Which statement best characterizes chemical reactions?

they always occur very quickly.
their rates vary significantly.
their rates are always slow.
they rarely occur in the natural environment.

Option (b) is the correct answer.

Explanation:

Chemical reactions are the reactions with the help of which rearrangement of molecular or ionic structure of a substance can be done.

The rate of a chemical reaction depends on various factors like nature of reactants, catalyst etc. Therefore some chemical reactions are fast, some are slow or some may be too slow.

Thus, we can conclude that their rates vary significantly.

answer : third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol

explanation :

let us write down the given equations

equation 1 :

equation 2 :

equation 3 :

let us use hess's law to find out which 2 equations can be added to get the third equation.

let us reverse equation 2 and multiply it by 2. the new equation that we get is,

equation 4 :

let us add equation 1 & 4

the above equation is similar to equation 3 but has different value for dhrxn.

therefore third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol