Net Ionic Equations Tutorial

Net Ionic Equations Tutorial
In a previous tutorial, you learned about double replacement reactions and how to predict what products will form. In this tutorial, you will learn that a reaction does not always occur when two solutions are mixed. You will also learn how to write what are called “net ionic equations” for the reactions that do occur. There are three possible reasons that a double replacement reaction might happen: The mixture of ions forms something insoluble. The mixture of ions forms a weak acid. The mixture of ion forms a compound that decomposes spontaneously.

First let’s learn more about solubility. These are the solubility rules: Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE These rules would enable you to predict, for example, that Mg(NO3)2 is soluble – since all NO31- compounds are soluble – no exceptions. On the other hand, CaS would be insoluble, since S2- compounds are insoluble. (There are exceptions, but Ca is not among them.) but Pb2+ is an exception Most Br1- compounds are soluble, , so that indicates that PbBr2 is insoluble.

Before we proceed, make sure you can apply the solubility rules. Determine whether each of the compounds below is soluble or insoluble: Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE BaSO4 (NH4)3PO4 SrCl2 K2CO3 Fe(OH)3 NaMnO4 Pb(C2H3O2)2 AgI Ca3(BO3)2 Insoluble (rule #3: SO4’s are soluble, but Ba is an exception.) Soluble (rule #1: NH4’s are soluble – no exceptions.) Soluble (rule #2: Cl’s are soluble, Sr is not an exception.) Soluble (rule #1: Group 1’s are soluble, no exceptions.) Insoluble (rule #4: OH’s are insoluble, Fe is not an exception.) Soluble (rule #1: C2H3O2’s are soluble, no exceptions.) Insoluble (rule #2: I’s are soluble, but Ag is an exception.) ???? (Rule #3 mentions Ca as an exception to the SO4 solubility, but that doesn’t necessarily apply to Ca3(BO3)2

But what does it mean that KF is soluble, but AgBr is insoluble? It means that when KF crystals are placed in water, they break apart and the ions distribute themselves throughout the sample of water: But an AgBr crystal does not: (at least not very much…) K+ F- K+ F- Ag+ Br- Ag+ Br- F- K+ F- K+ Br- Ag+ Br- Ag+ K+ F- K+ F- Ag+ Br- Ag+ Br-

Br- Ag+ Br- F- Ag+ K+ K+ F- F- K+ F- K+ Br- Ag+ Ag+ K+ Ag+ Br- K+ F- F- Br- Br- Ag+ Net Ionic Equations Tutorial A better way to think of this: if a bunch of dissolved K+ and F- ions are mixed together, nothing happens: they just stay dissolved – because KF is soluble. But, when a bunch of Ag+ & Br- ions are mixed together, most of them bond together into crystals, because AgBr is insoluble. Here is what that would look like: no reaction at all. Here is what that would look like: a definite chemical reaction. These tiny crystals reflect light and cause the mixture to turn instantly cloudy. This is known as a “precipitate.”

Now let’s look at the second reason a double replacement reaction might occur: the formation of a weak acid. An acid is a compound that has an H+ ion bonded to some negative ion: HNO3 for example is nitric acid. HF is hydrofluoric acid. All acids fall into one of two categories: strong acids and weak acids. A strong acid is one that dissociates 100% in water. That is, 100% of the molecules in solution are broken up into its ions. HNO3 breaks apart 100% into its ions: H+ and NO3-. that exist, only six are strong: HCl, HBr, HI, HNO3, HClO4, H2SO4. A weak acid is one that only dissociates this way to a small extent – usually less than 5%. All acids other than those six listed above are weak. For example, Of all the acids

But what does it mean that HNO3 is strong, but HF is weak? It means that when HNO3 molecules are dissolved in water, they break apart 100% like this: But when HF molecules are dissolved in water, only a relatively small percentage of them break apart into ions: H N O - + N O F - + H H F H + - + - + F N O N O H H H F F H H N O + - - F H H

- H + H + N O - + N O - H + H H + F - - F F - + + H N O - H N O - F - H + + + F - - H H F H + Net Ionic Equations Tutorial Again, starting with the ions: if a bunch of dissolved H+ and NO3- ions are mixed together , nothing happens: because HNO3 is a strong acid, these ions stay dissociated. But when H+ & F- ions are mixed together, most of them bond together into HF molecules, because HF is a weak acid. Even though it looks like nothing happened, this is a chemical reaction. Some- thing new was formed: HF molecules. These molecules are just too small to reflect light the way the insoluble precipitate did before. As before, this is what that would look like: no reaction at all.

