Question

When is enthalpy change ($\Delta H$):
(a)- Positive
(b)- Negative

Answer 1

The change in enthalpy of the reaction is a very important property by which we can tell the given reaction is exothermic and endothermic. The change in enthalpy of the reaction can be told by a change in the energy of the reaction.

Complete answer:
We know that the reactions can be categorized on the basis of many factors, so energy or heat is a factor in which the reactions are divided into two, i.e., exothermic reactions and endothermic reactions. If the reactions occur and energy or heat is released then it will be categorized as an exothermic reaction and if the reactions occur when the energy is supplied or absorbed then it will be categorized as an endothermic reaction.
This change in energy can be represented by enthalpy change ($\Delta H$) and there are two values of change in the energy of the reaction, i.e., positive and negative.
(a)- Positive
When the energy is supplied to the reaction and when this energy is supplied to the reaction only then the products can be formed. Then the enthalpy change is positive. These are endothermic reactions. For example:
$N{{H}_{4}}N{{O}_{3}}\to NH_{4}^{+}+NO_{3}^{-}\text{ }\Delta \text{H = + }x$
(b)- Negative
When the energy is released in the reaction and the reaction automatically occurs with release of energy. Then the enthalpy change is negative. These are exothermic reactions. For example:
$CaC{{l}_{2}}+2{{H}_{2}}O\to Ca{{(OH)}_{2}}+2HCl\text{ }\Delta \text{H = - }x$

Note:
The enthalpy change of the reaction can be zero also and this occurs when the reaction is in equilibrium which means the rate of the formation of product is equal to the rate of the formation of reactant.