Ionic precipitation reaction

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... 

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 ... 

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. 

A precipitation reaction occurs when two or more soluble species combine to form an insoluble product that we call a precipitate. The most common precipitation reaction is a metathesis reaction, in which two soluble ionic compounds exchange parts. When a solution of lead nitrate is added to a solution of potassium chloride, for example, a precipitate of lead chloride forms. We usually write the balanced reaction as a net ionic equation, in which only the precipitate and those ions involved in the reaction are included. Thus, the precipitation of PbCl2 is written as... 

Solid Compounds. The tripositive actinide ions resemble tripositive lanthanide ions in their precipitation reactions (13,14,17,20,22). Tetrapositive actinide ions are similar in this respect to Ce . Thus the duorides and oxalates are insoluble in acid solution, and the nitrates, sulfates, perchlorates, and sulfides are all soluble. The tetrapositive actinide ions form insoluble iodates and various substituted arsenates even in rather strongly acid solution. The MO2 actinide ions can be precipitated as the potassium salt from strong carbonate solutions. In solutions containing a high concentration of sodium and acetate ions, the actinide ions form the insoluble crystalline salt NaM02(02CCH2)3. The hydroxides of all four ionic types are insoluble ... 

Note that the brackets, [ ], refer to the concentration of the species. K,p is the solubility product constant hence [Cu " ] and [OH] are equal to the molar concentrations of copper and hydroxyl ions, respectively. The K p is commonly used in determining suitable precipitation reactions for removal of ionic species from solution. In the same example, the pH for removal of copper to any specified concentration can be determined by substituting the molar concentration into the following equation ... 

Write a net ionic equation for any precipitation reaction that occurs when solutions of the following ionic compounds are mixed. 

Although we have introduced net ionic equations to represent precipitation reactions, they have a much wider application. Indeed, we will use them for all kinds of reactions in water solution. In particular all of the chemical equations written throughout this chapter are net ionic equations. 

A complete ionic equation for a precipitation reaction shows all the dissolved ions explicitly. For example, the complete ionic equation for the silver chloride precipitation reaction shown in Fig. 1.5 is... 

Table 1.1 summarizes the solubility patterns of common ionic compounds in water. Notice that all nitrates and all common compounds of the Group 1 metals are soluble so they make useful starting solutions for precipitation reactions. Any spectator ions can be used, provided that they remain in solution and do not otherwise react. For example, Table 1.1 shows that mercury(I) iodide, Hg2I2, is insoluble. It is formed as a precipitate when solutions containing Hg22+ ions and I ions are mixed ... 

How would you use the solubility rules in Table 1.1 to separate the following pairs of ions In each case indicate what reagent you would add and write the net ionic equation for the precipitation reaction (a) lead(II) and copper(II) ions ... 

We saw in Section I that the net chemical change in a precipitation reaction is clarified by writing its net ionic equation. The same is true of neutralization reactions. First, we write the complete ionic equation for the neutralization reaction between nitric acid and barium hydroxide in water ... 

K.20 Classify each of the following reactions as precipitation, acid-base neutralization, or redox. If a precipitation reaction, write a net ionic equation if a neutralization reaction, identify the acid and the base if a redox reaction, identify the oxidizing agent and the reducing agent. 

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. 

To understand precipitation reactions, it is essential to work with the ionic species that exist in aqueous solution. For instance, mixing colorless solutions of lead(II) nitrate and potassium iodide causes a brilliant yellow solid to precipitate from the mixture (Figure 4-4t. To identity this yellow solid, we must examine the chemical species present in the solutions. 

The simplest balanced chemical equation for a precipitation reaction is a net ionic equation that has ions as the reactants and a neutral solid as the product. In a precipitation reaction, reactant ions combine to form a neutral ionic solid. One reactant carries positive charge and the other carries negative charge, but the product is electrically neutral. Because electrical charge always is conserved, the total positive charge of the reacting cations... 

Another example of a precipitation reaction is the process that occurs when we mix aqueous solutions of potassium hydroxide (KOH) and iron(III) chloride (FeCl3). A precipitate forms. A list of species present helps us to determine the net ionic equation describing this process ... 

A net ionic equation contains only those species that participate in a chemical reaction. Notice that neither K nor Cr appears in the equation for the precipitation of Fe (OH). Similarly, neither K nor NO3 appears in the equation for the precipitation of Pbl2. Although these other ions are present in the solution, they undergo no change during the precipitation reaction. Ions that are not involved in the chemical change are referred to as spectator ions. Spectator ions are omitted from net ionic equations. 

C04-0086. Write the baianced net ionic equation for each of these precipitation reactions. Also, identify the spectator ions. 

C04-0121. Lithopone, a brilliant white pigment used in paints, paper, and white rubber products, is a mixture of two insoluble Ionic solids, ZnS and BaSOq. Suggest how 1.0 kg of lithopone could be prepared by a precipitation reaction. 

These ionic equations summarise the following types of reaction (a) the precipitation reaction between aqueous silver and aqueous chloride ions (b) the redox reaction between zinc metal and dilute acid (c) the neutralisation reaction between an acid and an alkali. 

In this the determining factor mainly rests on the solubility product of the resulting nearly insoluble material generated in the course of a precipitation reaction and its ionic concentration at the equivalence point. It is, however, pertinent to mention here that the indicator electrode must readily come into equilibrium with one of the ions. 

In the previous Sample Problems, you saw that combining solutions of soluble ionic compounds can cause the precipitation of a slightly soluble ionic compound. Analytical chemists use precipitation reactions to remove ions from solution or to identify ions in an unknown solution. 

The scheme that is shown in Figure 9.9 is very simple. More complex qualitative analyses involve many more steps of isolation and identification, including some steps that are not precipitation reactions. For example, some ions, such as sodium, Na", and potassium, K, cannot he precipitated out of an aqueous solution, because the ionic compounds that contain them are always soluble. Instead, chemists identify these ions using a flame test. In the following ThoughtLab, you will simulate a qualitative analysis that includes a flame test. 

In Chapter 9, as in most of Unit 4, you learned about equilibrium reactions. In this section, you analyzed precipitation reactions. You mainly examined double-displacement reactions—reactions in which two soluble ionic compounds react to form a precipitate. You used the solubility product constant, Ksp, to predict whether or not a precipitate would form for given concentrations of ions. In Unit 5, you will learn about a class of reactions that will probably be new to you. You will see how these reactions interconvert chemical and electrical energy. 

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