Question

The `DeltaH` value for the reaction `1/2O_2 (g) + Hg(l) -> HgO (s)` is -90.8 kJ. How many kJ are released when 66.9 g `Hg` is reacted with oxygen?

Answer 1

`"30.3 kJ"`

Explanation:

Start by taking a look at the thermochemical equation given to you

`1/2"O"_ (2(g)) + "Hg"_ ((l)) -> "HgO"_ ((s))" "DeltaH = -"90.8 kJ"`

This equation tells you that when `1` mole of mercury reacts with `1/2` moles of oxygen gas, `"90.8 kJ"` of heat are being given off by the reaction.

Keep in mind that the minus sign used in the expression of the enthalpy change of reaction, `DeltaH`, symbolizes heat given off.

So, you now want to know how much heat will be released when `"66.9 g"` of mercury take part in the reaction. Since you know how much energy is released when `1` mole of mercury reacts, use the element's molar mass to convert the mass to moles

`66.9 color(red)(cancel(color(black)("g"))) * "1 mole Hg"/(200.6color(red)(cancel(color(black)("g")))) = "0.3335 moles Hg"`

You can now use the known `DeltaH` to determine how much heat is released when `0.3335` moles of mercury react

`0.3335 color(red)(cancel(color(black)("moles Hg"))) * "90.8 kJ"/(1color(red)(cancel(color(black)("mole Hg")))) = color(green)(|bar(ul(color(white)(a/a)color(black)("30.3 kJ")color(white)(a/a)|)))`

The answer is rounded to three sig figs.

This is equivalent to saying that when `"66.9 g"` of mercury undergo combustion, the enthalpy change of reaction is

`DeltaH_"rxn" = -"30.3 kJ"`

Once again, the minus sign symbolizes heat given off.