The reactions having low activation energy are……………reactions.

Answer

To solve the question "The reactions having low activation energy are……………reactions," we can follow these steps:

### Step 1: Understand Activation Energy
Activation energy is the minimum energy required for a chemical reaction to occur. It is the energy barrier that reactants must overcome to transform into products.

**Hint:** Recall that activation energy is crucial for determining how easily a reaction can proceed.

### Step 2: Identify the Relationship Between Activation Energy and Reaction Rate
Reactions with low activation energy require less energy to initiate, which means they can proceed more quickly. Therefore, the lower the activation energy, the faster the reaction.

**Hint:** Consider how energy barriers affect the speed of reactions.

### Step 3: Conclude the Type of Reactions
Since reactions with low activation energy are able to occur more readily and quickly, we can conclude that these reactions are referred to as "fast reactions."

**Hint:** Think about how speed is related to the energy required for reactions.

### Final Answer
The reactions having low activation energy are **fast reactions**.