Which of the following does not react with oxygen directly? $Zn$,$Ti$,$Pt$,$Fe$

What is the general molecular formula of the products obtained on heating lanthanoids $(Ln)$ with sulphur?

$(a)$ Transition metals can act as catalysts because they can change their oxidation state. How does $Fe(III)$ catalyze the reaction between iodide and persulphate ions?
$(b)$ Mention any three processes where transition metals act as catalysts.

Assertion : $PbI_4, BiCl_5, FeI_3, CuI_2$ do not exist. Reason : Higher oxidation states of metal ions are strong oxidising agents which give redox reactions with the iodide ion.

Arrange the oxides $CrO$,$CrO_3$,and $Cr_2O_3$ in the decreasing order of acidic strength.

Which of the following compounds reacts with $NaOH$ and $Na_2O_2$ to produce a yellow color?