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Let's do a (theoretical) experiment. We pour 250 mL of a colorless, silver nitrate solution (AgNO3) into a beaker. We then submerge a copper wire into the beaker. After waiting several minutes, we check back in on our solution, and it looks like this:Fig.1 Copper wire in silver nitrate solution So, what happened? Why is our copper wire suddenly silver? And…
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Jetzt kostenlos anmeldenLet's do a (theoretical) experiment. We pour 250 mL of a colorless, silver nitrate solution (AgNO3) into a beaker. We then submerge a copper wire into the beaker. After waiting several minutes, we check back in on our solution, and it looks like this:
So, what happened? Why is our copper wire suddenly silver? And why is our solution suddenly blue? Well, a single replacement reaction occurred. This means that silver crystals are forming on the wire (hence why it is silvery) and copper nitrate (Cu(NO3)2) is in solution, which is blue.
In this article, we will be learning all about both single and double replacement reactions, so we can better understand why reactions like this occur.
Let's start by looking at the definition of a single replacement reaction.
In a single replacement reaction, one element is swapped with another in a compound, creating a new compound
This reaction is sometimes called single displacement, since one element is "pushing out" (displacing) another.
The general formula for this reaction is:
$$A + BC \rightarrow AC + B$$
Essentially, element A is swapping with element B, which forms the new compound AC. This leaves element B as a lone element.
It's important to remember that the lone element will always swap with a species of like charge. Basically, only cation-forming (positively charged) species can displace another cation-forming species. For example, Iron (Fe), which can have a charge of +2 or +3, cannot displace oxygen (O), which can have a charge of -2.
Also, the element that does the replacing is always more reactive than the species it is replacing. We will talk about this more in the section "Activity Series".
Now that we've covered what the general reaction looks like, let's look at some specific examples.
Cation replacement reactions
$$Cu + AgNO_3 \rightarrow CuNO_3 + Ag$$
$$Zn + CuCl_2 \rightarrow ZnCl_2 + Cu$$
$$Zn + H_2S \rightarrow H_2 + ZnS$$
In the above reactions, the lone (neutral) element can form a cation, which will replace the cation in the compound.
The first and second reactions are actually the reactions we talked about in our introduction. These elements don't need to have the same charge, just have the same type of charge (i.e. positive or negative). In the third reaction, zinc (Zn) has a +2 charge in zinc sulfide (ZnS), while hydrogen (H) has a +1 charge when bonded to sulfur (S).
Anion replacement reactions
$$Br_2 + 2KI \rightarrow 2KBr + I_2$$
$$ F_2 + NaCl \rightarrow NaF + Cl_2$$
Anion replacement reactions are virtually the exact same as cation replacement reactions, except the negatively charged species is the one being replaced.
When an element is by itself (with no charged noted) it is neutral. It only has a charged when bonded. Because of this, you need to remember which species are cation forming and which are anion forming. For example, the following reaction isn't possible:
$$Zn + HCl \rightarrow H_2Zn$$
This is because zinc only forms a cation (+2 charge), so it can't replace chlorine (Cl), which has a -1 charge when bonded to hydrogen.
Single-replacement reactions can only happen when the element replacing another is more reactive. To help us remember what species is more reactive, we use an activity series.
An activity series is a chart listing elements from most to least reactive.
First up, here is the activity series for metals.
From this chart, we can see some general patterns:
Group 1 and 2 metals are more reactive than transition metals
The reactivity of transition metals decreases as you go down the rows
Let's work on an example:
Which of the following single replacement reactions is not possible?
$$2Na + ZnCl_2 \rightarrow 2NaCl + Zn$$
$$Cu + ZnCl_2 \rightarrow CuCl_2 + Zn$$
$$Pb + ZnCl_2 \rightarrow PbCl_2 + Zn$$
Each of these elements are swapping with zinc (Zn), so we need to compare their reactivities to that of zinc.
Since copper is below zinc, it is less reactive, and therefore that reaction isn't possible. Both sodium and lead are above zinc, so they are reactive enough for the reaction to occur.
And now for the non-metals:
Here are some general rules:
While these are a good rule of thumb, it's best to keep an activity series handy!
