What is the enthalpy of reaction from the following energy diagram?
a) -20 kj
b) +20 kj
c) +40 kj
d) +60 kj
e) +100 kj

The enthalpy of reaction is the amount of heat that is required or to be released in order to convert the reactants to the products. this can be calculated through the equation,
 
                            δh = h(products) - h(reactants)
or in this item,
                              δh = h(c + d) - h(a + b)

from the graph, h(c + d) = 20 and h(a + b) = 40.

substituting the known values,
                            δh = 20 kj - 40 kj
                                  δh = -20 kj

thus, the answer to this item is letter a.