"record the time to complete a chemical reaction" would most likely be part of a lab procedure.
answer : third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol
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