How can someone know whether a reaction is endothermic or exothermic.!?!

Exothermic Reactions:

• Observation: The surroundings (e.g., the solution in a beaker, the air around the reaction) will increase in temperature. You will feel the reaction vessel get hotter. This is because the reaction releases heat energy into the surroundings.
• Enthalpy Change: For an exothermic reaction, the enthalpy change ($\Delta H$) is negative ($\Delta H < 0$). This means the products have lower energy than the reactants.

Endothermic Reactions:

• Observation: The surroundings will decrease in temperature. You will feel the reaction vessel get colder. This is because the reaction absorbs heat energy from the surroundings.
• Enthalpy Change: For an endothermic reaction, the enthalpy change ($\Delta H$) is positive ($\Delta H > 0$). This means the products have higher energy than the reactants.