1. determine total δh for bonds broken in the reactants
(hint: there are two h2), answers: 935 kj/mol 1,371 kj/mol or 1,398 kj/mol

2. determine total δh for bonds formed in the products
(hint: there are two h2o), answers: 463 kj/mol 926 kj/mol or 1,852 kj/mol

3. determine overall change in δh, answers: positive or negative

4. record final answer with units, answers: 472 kj/mol 481 kj/mol or 917 kj/mol

- lead + phosphoric acid = hydrogen + lead(ii) phosphate

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.