The localization exact sequence.

Question

Let $X$ a scheme and $Y$ a close subscheme and $i:Y \to X$ the inclusion. Let $U:=(X-Y)$ and $j:U \to X$ the inclusion map. If I denote with $CH(-)$ the Chow group, the sequence $$ CH_r(Y) \stackrel{i_*}{\to} CH_r(X) \stackrel{j^*}{\to} CH_r(U) \to 0 .$$ is exact for every $r$. I have to prove that the proper push-forward $i_*$ and the flat pull-back $j^*$ are well-defined. My question is: why if $i$ is proper then $i_*$ is well.defined and why if $j$ is flat then $j^*$ is well-defined? Thanks in advance for attemption.