For millennia, all the way back to Ancient Egypt, alchemists have been attempting to synthesize gold from more common metals like lead. While their attempts always failed, the scientists of today have been able to do the impossible: make gold (though in a very, very expensive way).
Fig. 1: For many centuries, alchemists have been searching for ways to synthesize gold. Wikimedia Commons.
In 1980, Glenn Seaborg was able to "steal" protons and neutrons from bismuth, which did produce several thousand atoms of gold. However, this method was much too expensive for gold to be made this way.
While he was able to make gold, he didn't "synthesize" it in the traditional sense. In this article, we will be learning all about the synthesis reaction. After reading this article, you'll see why the alchemists dream of synthesizing gold was always destined to fail.
- This article cover the topic synthesis reactions.
- First, we will define what a synthesis reaction is.
- Next, we will look at the basic chemical equation for a synthesis reaction.
- Then, we will look at some different examples of these reactions.
- After that, we will summarize the characteristics of the synthesis reaction.
- Lastly, we will compare and contrast these reactions with another type of reaction called single replacement.
Synthesis reaction definition
A synthesis reaction (also called a combination reaction) is a reaction where two or more elements or compounds combine to form a new compound.
A key characteristic about synthesis reactions is that they only make one unique product. However, there can be multiple of that product produced.
As discussed in the intro, it is impossible to (traditionally) synthesis gold. Why? Because it is an element. When elements react, they will always form compounds.
There are ways for elements to combine to form other elements, but these methods are different from your typical reaction. For example, the sun fuses hydrogen nuclei into helium atoms by essentially smashing the nuclei together at very high temperatures.
Synthesis reaction chemical equation
All synthesis reactions follow this basic formula:
$$A + B \rightarrow AB$$
The basic structure is two (or more) elements and compounds combining to create one product.
Fig. 2: Synthesis reactions can be carried out in a chemical laboratory. Wikimedia Commons
Let's look at some examples, shall we?
Synthesis reaction examples
Let's have a look at some synthesis reaction examples to fully understand what synthesis reactions are and how they are represented.
First up, let's look at the synthesis of salt:
$$2Na_{(s)} + Cl_{2\,(g)} \rightarrow 2NaCl_{(s)}$$
As you can see, we have two elements (chlorine is naturally diatomic) forming one. Even though chlorine is a gas, it still forms a solid product. Synthesis reactions (and all reactions for that matter) don't always form a product of the same state as the reactants, so keep your eyes on the states listed in the equation.
Now let's look at a more complex example:
Here we have two compounds forming a product instead of elements. As long as one product is formed, it is a synthesis reaction, whether it is formed from elements or compounds.
$$CaO_{(s)} + H_2O_{(l)} \xrightarrow {heat} Ca(OH)_{2\,(aq)}$$
As a side not, the (aq) on Ca(OH)2 means "aqueous". This means that the compound is dissolved in water. Usually, but not always, reactions that contain water will produce aqueous products.
Also, "heat" is written above the arrow to show heat is required for the reaction to proceed.
Let's look at one last example:
$$2N_{2\,(g)} + 4H_2O_{(l)} + O_{2\,(g)} \rightarrow 2NH_4NO_{3\,(s)}$$
The above reaction has 3 reactants instead of 2. While two-reactant synthesis reactions are more common, it's important to keep in mind that these reactions can have more than 2 reactants.
Characteristics of a synthesis reaction
Synthesis reactions are pretty easy to spot when you know what to look for. As a summary, here are the characteristics of a synthesis reaction:
- Have 2 or more reactants.
- Reactants can be elements and/or compounds.
- Reactants combine to form one compound.
- The compound formed is more complex than the reactant.
Synthesis reactions in organic chemistry
When you read up on synthesis reactions, you might encounter a different type used in organic chemistry.
In organic chemistry, a synthesis reaction involves taking one simple compound and transforming through a series of steps to get a much more complex compound. While they are similar, they have some key differences, such as the fact that organic synthesis reactions are multiple steps a nd that they can produce side products.
Synthesis reaction vs single replacement
Synthesis reactions can often be confused with another reaction called single replacement.
In a single-replacement reaction, an element will displace another element present in a compound, "swapping" with it.
A general single-replacement reaction looks like this:
$$A + BC \rightarrow AB + C$$
The lone element essentially "kicks out" the like element (i.e. metals swap with metals, non-metals swap with non-metals).
Below is an example:
$$Zn_{(s)} + CuCl_{2\,(s)} \rightarrow ZnCl_{2\,(s)} + Cu_{(s)}$$
Here the zinc is swapping with the copper (both are transition metals).
Single replacement reactions are similar to synthesis reactions, since they involve 2 reactants coming together to form a new product. The key difference is that two products are formed instead of one. Also, this process involves swapping with synthesis involves only combining.
| Reaction | Representation | Example | Keyword |
| Synthesis reaction | $$A + B \rightarrow AB$$ | $$2Na_{(s)} + Cl_{2\,(g)} \rightarrow 2NaCl_{(s)}$$ | Combination |
| Replacement reaction | $$A + BC \rightarrow AB + C$$ | $$Zn_{(s)} + CuCl_{2\,(s)} \rightarrow ZnCl_{2\,(s)} + Cu_{(s)}$$ | Swap |
Table 1. Comparison between synthesis and replacement reactionsSynthesis Reaction - Key takeaways
- A synthesis reaction (also called a combination reaction) is a reaction where two or more elements or compounds combine to form a new compound.
- All synthesis reactions follow this basic formula:
$$A + B \rightarrow AB$$
Synthesis reactions are separate from single replacement reaction
In a single-replacement reaction, an element will displace another element present in a compound, "swapping" with it.
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