Acarbon-carbon double bond has a greater bond energy than a carbon-carbon single bond. because of the greater bond energy, a carbon-carbon double bond is and than a carbon-carbon single bond.

When a C atom forms a bond with another C atom , then the bond formed is a sigma bond. And we all know that if a bond is formed then energy is released. And stronger the bond ..it will be more stable so more energy will be released.

Now when the C atom forms the double bond then a pie bond is formed. Similarly for a triple bond also a pie bond is formed. And we read that a pie bond is weaker than a sigma bond because of less overlapping between the orbitals. So less energy will be released than the first bond.

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