Synthesis Reactions and Decomposition

Chemical Reactions and Equations Part 2

Lesson Target: Students will be able to complete a quizizz activity that describes the different types of chemical reactions (synthesis, decomposition, composition, and combustion) and by solving chemical equations.

​A synthesis reaction or direct combination reaction is one of the most common types of chemical reaction.

In a synthesis reaction, two or more chemical species combine to form a more complex product: A + B → AB.

In this form, a synthesis reaction is easy to recognize because you have more reactants than products. Two or more reactants combine to make one larger compound.

One way to think of synthesis reactions is that they are the reverse of a decomposition reaction.

​Synthesis Reactions

​Synthesis Reaction Examples

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In the simplest synthesis reactions, two elements combine to form a binary compound (a compound made of two elements). The combination of iron and sulfur to form iron (II) sulfide is an example of a synthesis reaction:

​8 Fe + S₈ → 8 FeS

Another example of a synthesis reaction is the formation of potassium chloride from potassium and chlorine gas:

2K_(s) + Cl_2(g) → 2KCl_(s)

As in these reactions, it's common for a metal to react with a nonmetal. One typical nonmetal is oxygen, as in the everyday synthesis reaction of rust formation:

4 Fe (s) + 3 O₂ (g) → 2 Fe₂O₃ (s)

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Multiple Choice

Balance out the following equation:

Na + Cl2 ------> NaCl

• 3Na + 2(Cl2) ----> NaCl

• 2(Na) + Cl2 -----> 2(NaCl)

• Na + 2(Cl2) ----> 2(NaCl)

• None of the answers are correct

Multiple Choice

Look at the following synthesis reaction below:

2(H2) + O2 ----> 2(H2O)

Which of the following would be labeled as a "compound"

• H2

• O2

• H2O

• None of the answers are correct

​A decomposition reaction starts from a single substance and produces more than one substance; that is, it decomposes. One substance as a reactant and more than one substance as the products is the key characteristic of a decomposition reaction. For example, in the decomposition of sodium hydrogen carbonate (also known as sodium bicarbonate),

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2 NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(ℓ)

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sodium carbonate, carbon dioxide, and water are produced from the single substance sodium hydrogen carbonate.

Composition and decomposition reactions are difficult to predict; however, they should be easy to recognize.

​Decomposition Reaction

Multiple Choice

Identify if the following chemical reaction equation is a synthesis reaction or a decomposition reaction:

C₃H₈ → C₃H₄ + 2 H₂

• Synthesis Reaction

• Decomposition Reaction

• Neither 

Multiple Choice

Carbonic acid (H₂CO₃) is an ingredient in soft drinks. A decomposition reaction takes place when carbonic acid breaks down to produce water (H₂O) and carbon dioxide (CO₂). This occurs when you open a can of soft drink and some of the carbon dioxide fizzes out. The equation for this reaction is:

H₂CO₃ → H₂O + CO₂

_{How many oxygen atoms are produced when you break down carbonic acid?}

• 1

• 2

• 3

• 4

​A combustion reaction occurs when a reactant combines with oxygen, many times from the atmosphere, to produce oxides of all other elements as products; any nitrogen in the reactant is converted to elemental nitrogen, N₂. Many reactants, called fuels, contain mostly carbon and hydrogen atoms, reacting with oxygen to produce CO₂ and H₂O. For example, the balanced chemical equation for the combustion of methane, CH₄, is as follows:

​

CH₄ + 2 O₂ → CO₂ + 2 H₂O

​Combustion Reactions

​Example of Combustion Reaction: Chimney Stacks

​A flue-gas stack, also known as a smoke stack, chimney stack or simply as a stack, is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is combusted in an industrial furnace, a power plant's steam-generating boiler, or other large combustion device.

​

​Flue gas is usually composed of carbon dioxide (CO₂) and water vapor as well as nitrogen and excess oxygen remaining from the intake combustion air. It also contains a small percentage of pollutants such as particulate matter, carbon monoxide, nitrogen oxides and sulfur oxides. The fuel gas stacks are often quite tall, up to 400 metres (1300 feet) or more, so as to disperse the exhaust pollutants over a greater area and thereby reduce the concentration of the pollutants to the levels required by governmental environmental policy and environmental regulation.

When the flue gases are exhausted from stoves, ovens, fireplaces, heating furnaces and boilers, or other small sources within residential abodes, restaurants, hotels, or other public buildings and small commercial enterprises, their flue gas stacks are referred to as chimneys.

​Kerosene can be approximated with the formula C₁₂H₂₆, and its combustion equation is

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2 C₁₂H₂₆ + 37 O₂ → 24 CO₂ + 26 H₂O

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Sometimes fuels contain oxygen atoms, which must be counted when balancing the chemical equation. One common fuel is ethanol, C₂H₅OH, whose combustion equation is

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C₂H₅OH + 3 O₂ → 2 CO₂ + 3 H₂O

​

If nitrogen is present in the original fuel, it is converted to N₂, not to a nitrogen-oxygen compound. Thus, for the combustion of the fuel dinitroethylene, whose formula is C₂H₂N₂O₄, we have

2 C₂H₂N₂O₄ + O₂ → 4 CO₂ + 2 H₂O + 2 N₂

Multiple Choice

The Following Equation is a combustion equation when kerosene is burned and releases carbon dioxide and water vapor:

2 C₁₂H₂₆ + __ O₂ → 24 CO₂ + 26 H₂O

Which of the following coefficient is missing for O2 on the reactant side of the chemical equation:

• 24

• 30

• 37

• 42

Multiple Choice

Sometimes fuels contain oxygen atoms, which must be counted when balancing the chemical equation. One common fuel is ethanol, C₂H₅OH, whose combustion equation is

C₂H₅OH + 3 O₂ →  CO₂ +  H₂O

Which of the following coefficients is missing for the product side of the chemical reaction?

• 3,2

• 4,2

• 2,3

• 3,3

Open Ended

How do you think that the climate can change due to combustion reaction? Please write 2 - 3 sentences on how you think a combustion reaction will affect the atm