CR_Double Displacement Lesson

Double Displacement

How Does Science Work?

Recall that in a double displacement reaction, two compounds react by "switching partners." There are several specific types that were mentioned earlier:

  • When one of the products formed comes out of solution, in the form of a solid, it is called a precipitation reaction.

LaTeX: {Ex. NaCl_(aq)+AGNO_{3(aq)} \rightarrow NaNO_{3(aq)} + AgCl_(s)}Ex.NaCl(aq)+AGNO3(aq)→NaNO3(aq)+AgCl(s)

  • When one of the products formed is a gas, this can be called a gas formation reaction.

LaTeX: Ex. HCl_(aq)+FeS_(s) \rightarrow FeCl_{2(aq)}+H_2S_(g)Ex.HCl(aq)+FeS(s)→FeCl2(aq)+H2S(g)

  • When one reactant is an acid and one is a base, this is called an acid-base or neutralization reaction.

LaTeX: {Ex.HCl_(aq)+NaOH_(aq)\rightarrow NaCl_(aq)+H_2O_(l)}\\ \,\,\,\,\,\,\,\,\,\,\,\, acid \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, baseEx.HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)acidbase

Precipitation Reactions

The formation of a precipitate is a driving force for a chemical reaction. A precipitate is an insoluble solid that is formed when two aqueous solutions are mixed. In the examples we have done so far, you have been told the state of matter for each chemical. This won't always be the case. So, how are you going to know if a chemical is soluble (aqueous) or if it forms a precipitate (solid)? You must learn some solubility rules!  

Solubility Rules 1. All common compounds of Group I and ammonium ions are soluble. 2. All nitrates, acetates, and chlorates are soluble. 3. All binary compounds of the halogens (other than F) with metals are soluble, except those of Ag, Hg(1), and Pb. Pb halides are soluble in hot water.) 4. All sulfates are soluble, except those of barium, strontium, calcium, lead, silver, and mercury (1). The latter three are slightly soluble. 5. Except for rule 1, carbonates, hydroxides, oxides, silicates, and phosphates are insoluble. 6. Sulfides are insoluble except for calcium, barium, strontium, magnesium, sodium, potassium, and ammonium.

The good news is that you don't need to memorize this entire list. You only need to memorize the fact that that all sodium, potassium, ammonium, and nitrate salts are soluble in water.  

Strong Acids

Strong Bases

HCl - hydrochloric acid

Hydroxides of group IA and IIA

HBr - hydrobromic acid

 

HI - hydroiotic acid

 

HNO3 - nitric acid

 

H2SO4 - sulfuric acid

 

HClO4 - perchloric acid

 

HClO3 - chloric aci

 

You will need to be able to apply the other rules using the chart. If there is ever a conflict when applying the rules, the lower rule number takes precedence over a higher number rule.

Gas Forming Reactions

Another driving force in a chemical reaction is the production of a gas. Some gases, like carbon dioxide, are familiar to you already. Other gases will require memorization so that you recognize them as such.

Some common gases at room temperature are:

CO2, SO2, SO3, H2S, NO2, NH3, O2, H2

Not only do you need to know these gases, you need to also be familiar with some specific ways they are produced.

  • Carbonic acid, H2CO3, is too unstable to remain in this form. Instead, it decomposes into H2O(l) and CO2(g). Any time you see H2CO3 as a product, you should cross it out and replace it with water and carbon dioxide.

LaTeX: H_2CO_3 \rightarrow H_2O_(l)+CO_2H2CO3→H2O(l)+CO2

  • Sulfurous acid, H2SO3, decomposes into water and sulfur dioxide gas.

LaTeX: H_2SO_3 \rightarrow H_2O_(l)+NH_{3(g)}H2SO3→H2O(l)+NH3(g)

  • Ammonium hydroxide, NH4OH, decomposes into water and ammonia gas.

LaTeX: NH_4OH \rightarrow H_2O_(l)+NH_{3(g)}NH4OH→H2O(l)+NH3(g)

Example: Hydrochloric acid reacts with sodium bicarbonate. Write the reaction. As always, start by writing the reactants. HCI (aq) + NaHCO3 → Since this is a double displacement reaction, switch the anions. HCI (aq) + NaHCO3 → NaCl + H2CO3 ⚫ Looking at the products, we see H2CO3. You should be on the lookout for this product because it is unstable and should be replaced according to the products mentioned earlier. H2CO3H2O) and CO2(g) HCI (aq) + NaHCO3 NaCl + H2CO3 H2O(1) + CO2(g) So, the final reaction is: HCI (aq) + NaHCO3 NaCl + H2O(l) + CO2(g) (FYI-This is what happens when you have an upset stomach and take an antacid. The result is that the excess acid in your stomach is neutralized, meanwhile producing CO2. This CO2 escapes your stomach in the form of a BURP! That's why your stomach often feels better after a burp!)

Before we discuss the last type of double displacement reaction, neutralization, you need some background information. Continue to the next page for this lesson. 

Remember to work on the module practice problems as you complete each section of content.

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