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There are many different types of chemical reactions. You can predict the outcome of a reaction based only on the reactants if you understand patterns in written chemical equations. 
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Synthesis reactions
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Although many different combinations of reactants are possible, you will see just a few structural patterns emerge. Understanding these patterns will help you predict the products of a reaction. Five types of chemical reactions are:
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Types of
chemical
reactions |
A + X → AX |
Synthesis |
| AX → A + X |
Decomposition |
| A + BX → AX + B |
Single replacement |
| AX + BY → BX + AY |
Double replacement |
X + O2 → XO
CxHx + O2 → CO2 + H2O |
Combustion, or
Combustion of a hydrocarbon |
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A synthesis reaction occurs when two or more reactants combine to form one product. The general form of a synthesis reaction is shown below. An example of a synthesis reaction is when hydrogen and oxygen combine to form water. Synthesis reactions combine different substances, so they are also known as combination reactions. The number of products is less than the number of reactants, however, the number of atoms remains the same.
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The example above shows the combination of two elements, but two or more larger compounds can also combine into one larger compound. One example of this is the reaction of magnesium oxide, MgO, with water to make magnesium hydroxide, Mg(OH)2, also known as milk of magnesia: MgO + H2O → Mg(OH)2. Milk of magnesia is a common over-the-counter medication used to neutralize excess stomach acid.
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| Find the product that forms when potassium and chlorine react, then balance the reaction.
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| Relationships |
You can tell this is a synthesis reaction because the question asks you to find "the" product;
this means there is only one product. |
| Solve |
- Write the formula for each reactant. Both reactants are elements, so apply the diatomic molecule rule. Chlorine forms a diatomic molecule, but potassium does not: K + Cl2 → ?
- Combine K and Cl to form a compound using ionic formula rules:
- Write the ion form for each substance: K+ and Cl−. Ignore the fact that chlorine is in diatomic form.
- Criss-cross charges. In this case both charges are "1": KCl
- Rewrite the reaction with the new compound: K + Cl2 → KCl
- Balance the reaction: 2K + Cl2 → 2KCl
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| Answer |
The balanced synthesis reaction is: 2K + Cl2 → 2KCl |
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