Given the balanced ionic equation representing a reaction:
2a1(s) + 3cu2+(aq) ==> 2a13+(aq) + 3cu(s)
which half-reaction represents the reduction that occurs? (i ) al ==> al3+ + 3e-
(2) al3+ + 3e- ==> al
(3) cu ==> cu2+ + 2e-
(4) cu2+ + 2e- ==> cu

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