Which of the following statements is true about chemical reactions?

endothermic reactions absorb heat and exothermic reactions give off heat
exothermic reactions absorb heat and endothermic reactions give off heat
endothermic and exothermic reactions absorb heat
endothermic and exothermic reactions give off heat

you didnt put any of the following

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