How to solve double replacement problems?

Question

how do you know if the reaction will take place or not?

How do you identify a gas?

indicate all gaseous products by (g) and all unionized or insoluble products by (^). if the reaction doesn't take place, indicate the reason

1) CuS + 2KCl ---> CuCl2 + K2S

2) Hg2Cl2 + HCl -->

3) H2O + K2SO4 --> H2SO4 + K2O

4) NH4Cl + KOH --> KCL + NH4OH

5) NaNO3 + KC2H3O2 --> NaC2H3O2 + KNO3

6) 2NH4OH + BaCl2 --> 2NH4Cl + Ba(OH)2

7) Ca(NO3)2 + Na2SO4 --> CaSO4 + 2NaNO3

8) Hg2(NO3)2 + K2SO4 -->

9) Pb(NO3)2 + Al(NO3)3 -->

10) KCl + Hg2SO4 --> K2SO4 + HgCl

Answer 1

1) CuS(^) + 2KCl → no reaction

A reaction will attempt to go into the direction with the product of lowest solubility. CuCl2 is soluble, so the reaction will not proceed.

2) Hg2Cl2(^) + HCl → no reaction

Similar reasons.

3) H2O(^) + K2SO4 → no reaction

K2O is a base, and H2SO4 is an acid. It is more favourable for them to neutralise each other to form water and a salt.

4) NH4Cl + KOH → KCl + NH4OH

NH4OH will decompose. NH4OH → NH3(g) + H2O

5) NaNO3 + KC2H3OH → will not proceed

An equilibrium may form, where the Na and K interchange, but it will not form a full reaction.

6) 2NH4OH + BaCl2 → 2NH4Cl + Ba(OH)2(^)

Excess barium hydroxide will precipitate.

7) Ca(NO3)2 + Na2SO4 → CaSO4(^) + 2NaNO3

CaSO4 is insoluble and will precipitate.

8) Hg2(NO3)2 + K2SO4 → Hg2SO4(^) + 2KNO3

Nitrates are soluble, but most mercury(I) salts are not. Hg2SO4 will precipitate.

9) No reaction. Both salts are similar in that they are nitrates. Replacement will have no effect.

10) No reaction. Hg2SO4 is insoluble and tries to remain that way.

Answer 2

I was very pleased to see the response from Kumorifo on this subject. Too often answers are give that a doule displacement reaction has taken place, merely because the ions of the two compounds have been swapped, supposedly making two new compounds . This is not correct!!. What is the real situation:

A typical double replacement reaction can occur when two ionic compounds are mixed together. In water these ionic compounds split apart into their respective anions and cations. The cations now have an opportunity to swap anions.

NOW THIS IS THE CRITICAL BIT:

A reaction occurs if , by swapping anions, a product is formed that cannot split apart into anions and cations. Look at the following example: the reaction of silver nitrate and potassium chloride.

The silver, nitrate, potassium and chloride ions all begin in solution.When a silver ion combines with a chloride ion, it leaves the solution and becomes a solid. This drives the reaction to completion!!!

AgNO3(aq) + KCl(aq)� ‑‑‑‑>� AgCl(s) + KNO3(aq)

The reaction must be driven by one of three driving forces:

Formation of a solid,

Formation of a gas

Formation of a weakly‑ionizing compound such as water. Let's look to see what happens when the driving forces are not there.

NaNO3(aq) + KCl(aq →   Na+  +  Cl- + K+  +   NO3-

Nothing has actually happened!! Without a driving force there is no change in the solution so we say no reaction has taken place. Note that even though we can write an equation for the reaction, no reaction takes place without a driving force!!

How can we tell if an ionic compound will be soluble in water? There are a series of guidelines in your class chemistry book. Here are some solubility rules to guide you:

Solubility Rules

1. All compounds containing Na+, K+, or NH4+ ions are soluble in water.

2. All nitrates (NO3‑) are soluble in water.

3. Most chlorides (Cl‑ ), and sulfates (SO4 2‑) are soluble. Some important exceptions are silver chloride (AgCl), barium sulfate (BaSO4), and lead sulfate (PbSO4) which are insoluble.

4. Most carbonates (CO32‑), phosphates (PO43‑), sulfides (S2‑), and hydroxides (OH‑) are insoluble in water. Important exceptions are those of Na+, K+, and NH4+, as well as barium hydroxide, Ba(OH)2.

How do you know if a gas will form?

If H2CO3 is formed it will form a gas by decomposing to H2O and CO2. You will see that in this lab.

Other compounds that release water or a gas are

NH4OH and H2SO3.

When an strong acid and a strong base react, in an aqueous solutions, they produce a salt and water. This is an example of a double displacement reaction where the driving force is the formation of the covalent compound water.

Often, the only way to tell if one of these reactions has occurred is by looking for the formation of heat or by using an acid/base indicator to tell if the base has neutralized the acid.

If you do not see a reaction occurring you may wish to ask your instructor for an appropriate indicator. (How can you tell what the color of the indicator will be in an acidic solution and a basic solution? Answer: Test the solutions individually.)

Now with this information you should be able to go through the list by yourself, check the data given to you above, and work this out by yourself. Good luck

Answer 3

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