definition of synthesis reaction in chemistry

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What Is a Synthesis Reaction? Definition and Examples

A synthesis reaction is one of the four main types of chemical reactions , along with decomposition, single replacement , and double replacement reactions. Here is the synthesis reaction definition, examples of the reaction using elements and compounds, a look at how many reactants are involved, and how to recognize a synthesis reaction.

Synthesis Reaction Definition

A synthesis reaction is a chemical reaction that combines two or more simple elements or compounds to form a more complex product . A + B → AB This type of reaction is also called a direct combination reaction or simply a combination reaction. It’s the type of reaction that forms compounds from their elements. Synthesis reactions also make large molecules from smaller ones. A synthesis reaction is the opposite of a decomposition reaction , which breaks complex molecules into simpler ones.

Synthesis Reaction Examples

There are many examples of synthesis reactions. Some involve elements. In others, an element reacts with a compound. In still other cases, compounds react with other compounds to form larger molecules.

Synthesis Reactions Between Elements

• Iron and sulfur react to form iron sulfide. 8 Fe + S 8  → 8 FeS
• Potassium and chlorine react to form potassium chloride. 2K (s)  + Cl 2(g)  → 2KCl (s)
• Iron and oxygen react to form rust. 4 Fe (s) + 3 O 2  (g) → 2 Fe 2 O 3  (s)
• Hydrogen reacts with oxygen to form water. 2 H 2 (g) + O 2 (g) → 2 H 2 O(g)

Synthesis Reactions Between an Element and a Compound

• Carbon monoxide reacts with oxygen to form carbon dioxide. 2 CO(g) + O 2 (g) → 2CO 2 (g)
• Nitric oxide reacts with oxygen to form nitrogen dioxide. 2NO + O 2  → 2NO 2
• CH 2 CH 2 (g) + Br 2 (ℓ) → CH 2 BrCH 2 Br

Synthesis Reactions Between Compounds

• Sulfur oxide reacts with water to form sulfuric acid. SO 3  (g) + H 2 O (l) → H 2 SO 4  (aq)
• Calcium oxide reacts with water to form calcium hydroxide. 2CaO (s) + 2H 2 O (l) → 2Ca(OH) 2 (aq)
• Iron oxide and sulfur oxide react to form iron sulfate. Fe 2 O 3  + 3SO 3  → Fe 2 (SO 4 ) 3

How Many Reactants Are There?

Usually, there are two reactants in a synthesis reaction. They could be two elements, an element and a compound, or two compounds. However, sometimes more reactants combine to form a product. Here are examples of synthesis reactions involving three reactants:

• Sodium carbonate reacts with water and carbon dioxide to form sodium bicarbonate. Na 2 CO 3  + H 2 O + CO 2 → 2NaHCO 3
• Nitrogen reacts with water and oxygen to form ammonium nitrate. 2N 2 (g) + 4H 2 O(g) + O 2 (g) → 2NH 4 NO 3 (s)

How to Recognize a Synthesis Reaction

The easiest way to recognize a synthesis reaction is to look for a reaction where multiple reactants produce a single product. However, sometimes a synthesis reaction equation includes multiple products and reactants. A good example is the overall reaction for photosynthesis, in which carbon dioxide and water combine to form glucose and oxygen. CO 2  + H 2 O → C 6 H 12 O 6  + O 2 But, even in this case, two simpler molecules react to form a more complex one. So, this is the key in synthesis reaction identification.

Some synthesis reactions form predictable products. If you recognize them, it’s easy to recognize the reaction type:

• Reacting two elements forms a binary compound. For example, hydrogen and oxygen react to form water.
• When two nonmetals react, more than one product is possible. For example, sulfur and oxygen react to form sulfur dioxide or sulfur trioxide.
• Alkali metals react with nonmetals to form ionic compounds. For example, sodium and chlorine form sodium chloride.
• Transition metals react with nonmetals to form more than one possible product. To predict the product, you need to know the oxidation state (charge) or the metallic cation.
• Nonmetal oxides react with water to form acids. For example sulfur dioxide reacts with water to make sulfurous acid.
• Metallic oxides react with water to form bases.
• Nonmetal oxides react with one another to form salts.

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Synthesis Reaction Description Plus Examples

Two or more simple substances combine to form more complex products

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While there are many types of chemical reactions, they all fall into at least one of four broad categories: synthesis reactions, decomposition reactions, single displacement reactions, and double displacement reactions.

