CHEM 1201 : Mostly Finished Final Review

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azuremammoth471

15 Mar 2019

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Vocabulary: electrolyte: a substance whose aqueous solutions contain ions, solvation: helps to stabilize the ions in a solution and prevent cations and anions from recombining, water is a very effective solvent for ionic compounds (aka. Electrolytes: be able to identify strong, weak and non-electrolytes, when an electrolyte is added to water it produces ions. Some electrolytes ionize completely (100% dissociation) producing a strong electrolyte. Weak electrolytes only ionize to a small extent so that just a (relatively) few of its molecules produce ions. When a compound dissolves and produces ions it will then allow an electric current to flow through the solution. A solution in which an electric current flows is called an electrolyte. The main difference between strong and weak electrolytes is the amount of electricity that is allowed to flow. It is the number of ions in solution that determines the amount of electricity that can flow through a solution: strong electrolyte.

Related Questions

1. Please write balanced net ionic equations for some of the ionic solids that formed in the precipitation lab. (Use the lowest possible coefficients. Omit states-of-matter from your answer. Enter "NONE" if no reaction occurred.)

2. Identify the ionic solids below as soluble or insoluble in water.

3. During lab, students mixed two solutions of soluble ions in a ceramic well to determine if a precipitate forms. Write the dissolution reaction for the ionic solids below. (Use the lowest possible coefficients. Include states-of-matter under the given conditions in your answer.)

The two solutions, when mixed, will have two cations and two anions.

c. Based on your lab results, enter the cation and anion for which a precipitate will form. (Separate substances in a list with a comma.)

d. Write the net precipitation reaction that occurs. (Use the lowest possible coefficients. Include states-of-matter under the given conditions in your answer.)

4. During lab, students mixed two solutions of soluble ions in a ceramic well to determine if a precipitate forms. Based on extrapolation of your lab results, write the dissolution reaction for the ionic solids below. (Use the lowest possible coefficients. Include states-of-matter under the given conditions in your answer.)

The two solutions, when mixed, will have two cations and two anions.

c. Based on your lab results, enter the cation and anion for which a precipitate will form. (Separate substances in a list with a comma.)

d. Write the net precipitation reaction that occurs. (Use the lowest possible coefficients. Include states-of-matter under the given conditions in your answer.)

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Qualitative Analysis of Group I Cations

Figure 1. Snack Mix

Are There any Cheese Flavored Snacks in the Mix?

You may have seen bowls of mixed snacks such as the one in Figure 1 at parties or in offices. Which one is your favorite? Some people prefer the salty pretzels, and some like the honey nuts, whereas others prefer the cheese flavored chips.

How do you tell if the bowl contains your favorite flavor snack? You will probably start by looking at the mix and see if you can spot it. If you do, then your search is done. You may however spot a similar snack but still be unsure about the flavor. You could then try to smell it to see if it smells familiar or go straight for the ultimate test: the taste test! Suppose you do not find the snack you are looking for, but you see another one that appeals to you. You will probably follow a similar pattern to identify the flavor of the snack. Similarly, qualitative analysis is used to check for the presence of ions in a mix or to identify unknown ions.

Qualitative Analysis of Ions

Qualitative analysis is used to separate and detect cations (positively charged ions) and anions (negatively charged ions) in a sample substance. The tests that are performed are not concerned with the amount of a substance in the sample as opposed to quantitative analysis, but only with a qualitative answer to the question of whether the substance is present in the sample or not.

The positively charged cations are divided into analytical groups. The groups are defined as sets of substances that can be precipitated with the addition of a single reagent. These groups are not to be confused with periodic system groups. Each cation group can be analyzed with a set of reactions to identify the presence or absence of each member of the group.

Group I Cation Separation and Identification

Qualitative analysis is made easier by the development of a standard scheme that can be followed step by step until all of the cations have either been detected or ruled out. For each group of cations, a flowchart of the scheme is prepared, and one can simply follow the steps and the decision branches in order. The flowchart for the group I cations is shown in Figure 2 below.

Figure 2. Flowchart for Separating and Identifying Group I Cations

Group I cations are silver (Ag+), lead (II) (Pb+2) and mercury (I) (Hg2+2). They can be precipitated from a solution as chlorides in a reaction with HCl. Among these three insoluble chlorides, lead chloride dissolves in hot water. Subsequently, to test whether the lead cation is present, potassium chromate (K2CrO4) is used. Lead cations react with chromate ions to form lead chromate (PbCrO4) as a yellow precipitate. Detection of the mercury (I) cation requires the addition of ammonia solution. This causes mercury (I) chloride to precipitate as the ammonium complex HgNH2Cl, and solubilizes silver chloride as an ammonium complex, [Ag(NH3)2]Cl. Last, the addition of nitric acid (HNO3) confirms the presence of silver ion as it precipitates as AgCl.

About This Lab

In this lab, you will test the identification reactions on individual samples of all three group I cations. You will then use your observations to detect the presence of the Group I cations in two unknown samples.

CHEM 1201 : Mostly Finished Final Review

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