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I don't think you can tell whether a reaction is exothermic or endothermic by the equation alone. You have to either measure the heating to cooling or know enough about similar types of reactions.
I know the triangle on top of the arrow indicates there is heat involved, so you know that equation can be endothermic or exothermic. However, to determine whether a reaction is exothermic or endothermic, I would assume we would be given delta H or some variation of it.
You can predict if a reaction is endothermic or exothermic by looking at the energies of the reactants and products (but this way is really hard with more complicated reactions). For example if you're given H2 = 2H, it would probably be endothermic since H2 is lower in energy than 2 H.
A quick difference between endothermic and exothermic involves reactions in the environment. An endothermic reaction takes place when energy is absorbed from surroundings in the form of heat, and exothermic is when energy is released from the system into the surroundings.
We can know if a reaction is endothermic when heat is absorb and in contrast an exothermic reaction is when heat is released. This is how one can environmentally tell if a reaction is exothermic and endothermic. Other than that, I think one has to know if delta H is negative or positive.
It helps to see an energy graph to think about why exothermic has a negative change in H and endothermic has a positive change in H. In exothermic reactions, heat is leaving the system, so the graph will end up at a lower delta H value in the products compared to the reactants. Therefore, the change in H will be negative (smaller H - larger H < 0). In endothermic reactions, heat is being put into the system/the system absorbs hear, so the graph will end up at a higher delta H value in the products compared to the reactants. Therefore, the change in H will be positive (larger H - smaller H > 0).
When a reaction gives off heat it is exothermic, but when a reaction requires heat it is endothermic. SO for a reaction the be exothermic the change in H must be less than than 0. For a reaction to be endothermic the change in H is greater than 0.
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