Now: the third and final cause of a double replacement reaction: the formation of a compound that decomposes spontaneously. This one is fairly simple: there are three specific compounds that decompose spontaneously : H2CO3, H2SO3 and NH4OH. Each one decomposes into water (H2O) and a specific gas. Here are those three decomposition reactions: H2CO3  H2O(l) + CO2(g) H2SO3  H2O(l) + SO2(g) NH4OH  H2O(l) + NH3(g) When one of these specific compounds forms, you just have to recognize it as one that undergoes this type of decomposition. By the way, only the first one would show visible bubbling and the CO2 gas forms. The other two don’t show bubbling: the gases are given off at the surface of the liquid, but they do have very distinctive odors they produce: SO2 smells like burnt matches and NH3 is ammonia and has the pungent smell associated with Windex.

NO3- K+ Go Br-! Solubility RULES! Yay Ag+! You da best! So, how do we represent these reactions? These reactions are best represented with net ionic equations. Net ionic equations show just those ions that are involved in the reaction. For example, when AgNO3 solution and KBr solution are mixed together, the Ag+ ions react with the Br- ions to form the insoluble compound: AgBr The net ionic equation is simply: Ag+(aq) + Br-(aq)  AgBr(s) Notice how the NO3- and K+ ions are left out of the net ionic equation. Because they remained dissolved in the solution, they really don’t do anything. Such do-nothing ions are known as “spectator ions,” and they are left out of the net ionic equation. The “net” weight for a bag of chips refers to just the weight of the chips – not the bag, since the bag “doesn’t count.” In a similar way, the “net” ionic equation refers to just the ions that are doing something – not the spectator ions, since they don’t count.

So, let’s work through some problems to see how this works: Pb(NO3)2 + NaI  When we mix the positive ion from one compound with the negative ion from the other, we get what are sometimes referred to as “cross-products.” and vise-versa, Here the possible cross-products are Pb with I and Na with NO3. Let’s check whether either of these meets one of the three criteria: something insoluble, a weak acid, or something that decomposes. NaNO3 is not insoluble (by rule #1); it is not a weak acid (since it doesn’t start with H, it’s not an acid at all); and it is not a decomposer. If a reaction does happen here it will not include the Na+ or the NO3-. So let’s check the other cross-product: the Pb with I. We can see by rule #2 that Pb with I is insoluble, so we have found the two ions that react. Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

So, let’s work through some problems to see how this works: Pb(NO3)2 + NaI  Pb I When we mix the positive ion from one compound with the negative ion from the other, we get what are sometimes referred to as “cross-products.” and vise-versa, So let’s pull these two ions out of the problem with their charges. First the Pb. Pb sometimes has a charge of 2+ and sometimes 4+, but here we know its charge is 2+ since it took two NO3’s to balance it. Now the I. I always has a charge of 1-. And they both start off dissolved in water. They react to form the insoluble compound: PbI PbI must be charge balanced: And finally, let’s balance the entire equation: (s) for “solid” to show that the reaction has formed crystals that are no longer dissolved. 2+ 1-  PbI (s) 2 This is the completed net ionic equation: (aq) (aq) 2 Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