A synthesis reaction or direct combination reaction is a type of chemical reaction in which two or more simple substances combine to form a more complex product. The reactants may be elements or compounds, while the product is always a compound.

General Form of Synthesis Reactions

The general form of a synthesis reaction is:

A + B → AB

Examples of Synthesis Reactions

Here are some examples of synthesis reactions:

• Water: 2 H 2 (g) + O 2 (g) → 2 H 2 O(g)
• Carbon dioxide: 2 CO(g) + O 2 (g) → 2CO 2 (g)
• Ammonia: 3 H 2 (g) + N 2 (g) → 2 NH 3 (g)
• Aluminum oxide: 4 Al(s) + 3 O 2 (g) → 2 Al 2 O 3 (s)
• Iron sulfide: 8 Fe + S 8 → 8 FeS
• Potassium chloride: 2 K(s) + Cl 2 (g) → 2 KCl(s)

Recognizing Synthesis Reactions

The hallmark of a synthesis reaction is that a more complex product is formed from the reactants. One easy-to-recognize type of synthesis reaction occurs when two or more elements combine to form a compound. The other type of synthesis reaction happens when an element and a compound combine to form a new compound.

Basically, to identify this reaction, look for a product that contains all the reactant atoms. Be sure to count the number of atoms in both the reactants and the products. Sometimes when a chemical equation is written, "extra" information is given that might make it hard to recognize what is going on in a reaction . Counting numbers and types of atoms makes it easier to identify reaction types.

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Synthesis Reactions — Definition & Examples - Expii

Synthesis reactions — definition & examples.

In a synthesis reaction, or combination reaction, simpler reactants combine to form more complex products. The general equation is A + B → AB.

Explanations (4)

Synthesis reactions.

What are Synthesis Reactions?

• Synthesis reactions are reactions that involve multiple reactants reacting to form one single product.

Example: A + B → AB

• Synthesis reactions are exothermic reactions. So, they release energy as heat or light .

They involve the formation of either ionic or covalent bonds. The formation of a bond releases energy and increases stability. In contrast, breaking bonds requires energy.

Image source: Caroline Monahan

Synthesis Reaction Practice Problem

Which of the following are synthesis reactions?

2S + 3O2 → 2SO3

HCl + NaOH → NaCl + H2O

MgO + CO2 → MgCO3

CO2 + H2O → H2CO3

2Ni2O3 → 4Ni + 3O2

Related Lessons

(Video) Synthesis Reactions: Chemical Reaction (5 of 11) Synthesis Reactions, an Explanation

By Step by Step Science

In this video, you will learn that a synthesis reaction occurs when two or more compounds or elements react with each other to form a singular compound. You will also learn different examples of synthesis reactions in this video.

Synthesis Reactions: What are synthesis reactions?

Image Source: Leo Dong

What is Chemical Synthesis like for Professional Chemists?

In general chemistry, synthesis reactions are the least complicated chemical reaction. You take two reactants and make one product. A generic balanced chemical equation would be A+B→AB. Unfortunately, in the real world, chemical synthesis is rarely so straightforward.

I remember a nightmare scenario from my organic chemistry class. I had one question left on our online homework. I was so close to finishing! I clicked the next button and saw the final problem. It was a fifteen-step synthesis! When you're learning organic chemistry, that's not a short puzzle. Often in chemical synthesis, there are many steps. Why? There are a couple of possible reasons. Sometimes, the reactants that produce an easy synthesis reaction are expensive. So, you might have to start with cheap reactants and build up your molecules. Other times, the reaction intermediates may be unstable. They only form during a series of chemical reactions. So, again we have to build up our reactants.

Planning a Chemical Synthesis

Chemical synthesis is the heart of applied chemistry . The chemist works to develop a procedure to produce their desired compound. What's the result? They construct the reaction mechanism , piece by piece. Along the way, they often have to manipulate specific steps to achieve their goals. They use their knowledge of chemistry principles to influence the reaction.

How would a chemist go about planning a synthesis? Most often, they start at the final product and work backward. Each step focuses on changing one specific bond or group of atoms . In organic chemistry, a compound's reactivity comes from functional groups. They are groupings of atoms with set chemical properties . For example, a carbon - oxygen - hydrogen set is the alcohol group. Because oxygen is bound to hydrogen, it is always a polar group . The oxygen-hydrogen bond is also a weak acid . So, we might manipulate it with a strong base . Other chemists, like inorganic chemists, often focus on individual atoms. But they also have to consider the reactivity of the element .

