A chemical reaction between iron atoms and oxygen molecules can only occur if

(1) the particles are heated

(2) the atmospheric pressure decreases

(3) there is a catalyst present

(4) there are effective collisions between the particles

Answer: option (4) there are effective collisions between the particles

Justification:

The collision theory explains the chemical reactions in terms of collisions. A chemical reaction takes place if two molecules collide effectively.

So, the first conditon is that the collision occur.

The second condition, that the collision be effective requires two conditions:

1) that the collision occurs with the proper orientation of the molecules, and

2) that the energy of collision be high enough to start the break of the original bonds ant the creation of the new bonds in the activated complex.

The energy required to form this activated complex is the activation energy.

So, an effective collision requires the proper orientation of the molecules when they collide and that the energy be equal or greater than the activation energy.

Temperature and catalyst are factors that affect the occurrence of those collisions, so they affect the rate of the reaction, by modifiying the number of collisions.

The answer is D because in order for a reaction to take place there must be collisions between the iron and oxygen atoms