Merlin's Help in Chemistry

First of all, the definition of a synthesis reaction is when two or more substances combine to form a new compound. This reaction can be represented by this equation:

A + B --> AB

element + element --> compound

Water can be used to demonstrate this type of reaction. When the element hydrogen combines with the element oxygen, water is formed. Coefficients are placed in front of the substances in order to balance the equation.

2H₂ + O₂ --> 2H₂O

The next type of reaction is decomposition. It is the exact opposite of a synthesis reaction. The proper definition for a decomposition reaction is a single compound that undergoes a reaction that produces two simpler substances. It can be represented like this:

AB --> A + B

compound --> element + element

Water can also be used to demonstrate this type of reaction, but this time the water molecule is being broken down. Like this:

2H₂O --> 2H₂ + O₂

Another type of reaction is the single displacement reaction. The common definition for a single displacement reaction is a reaction in which one element replaces another element in a compound. It can be shown below:

AB + C --> AC + B

compound + element --> compound + element

With the single displacement reactions, you have to determine the positive and negative ions in the compound and make sure the elements are combined correctly. For example, take the single displacement reaction below:

2HCl + Zn --> ZnCl₂ + H₂

Hydrogen is positive and chlorine is negative. They are combined to form hydrogen chloride. The element zinc that is alone is also positive. Therefore, when the reaction takes place zinc has to combine with chlorine, since chlorine is negative.

Double displacement reactions are yet another type of reaction. Its definition is a reaction when the ions of two compounds exchange places in a solution to form two entirely new compounds. Below is the equation for double displacement reactions:

AB + CD --> AC + BD

compound + compound --> compound + compound

Using compounds, a double displacement reaction is shown below:

HCl + NaOH --> NaCl + H(OH)

This is a double displacement reaction because the two beginning compounds are interchanged to form new compounds.

The last type of reaction is called a combustion reaction. A combustion reaction occurs when a substance combines with oxygen releasing a large amount of energy in the form of light and heat. A combustion reaction is in the form of this equation:

hydrocarbon + O₂ --> CO₂ + H₂O

A hydrocarbon is any compound made up of carbon and hydrogen. One example of a hydrocarbon is pentane, C₅H₁₂. The combustion of pentane occurs like this:

C₅H₁₂ + 8O₂ --> 5CO₂ + 6H₂O

Now it is time for you to try some on your own. Below are some problems. Determine what type of reaction has occurred in each of the following.

1) K + Cl₂ --> 2KCl

2) AgNO₃ + NaCl --> AgCl + NaNO₃

3) CH₄ + 2O₂ --> CO₂ + 2H₂O

4) MgCl₂ --> Mg + Cl₂

5) Cl₂ + 2NaBr --> 2NaCl + Br₂

Now write the equation and try to determine what the products would be.

1) sodium iodide + bromine -->

2) calcium hydroxide + sulfuric acid -->

3) potassium iodide -->

4) octane + oxygen -->

5) iron + oxygen -->

Return to the GCHS Chemhelp Page

For answer page click here

Back