Which of the following can occur as a result of a chemical reaction?

Question

select all that apply

1. Energy is absorbed (endothermic) 
2. new substances are formed from the old substances. 
3. Energy is created or destroyed
4. Energy is released (exothermic) 

Answer 1

From the 1st law of Thermodynamics aka Conservation of Energy, we know that energy cannot be created or destroyed in an isolated system. Additionally we know about the following examples of the remaining types of reactions:

1. Energy is absorbed (endothermic) - think of an instant ice/cold pack, the kind you hit and then gets cold. The way these work is that when you hit it you open a pouch that mixes two substances, typically water and ammonium-nitrate. This mixture creates an endothermic reaction which absorbs heat creating the cold feeling.
2. Energy is released (exothermic) - putting steel wool in vinegar, or the more popular elephant toothpaste experiment.
3. New substances are formed from old substances - one of the most visible examples of this is rust. A reaction between iron and oxygen to form iron oxide aka rust.