How many kilojoules are released when 111 g of #Cl_2# reacts with silicon in the reaction #Si(s)+2Cl_2(g) ->SiCl_4(g)# #ΔH = - 657kJ?#

Notice that the problem provides you with the thermochemical equation for this synthesis reaction.

A thermochemical equation is simply a balanced chemical equation that includes the enthalpy change of reaction, #DeltaH_"rxn"#, for that given chemical reaction.

In this case, you have

#"Si"_text((s]) + color(red)(2)"Cl"_text(2(g]) -> "SiCl"_text(4(g])", "DeltaH_text(rxn) = -"657 kJ"#

This means that when one mole of silicon tetrachloride, #"SiCl"_4#, is formed, the reaction gives off #"657 kJ"# of heat. Since heat is being given off, hence the negative sign for the enthalpy change oi reaction, you know that you're dealing with an exothermic reaction.

Your goal now will be to determine how many moles of silicon tetrachloride will be produced by that #"111-g"# sample of chlorine gas.

To do that, start by figuring out how many moles of chlorine gas are found in that many grams

#111 color(red)(cancel(color(black)("moles Cl"_2))) * "1 mole Cl"_2/(70.906color(red)(cancel(color(black)("moles Cl"_2)))) = "1.565 moles Cl"_2#

Notice that you have a #color(red)(2):1# mole ratio between chlorine gas and silicon tetrachloride. This means that the reaction will produce

#1.565 color(red)(cancel(color(black)("moles Cl"_2))) * "1 mole SiCl"_4/(color(red)(2)color(red)(cancel(color(black)("moles Cl"_2)))) = "0.7825 moles SiCl"_4#

So, if the reaction gives off #"657 kJ"# of heat when one mole of product is formed, it follows that producing #0.7825# moles of silicon tetrachloride will give off

#0.7825 color(red)(cancel(color(black)("moles SiCl"_4))) * "657 kJ"/(1color(red)(cancel(color(black)("mole SiCl"_4)))) = color(green)("514 kJ")#

The answer is rounded to three sig figs.

So, you can say that

The reaction gives off #"514 kJ"# of heat when #0.7825# moles of silicon tetrachloride are produced

or

The reactions has #DeltaH_"rxn" = -"514 kJ"# when #0.7825# moles of silicon tetrachloride are produced