Answer:
Explanation:
(a) Part 1: 
reaction. This is a nucleophilic substitution reaction in which we have two steps. Firstly, chlorine, a good leaving group, leaves the carbon skeleton to form a relatively stable secondary carbocation. This carbocation is then attacked by the hydroxide anion, our nucleophile, to form the final product.
To summarize, this mechanism takes places in two separate steps. The mechanism is attached below.
Part 2: 
reaction. This is a nucleophilic substitution reaction in which we have one step. Our nucleophile, hydroxide, attacks the carbon and then chlorine leaves simultaneously without an intermediate carbocation being formed.
The mechanism is attached as well.
(b) The rate determining step is the slow step. Formation of the carbocation has the greatest activation energy, so this is our rate determining step for . For , we only have one step, so the rate determining step is the attack of the nucleophile and the loss of the leaving group.
Answer:
Y Q W Z X
Explanations:
The most reactive element is the element that will displace an element from it compound . The most reactive element will replace the less reactive element in it compound.
Q+ + Y Reaction occurs
Since the reaction occurs the element Y which is more reactive displaced element Q from it compound.
Q+W+ Reaction occurs
The reaction occurs, that means element Q replaces element w from it compound. Element Q is therefore more reactive than element W.
W+Z+ Reaction occurs
The reaction also occurs . This is an indication that element W replaces element Z in it compound. This means element W is very reactive than element Z.
X+Z+ No reaction
There is no reaction here. This is an indication that element X is less reactive than element Z. This is why element X can't displace element Y in it compound.
What they have in common is that they both have the same number of atoms.
I think it might be Selenium.
