Quick hess's law question

Monoclinic sulphur is formed in volcanic regions by reaction between sulphur dioxide and hydrogen sulphide according to the equation.
SO2 + 2H2S --> 2H2O + 3S

Draw an enthalpy diagram or cycle and calculate the standard enthalpy change for this reaction.

Here are some values.

Standard enthalpy of formation
H2O(l) -286
H2S(g) - 20.2

Standard enthalpy of combustion
S (monoclinic) -297.2

Widowmaker

Monoclinic sulphur is formed in volcanic regions by reaction between sulphur dioxide and hydrogen sulphide according to the equation.
SO2 + 2H2S --> 2H2O + 3S

Draw an enthalpy diagram or cycle and calculate the standard enthalpy change for this reaction.

Here are some values.

Standard enthalpy of formation
H2O(l) -286
H2S(g) - 20.2

Standard enthalpy of combustion
S (monoclinic) -297.2

equation 1: H2 + 1/2O2 --> H2O -286
equation 2: H2 + S --> H2S -20.2
equation 3: S + O2 --> SO2 -297.2

multiply E2 by 2
2H2 + 2S --> 2H2S -40.4

add E3
equation 4: 2H2 + 3S + O2 --> SO2 + 2H2S -337.6

multiply E1 by 2
equation 5: 2H2 + O2 --> 2H2O -572

subtract 4 from 5
- 3S ---> 2H2O - SO2 - 2H2S -234.4

rearrange
SO2 + 2H2S --> 2H2O + 3S -234.4

Reply 2

Christophicus

OP

18

charco

equation 1: H2 + 1/2O2 --> H2O -286
equation 2: H2 + S --> H2S -20.2
equation 3: S + O2 --> SO2 -297.2

multiply E2 by 2
2H2 + 2S --> 2H2S -40.4

add E3
equation 4: 2H2 + 3S + O2 --> SO2 + 2H2S -337.6

multiply E1 by 2
equation 5: 2H2 + O2 --> 2H2O -572

subtract 4 from 5
- 3S ---> 2H2O - SO2 - 2H2S -234.4

rearrange
SO2 + 2H2S --> 2H2O + 3S -234.4

nice method!
I couldn't have done that!

Hess' law allows you to manipulate chemical equations in the same way as maths equations... The beauty of it is that you always arrive at the correct answer as if by magic providing that you follow the four rules of number.

Look at your target equation and construct it bit by bit by manipulating the other equations.

For example
Target equation is SO2 + 2H2S --> 2H2O + 3S

So I need to get SO2 and 2H2S on the same side of the reaction arrow
I just look for other equations that have these components and combine them.

Having done that I arrive at equation 4: 2H2 + 3S + O2 --> SO2 + 2H2S
Now its easy to see that there are things that I don't want here i.e. the 2H2 + O2 so I look to find another equation with these components in to subtract (there will always be one, although you might have to multiply or divide another equation to get there)

In this case doubling the enthalpy of formation of water gives me 2H2 + O2 --> 2H2O and this is perfect for my purposes

Subtract one from the other (it really dpoesn't matter which way round as long as you are careful with the signs) and Robert is your mother's sisters husband!

There are many different ways to combine the other equations but they will all give the correct answer eventually. If you inspect the equations given you can usually see the fastest way to the target equation.

It's a question of practice I suppose...

Quick hess's law question

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