Which of the following reactions is not a replacement reaction?
fe + cuso4 → feso4 + cu
2na + 2h2o → 2naoh + h2
zn + 2hcl → zncl2 + h2
2na + cl2 → 2nacl

the last one because it has only has one product!

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