How do you know if enthalpy is positive or negative ?

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Solution

The enthalpy is positive in case of an endothermic reaction where the energy is taken from the surroundings
The given example shown below the dissociation of ammonia into nitrogen and hydrogen is an example of endothermic reaction.
$2 {NH}_{3} (g) \to_{}^{} N_{2} (g) + 3 H_{2} (g); ∆ H = + 92.2 kJ$
In case of an exothermic reaction, the enthalpy is negative as the energy is released from the reaction.
The given example below shown the formation of ammonia from nitrogen and hydrogen is an example of exothermic reaction:
$N_{2} (g) + 3 H_{2} (g) \to_{}^{} 2 {NH}_{3} (g); ∆ H = - 92.2 kJ$

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