Question

What does it mean when delta H is negative?

Answer 1

Hint: The symbol is used to denote enthalpy of a thermodynamic system which is defined as the sum of its internal energy U and the work required to achieve its pressure and volume.
 $H=U+pV$
where p is pressure, and V is the volume of the system.
Delta H ( $△H$ ) is a symbol which is used to denote the enthalpy change in a chemical reaction.

Complete answer:
Under suitable conditions compounds tend to react with one other.This process is vaguely termed as chemical reaction.When compounds chemically react with one another they form products which have completely different chemical and physical properties than their reactants.The general representation can be given as-
 $A+B\to C+D$
Where A and B are reactants and C and D are products.Whenever chemical reactions occur there are two possible scenarios-
Case 1-The reactants are at a higher energy level(Exothermic reaction)
Case 2-The products have higher energy level(Endothermic reaction)
In order to better understand the above two scenarios a parameter known as enthalpy change ( $△H$ ) was introduced.The amount of heat absorbed or evolved during a chemical reaction carried out at constant pressure is known as enthalpy change.Delta H can be negative or positive,let us look at the possible situation.
1)Delta H is positive-The products are at a higher energy level as compared to the reactants the net heat was absorbed,it’s an endothermic reaction
2)Delta H is negative- The products are at a lower energy level as compared to the reactants. The net heat evolved,it’s an exothermic reaction.

Note:
Delta H gives us a clear picture about the energy levels of the products and reactants.It also indicates the energy gained or lost by the species involved in a chemical reaction.The unit of enthalpy change is Joules.