Performing the Lab Work

In college, I had a friend that was doing a research project. One of the steps in his synthesis formed a hydrate . But, before he could proceed to the next step, he needed to remove the water. So, his compound had to sit in a chemical oven for twelve hours. He didn't get it in until after his afternoon classes. So, he had to return to the lab at 3:00 am to take it out! In chemical synthesis, the lab work is often the most challenging part. Why? A chemical reaction gets influenced by so many factors! We must consider temperature , pressure , pH , time, and other factors. Often we have to manipulate many factors along the way.

Sometimes, we use chemical manipulation . For example, maybe step seven of a fifteen-step reaction is the rate-limiting step . What could we do? We might try to develop a catalyst . Remember, they lower a reaction's activation energy and improve its rate . Sometimes, we can research the published work of other chemists for possible catalysts. Other times, we may need to use theoretical modeling to develop a catalyst. Sometimes, we even have to synthesize the catalyst!

Other times, we need to use physical manipulations. For example, one of our steps could be an equilibrium reaction. But we want it to go to completion. How do we manipulate the equilibrium? We take advantage of Le' Chatlier's principle . If the reaction is endothermic, we could apply heat. What if it's exothermic? We could cool it. Sometimes we can physically separate our products. By removing the products, we alter the equilibrium.

What are Your Results?

Chemistry is complex. For example, sometimes, a reaction can produce multiple products. So, even if we successfully implement our plan, we may not get our desired result. Why? We also have to worry about the percent yield . Part of planning the mechanism is calculating the theoretical yield . But often, one of the steps will act as a limiting reagent . So, very rarely are the theoretical and actual yields equivalent. Sometimes, experimental errors mean we don't produce our desired amount.

A + B ---> AB

Mg + O 2 ---> MgO H 2 + O 2 ---> H 2 O K + Cl 2 ---> KCl Fe + O 2 ---> Fe 2 O 3

CaO + CO 2 ---> CaCO 3 Na 2 O + CO 2 ---> Na 2 CO 3 KCl + O 2 ---> KClO 3 Ba(ClO 3 ) 2 ---> BaCl 2 + O 2

CO 2 + H 2 O ---> C 6 H 12 O 6 + O 2

H 2 + O 2 ---> H 2 O 2

1) Direct union of two elements will produce a binary compound. 2) Metallic oxides and carbon dioxide react to produce carbonates. 3. Binary salts and oxygen react to produce a chlorate.

CaO + H 2 O ---> Ca(OH) 2 Na 2 O + H 2 O ---> NaOH N 2 O 5 + H 2 O ---> HNO 3 P 2 O 5 + H 2 O ---> H 3 PO 4

1) LiCl + O 2 ---> 2) Na 2 O + CO 2 ---> 3) SO 3 + H 2 O ---> 4) N 2 + H 2 --->

LiCl + O 2 are the products of a chlorate decomposing.

Chlorate is always ClO 3 ¯ Li is plus one

LiCl + O 2 ---> LiClO 3

Na 2 O + CO 2 are the products of a carbonate decomposing.

Carbonate is always CO 3 2 ¯ Na is plus one

Na 2 O + CO 2 ---> Na 2 CO 3

SO 3 + H 2 O are the products of an acid decomposing.

In SO 3 the S has an oxidation number of +6 H has its usual value of +1 and O has its usual value of -2

SO 3 + H 2 O ---> H 2 SO 4

N 2 + H 2 are the products of a binary compound decomposing.

N has a charge of -3 H has its usual value of +1

N 2 + H 2 ---> NH 3

Synthesis Reactions

Synthesis reactions, also known as combination reactions, are chemical reactions in which two or more reactants combine to form a single product. This type of reaction is represented by the general equation: A + B → AB

Formation of a new substance: Synthesis reactions result in creating a new compound or molecule from simpler reactants.

Energy absorption: These reactions often require an input of energy, typically in the form of heat or light, to initiate the reaction.

Common examples of synthesis reactions include the formation of water (2H 2 + O 2 → 2H 2 O), the synthesis of ammonia (3H₂ + N₂ → 2NH₃), and photosynthesis (6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 + 6H 2 ).