Let's do another example, this time using the nonmetal series:
Which of the following reactions isn't possible?
$$ Br_2 + CuO \rightarrow CuBr + O_2$$
$$ F_2 + CuO \rightarrow CuF + O_2$$
$$ I_2 + CuO \rightarrow CuI + O_2$$
Since Iodine (I) is below oxygen in the activity series, it is less reactive than oxygen and therefore this reaction cannot occur (both bromine (Br) and fluorine (F) are above oxygen).
Now let's move on to double replacement reactions. Its definition is as follows:
Double replacement reactions are those that occur between two ionic compounds. Ions of the same type (cations or anions) are swapped between the two compounds, forming new ones.
The general formula for double replacement reactions is:
$$AB + CD \rightarrow CB + AD$$
Where A and C are both cations and B and D are both anions.
In a double replacement reaction, either both cations are swapped or both anions. Unlike with single replacement reactions, you can't really tell which ones have been swapped, because you would end up with the same product regardless.
Below are some examples of double replacement reactions:
$$(NH_4)_2CO_3 + Ba(NO_3)_2 + 2NH_4NO_3 + BaCO_3$$
$$2KI + Pb(NO_3)_2 \rightarrow PbI_2 + 2KNO_3$$
$$Ca(OH)_2 + HCl \rightarrow H_2O + CaCl_2$$
Precipitation reactions are a special type of double replacement reactions. In these reactions, two aqueous (dissolved in water) solids react to form an insoluble solid called a precipitate.
Below are some examples of a precipitation reaction:
$$AgNO_{3\,(aq)} + NaCl_{(aq)} \rightarrow NaNO_{3\,(aq)} + AgCl_{(s)}$$
$$CuSO_{4\,(aq)} + 2NaOH_{(aq)} \rightarrow Na_2SO_{4\,(aq)} + Cu(OH)_{2\,(s)}$$
Often when looking at precipitation reactions, we use the net ionic equation instead of the normal chemical equations as shown above. In net ionic equations, only the ions that form the precipitate are shown, since this is the direct reaction occurring.
For example, the net ionic equation for our first example is:
$$Ag^+_{(aq)} + Cl^-_{(aq)} \rightarrow AgCl_{(s)}$$
Basically, any ions that stay in solution for the entire process are not included. These ions are called spectator ions, since they only "spectate" the reaction and don't directly participate.
Another special type of double replacement reaction is the neutralization reaction.
A neutralization reaction is a reaction between an acid (H+ donator) and a base (H+ acceptor) that forms a salt (ionic solid) and water
The general formula for this type of reaction is:
$$HA + BOH \rightarrow H_2O + BA$$
Where A is the anion in the acid and B is the cation in the base.
The reaction is called a neutralization reaction because both salts (here shown as BA) and water (H2O) have a neutral pH (pH=7)
Below are some examples:
$$HCl + NaOH \rightarrow H_2O + NaCl$$
$$HBr + KOH \rightarrow H_2O + KBr$$
$$A + BC \rightarrow AC + B$$
An activity series is a chart listing elements from most to least reactive.
For a single replacement reaction to occur, the replacement element must be more reactive than the element being replaced
Double replacement reaction are reactions that occur between two ionic compounds. Ions of the same type (cations or anion) are swapped between the two compounds, forming new ones.
$$AB + CD \rightarrow CB + AD$$
Precipitate reactions are a special type of double replacement reactions where two aqueous (dissolved in water) solids react to form an insoluble solid called a precipitate
A neutralization reaction is a reaction between an acid (H+ donator) and a base (H+ acceptor) that forms a salt (ionic solid) and water
In a single replacement reaction, one lone element is swapping with an element in a compound of like charge (i.e. positive or negative). Double replacement reactions also have a swapping of like charges, except it is two elements within compounds swapping,
An example of a single replacement reaction is:
Cu + AgNO3 --> Cu(NO3)2 + Ag
An example of a double replacement reaction is:
AgNO3 + NaCl --> NaNO3 + AgCl
The two types of replacement reaction are single and double replacement
Double replacement reaction are reactions that occur between two ionic compounds. Ions of the same type (cations or anion) are swapped between the two compounds, forming new ones.
The general formula is:
AB + CD --> CB + AD
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