How do you know it is a exothermic reaction?

Answer 1

If you are in a lab and want to tell whether a reaction was exothermic, carefully check the temperature of the reaction vessel. If the reaction was exothermic, the vessel will be warmer than it was before the reaction occurred.

If you are looking at a reaction on paper, you will need a table of thermodynamic data. It should contain the molar enthalpies of formation, represented by H, of each of the reactants and products (note: if a reactant or product is an element in its most stable state, like H2(g) or Zn(s), the enthalpy of formation is defined as zero, so they may be left out of the table). Multiply the number of moles of each product and reactant by its molar enthalpy. Add up all the results for the products and subract all the results for the reactants. If your net result is negative, the reaction was exothermic. If it is positive, the reaction was endothermic.

Example:

CH4(g)+2O2(g)-->CO2(g)+2H2O(g)

Delta H (kJ/mol)

CH4(g) -74.8

O2(g) 0

CO2(g) -393.5

H2O(g) -241.82

Products: 2(-241.82)+(-393.5)= -877.14 kJ

Reactants: -74.8+2(0)= -74.8 kJ

Total: -877.14-(-74.8)= -802.34 kJ

The reaction was exothermic.

Answer 2

You can tell if it is exothermic when it starts to lose heat.

Answer 3

If a chemical reaction proceeds by releasing heat in the surroundings, than the reaction is an exothermic reaction and if it needs heat to proceed than it is an endothermic reaction.

Answer 4

An exothermic reaction release heat.