Given the enthalpies of reaction: 2p(g) + 3cl2(g) → 2pcl3(g) dh = –574 kj 2p(g) + 5cl2(g) → 2pcl5(g) dh = –887 kj what is the enthalpy change of the following reaction: pcl3(g) + cl2(g) → pcl5(g

Explanation: It's based on Hess's law. We will rearrange the two given equations and make the equation to which it asks to calculate the enthalpy change.

First equation needs to be reversed and divided by 2 so that we could get one on reactant side. When we reversed an equation then the sign of  enthalpy change is also changed.

Second equation also needs to be divided by 2 and its not reversed.

Both the equations are added now:

So, the enthalpy change for the equation is -156.5 kJ.

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.