Why can a reaction feel cold even while chemistry is happening fast?
Your first clue about energy change is often your hand. If your beaker or cup gets warmer, energy has moved out into the surroundings; if it feels cooler, energy has been pulled in from the surroundings.
When your reaction gives energy to everything around it, that is an Exothermic reaction: exothermic reaction happens when the reacting chemicals end up passing energy, usually as heat, to the space around them, so your surroundings warm up while the reacting system loses energy overall. When your reaction takes energy in from outside, that is an Endothermic reaction: endothermic reaction is a process where the reacting substances draw in energy from their surroundings, so the system gains energy and the nearby temperature drops.
You can track this with temperature. In an exothermic change, your surroundings heat up and the temperature goes up. In an endothermic change, your surroundings cool down and the temperature goes down.
The sign of ΔH matches this pattern. If ΔH is negative, your reaction is exothermic. If ΔH is positive, your reaction is endothermic.
Think of your reaction like your phone battery and a charger. In an exothermic reaction, the chemicals are like a battery powering something else, so energy flows out. In an endothermic reaction, the chemicals are like a phone being charged, so energy has to flow in.
A classic reaction that takes in energy is photosynthesis:
Your plants are not “making energy from nothing”. They take in light energy and lock it into the bonds of glucose, so this process is endothermic.
A well-known reaction that gives out lots of energy is methane burning:
Your gas cooker works because this reaction releases a large amount of heat, which is why methane is useful as a fuel.
One sneaky point: not every energy change gives you a dramatic temperature jump. In dilute solutions, your thermometer reading may shift only a little, so you need careful measurements.
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