CHE 106 Test #2 Lecture 9

Can a Nonelectrolyte dissolve in water? Can it be a good conductor?

-A nonelectrolyte may dissolve in water, but it does not dissociate into ions when it does so. This yields a nonconducting solution.

Weak Electrolyte

-A weak electrolyte is an electrolyte that dissolves in water to give a relatively small percentage of ions. -Most soluble molecular compounds are either nonelectrolytes or weak electrolytes. *Solutions of weak electrolytes contain only a small percentage of ions. We denote this by writing the equation with a DOUBLE ARROW.

Nonelectrolytes

-Some molecular compounds dissolve, but do not dissociate into ions. -They dissolve in water to give a nonconducting solution.

Electrolytes

-Those compounds that "freely" dissociate into independent ions in aqueous solution. -Their aqueous solutions are capable of conducting an electric current.

Strong Electrolyte

-is an electrolyte that exists in solution almost entirely as ions -Most ionic solids that dissolve in water do so almost completely as ions, so they are strong electrolytes.

Molecular Equation

-is one in which the reactants and products are written as if they were molecules, even though they may actually exist in solution as ions. -Note that Ca(OH)2, Na2CO3, and NaOH are all soluble compounds but CaCO3 is not

SUMMARY of writing net ionic equations

1-Write a balanced molecular equation. 2-Dissociate all strong electrolytes. 3-Cross out anything that remains unchanged from the left side to the right side of the equation. 4-Write the net ionic equation with the species that remain.

Step 2: (in checking if the reaction occurs on the ionic level)

After canceling the spectator ions, you obtain the net ionic equation.

Precipitation Reaction

A precipitation reaction occurs in aqueous solution because one product is insoluble.

If you have equal concentrations (e.g. 1 mole per liter of solution) of the following aqueous solutions, which will have the highest conductivity? a) CaCl2 b) KBr c) Na2SO4 d) Al(NO3)3 e) LiCH3OO

Al(NO3)3 (4 IONS) *more ions= higher conductivity

Describe water in terms of its partial charge and whether or not it's neutral?

Although water is an electrically neutral molecule, one end of the molecule (the O atom) is rich in electrons and has a partial negative charge, denoted by δ-.

Exchange Reaction

An exchange (or metathesis) reaction is a reaction between compounds that, when written as a molecular equation, appears to involve an exchange of cations and anions.

____ can be strong or weak. Almost all ____ that dissolve are strong electrolytes. Molecular substances that dissolve are either ____ or weak ____.

Electrolytes/ ionic substances/ nonelectrolytes/ weak electrolytes

How do we see if the reaction occurs on the ionic level?

To see the reaction that occurs on the ionic level, we must rewrite the molecular equation as an ionic equation.

Step 1: (in checking if the reaction occurs on the ionic level)

First write strong electrolytes (the soluble ionic compounds) in the form of ions to obtain the complete ionic equation.

Can Ionic Compounds be Electrolytes?

Many ionic compounds dissociate into independent ions when dissolved in water So YES

Metathesis

Metathesis comes from a Greek word that means "to transpose." It appears the ions in the reactant compounds exchange, or transpose, ions.

Types of Chemical Reactions

Most of the reactions we will study fall into one of the following categories: 1-Precipitation Reactions 2-Acid-Base Reactions 3-Oxidation-Reduction Reactions

Solubility

Substances vary widely in their solubility (ability to dissolve) in water. For example, NaCl is very soluble in water whereas calcium carbonate, CaCO3, is insoluble in water.

Are all electrolytes ionic compounds?

Not all electrolytes are ionic compounds. Some molecular compounds dissociate into ions. -The resulting solution is electrically conducting, and so we say that the molecular substance is an electrolyte.

Step 2: in telling if a precipitation reaction occurs, and if it does, what products to expect

Now that we have predicted potential products, we must balance the equation 3NiCl2+2Na3PO4--->Ni3(PO4)2+6NaCl

How do we predict that precipitate will form?

To predict whether a precipitate (insoluble solid compound) will form, we need to look for potential insoluble products. Table 4.1 lists solubility rules for ionic compounds. These rules apply to the most common ionic compounds.

Can molecular compounds be considered electrolytes? If so which ones?

Some molecular compounds, such as acids, also dissociate in aqueous solution and are considered electrolytes.

Solvent vs. Solute

The solvent is present in greatest abundance in a solution. All other substances are solutes.

What is a very effective solvent for Ionic Compounds?

Water

How do we predict that a reaction will occur specifically a precipitation reaction

We predict that a reaction occurs because nickel(II) phosphate is insoluble and precipitates from the reaction mixture.

Dissociation

When an ionic substance dissolves in water, the solvent pulls the individual ions from the crystal and solvates them.

Solutions

defined as homogeneous mixtures of two or more pure substances

Net Ionic Equation

is a chemical equation from which the spectator ions have been removed

Complete Ionic Equation

is a chemical equation in which strong electrolytes (such as soluble ionic compounds) are written as separate ions in solution.

Precipitate

is an insoluble solid compound formed during a chemical reaction in solution. For example, the reaction of sodium chloride with silver nitrate forms AgCl(s), an insoluble precipitate. (as shown in the above reaction)

Spectator

is an ion in an ionic equation that does not take part in the reaction. *remember in the chem lab when we mad alum out of aluminum and clay where in the balanced reaction only the aluminum and clay were important on the reactants side and alum was the only important part on the products side.

If the solution is __________, the circuit is incomplete and the bulb does not light. If the solution is __________, the circuit is complete and the bulb lights.

nonconducting/ conducting

Ionic Equation

represents strong electrolytes as separate independent ions. This is a more accurate representation of the way electrolytes behave in solution.

Predicting precipitation reactions Suppose you mix together solutions of nickel(II) chloride, NiCl2, and sodium phosphate, Na3PO4. NiCl2+Na3PO4---> How can you tell if a reaction will occur, and if it does, what products to expect?

step 1: Precipitation reactions have the form of an "exchange reaction." determine if this is true

Step 3: in telling if a precipitation reaction occurs, and if it does, what products to expect

step 3: We must then verify that NiCl2 and Na3PO4 are soluble and then check the solubilities of the products. Table 4.1 indicates that our reactants, nickel(II) chloride and sodium phosphate are both soluble. 3NiCl(aq)+ 2Na3PO4(aq)--->Ni3(PO4)2(s)+6NaCl(aq) Looking at the potential products, we find that nickel(II) phosphate is not soluble although sodium chloride is.

COMPLETE and NET IONIC EQUATIONS Let's try an example to write the net ionic equation of the reaction below. First, we start with a molecular equation. 2HNO3(aq)+Mg(OH)2(s)--->2H2O(l)+Mg(NO3)2(aq) Nitric acid, HNO3, and magnesium nitrate, Mg(NO3)2, are both strong electrolytes. -remember what strong electrolytes are compounds that dissolve that almost completely dissolves in water and exists in a solution almost entirely as ions.

to write the net ionic equation we have to remove the spectator ions. The spectator ion is an ion in an ionic equation that does not take part in the reaction After writing the molecular equation we then have to separate the strong electrolytes into separate ions, we obtain the complete ionic equation. Before 2HNO3(aq)+Mg(OH)2(s)--->2H2O(l)+Mg(NO3)2(aq) After (the image above) Note that the nitrate ions did not participate in the reaction. These are spectator ions. (continue on next card)

In summary, substances that dissolve in ____ are either electrolytes or nonelectrolytes. Nonelectrolytes form ____ solutions because they dissolve as molecules. Electrolytes form conducting solutions because they dissolve as ____.

water/ nonconducting/ ions