By the way, the order of the reactants doesn’t matter. Here’s another one: 2) K3PO4 + Ag2SO4  Is this what you came up with: 3 Ag1+(aq) + PO43-(aq)  Ag3PO4(s) This answer is just as correct. PO43- (aq) Ag1+(aq)  Ag3PO4(s) This time the possible cross-products are K with SO4 and Ag with PO4. Let’s check whether either of these meets one of the three criteria: K-SO4 is not insoluble (by rule #1); it is not a weak acid (no H); and it is not a decomposer. So we know K and SO4 are out of the running. We can see by rule #4 that Ag with PO4 is insoluble, so we have found the two ions that react. Try right now to write the balanced net ionic equation. Did you include charges on both ions? Did you balance charges? Did you include (s), (aq)…? And did you balance the overall equation? Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

Since H3BO3 is soluble, we keep this as “aq”. Here’s another one: 3) HNO3 + Li3BO3  Is this what you came up with: 3 H1+(aq) + BO33-(aq)  H3BO3(aq) This time the possible cross-products are H with BO3 and Li with NO3. Let’s check whether either of these meets one of the three criteria: H-BO3 is not insoluble (by rule #1); but it is a weak acid (notice how it starts with H and it’s not on the list below), so we can stop right there. We have found the two ions that react. H and BO3 Try writing the balanced net ionic equation for this reaction. (Hint: if you do not know the charge on BO3, look at the compound you are given above. The number of Li’s should give away the charge.) Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

Here’s another one: 4) (NH4)2S + KBr  NR NR stands for NO REACTION. When neither of the cross products meet any of the the three criteria, it is an “NR” When everyone is sitting around spectating, nothing gets done! This time the possible cross-products are NH4 with Br and K with S. NH4-Br is not insoluble (by rule #1); it is not a weak acid; and it is not a decomposer. So we know NH4 and Br are not involved in this reaction. How about K-S? K-S is not insoluble (again, by rule #1); ); it is not a weak acid; and it is not a decomposer. So it looks as though none of the ions is doing anything in this problem. So, what do we write for the balanced net ionic equation????? Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

Did you remember to include the ( ) around the OH ion? Here’s another one: 5) MgCl2 + NaOH  Is this what you came up with: Mg2+(aq) OH1-(aq)  Mg(OH)2(s) This time the possible cross-products are Mg with OH and Na with Cl. Let’s check whether either of these meets one of the three criteria: Mg-OH is insoluble (by rule #4); so we can stop there. Go ahead and write the balanced net ionic equation. Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

This is how we show a compound forming but then right away decomposing into something else Here’s another one: 6) KOH + NH4NO3  Is this what you came up with: NH41+ (aq) + OH1-(aq)  NH4OH  H2O(l) + NH3(g) This time the possible cross-products are K with NO3 and NH4 with OH. K-NO3 is not insoluble (by rule #1); it is not a weak acid (no H); and it is not a decomposer. So we know K and NO3 are sitting this one out. NH4-OH is not insoluble (by rule #1); it is not a weak acid (it doesn’t start with an H); and it is not … wait a second… IT IS A DECOMPOSER. Try right now to write the balanced net ionic equation. Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

This one was somewhat trickier to balance. Here’s another one: 8) K2CO3 + AlBr3  Is this what you came up with: 2 Al3+(aq) + 3 CO32-(aq)  Al2(CO3)3(s) This time the possible cross-products are K with Br and Al with CO3. K-Br is not insoluble (by rule #1); it is not a weak acid (no H); and it is not a decomposer. So K and Br are sitting this dance out. We can see by rule #4 that Al with CO3 is insoluble, so we have found the two ions that react. Try right now to write the balanced net ionic equation. Strong acids (weak acids are the ones that are not on this list) HCl HBr HI HNO3 HClO4 H2SO4 Group I1+, NH41+, NO31-, C2H3O21- Cl1-, Br1-, I1- SO42- (no exceptions) O2-, S2-, OH1-, CO32-, CrO42-, PO43- Pb2+, Ag1+, Hg22+ Pb2+, Hg22+ Ca2+, Sr2+, Ba2+ Group I1+, NH41+ 1) 2) 3) 4) except INSOLUBLE SOLUBLE Compounds that decompose H2CO3 H2SO3 NH4OH

Net Ionic Equations Tutorial

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