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

Md Ariful Islam

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Md Ariful Islam

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

A thermochemical equation is simply a balanced chemical equation that includes the 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 .

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 $color(red)(2):1$ 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 .

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

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