The other silver(I) halides are traditionally prepared by ionic precipitation, on account of their insolubility ... 

Seven chemical reactions were identified from the chemistry syllabus. These chemical reactions were selected because they were frequently encountered during the 2-year chemistiy course and based on their importance in understanding concepts associated with three topics, namely, acids, bases and salts, metal reactivity series and inorganic chemistry qualitative analysis. The seven types of chemical reactions were combustion of reactive metals in air, chemical reactions between dilute acids and reactive metals, neutralisation reactions between strong acids and strong alkalis, neutralisation reactions between dilute acids and metal oxides, chemical reactions between dilute acids and metal carbonates, ionic precipitation reactions and metal ion displacement reactions. Although two of the chemical reactions involved oxidation and reduction, it was decided not to include the concept of redox in this study as students had only recently been introduced to ion-electron... 

A similar observation was made in the ionic precipitation of lead(ll) iodide. When aqueous solutions of potassium iodide and sodium iodide were separately added to aqueous leadfll) nitrate, 12% of students believed that the ionic equation for the precipitation reactions was different in the two instances even though the stoichiometry of the two chemical reactions had no influence on the ionic equation. 

Ionic precipitation process is accomplished by adding a reagent instead of water to a metal ion-containing solution, leading to the formation of a compound whose solubility is very low so that it readily precipitates. The added reagent functions by contributing anionic species. 

Ionic precipitation involving addition of reagent contributing to the anionic species, OH-ions, in the aqueous medium and interaction with the cationic species, the metal ions, to result in the formation of a compound which, on account of its poor solubility in the medium, precipitates rapidly and is generically alternatively embodied in the description on hydrolysis. 

Table 5.9 Comparison of various ionic precipitation processes for iron removal.

Covalent substances and ionic precipitates are written in full. 

Why are many ionic precipitates washed with electrolyte solution instead of pure water ... 

Heterogeneous nucleation of an ionic precipitate can be viewed as a sequence involving the diffusion of ions or ion pairs to a surface and their adsorption and surface diffusion to form a two-dimensional cluster or island. The critical nucleus so formed probably consists of relatively few ions. If the lattice spacings of the... 

Colloidal dispersions owe their stability to a surface charge and the resultant electrical repulsion of charged particles. This charge is acquired by adsorption of cations or anions on the surface. For example, an ionic precipitate placed in pure water will reach solubility equilibrium as determined by its solubility product, but the solid may not have the same attraction for both its ions. Solid silver iodide has greater attraction for iodide than for silver ions, so that the zero point of charge (the isoelectric point) corresponds to a silver ion concentration much greater than iodide, rather than to equal concentrations of the two ions. The isoelectric points of the three silver halides are ° silver chloride, pAg = 4, pCl = 5.7 silver bromide, pAg = 5.4, pBr = 6.9 silver iodide, pAg = 5.5, pi = 10.6. For barium sulfate the isoelectric point seems to be dependent on the source of the product and its de ee of perfection. ... 

The common-ion effect is responsible for the reduction in solubility of an ionic precipitate when a soluble compound combining one of the ions of the precipitate is added to the solution in equilibrium with the precipitate (see color plate 4]. 

Most H2S scavengers function on a surface adsorption manner or through ionic precipitation. If the scavenger being used is based on the surface adsorption technique, the mud must be in constant flow to assure that the additive and the sulfides actually collide with one another in order for the necessary reactions to take place. A turbulent flow type of situation would be ideal for this and would assure many random collisions of the two particles. When the scavenger is based on an ionic reaction, properties of the scavenger must be understood to assure that variables such as pH and salinity are conducive to the usage of the additive. 

Students may have some experience of ionic precipitation, acid-base, thermal decomposition and redox reactions from lower secondary science. If they have already learnt the topics Acids, bases and salts (covered here in Chapter 6) and Redox reactions ... 

Students will need to be familiar with the main reactions involved in inorganic chemical analysis. These are ionic precipitation, acid-base, thermal decomposition and redox reactions. It is hoped that the formation and colour of products in the activities below will capture students attention and interest and motivate them to learn the reactions involved. 

The following procedure outlines a common ionic precipitation reaction in which two soluble compormds react to form an insoluble compound (precipitate) and a soluble compound. 

Students may have learnt about ionic precipitation reactions and the formation and properties of ammonia in the topic on Acids, bases and salts . They also need to draw upon knowledge of amphoteric compounds (those that react with both acids and alkalis) to understand the further reactions of the insoluble hydroxides of zinc, aluminium and lead with excess sodium hydroxide solution to form soluble salts. The reactions of the insoluble hydroxides of copper and zinc with excess ammonia solution are similar in that soluble salts are also formed. 

A brown precipitate is formed when the two drops of sodium hydroxide or ammonia solution are added to the iron chloride solution. An ionic precipitation reaction has occurred forming insoluble iron(lll) hydroxide which does not react with excess sodium hydroxide or ammonia solution. Ammonia reacts with water to produce ammonium ions and hydroxide ions which react with the ions. The brown precipitate indicates the presence of iron(... 

A white precipitate is formed when the two drops of sodium hydroxide solution are added to the zinc chloride solution. An ionic precipitation reaction has occurred forming insoluble zinc hydroxide. 

Research has shown that students tend to regard ionic precipitation reactions involving sodium hydroxide solutions as displacement reactions. This is most likely because they learn that sodium is a reactive metal and will displace less reactive metal ions from their compounds, and because they do not differentiate clearly between metals and metal ions. Thus, you may want to compare the two reactions to clarify that an ionic precipitation reaction involves metal ions and not metals, and also that it does not involve a redox reaction ... 

The anions that are involved in secondary inorganic chemical analysis are, in general, carbonate, chloride, iodide, nitrate(V), sulfate(IV) and sulfate(VI). Students may be familiar with the carbonate ion from lower secondary science the reactions of the remaining anions have been introduced in the tests for gases or ionic precipitation (iodide). Silver nitrate(V) solution, barium chloride/nitrate(V) solution, lead(II) nitrate(V) solution and dilute acids are generally used to test for anions. 

Generally, tests for anions involve the formation of precipitates in ionic precipitation reactions with silver nitrate(V), barium nitrate(V)/chloride or lead(ll) nitrate and whether the anions react with acid if they do, a gas is usually evolved and has to be identified. Barium ions form precipitates with carbonate, sulfate(IV) and sulfate(VI) ions, while silver ions form precipitates with chloride, iodide, carbonate and sulfate(IV) ions silver sulfate(VI) is sparingly soluble and a precipitate may not be formed if the concentrations of the reagents used are low. Lead(ll) nitrate(V) is used to determine the presence of iodide and chloride ions. Lead(ll) iodide and lead(ll) chloride precipitates are soluble in hot water (when the mixture is heated) but will recrystallise when cooled (as discussed in section 9.1). The equations for some of these reactions are given overleaf. 

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