CONTINUE>>>>>>>>>>>

The enthalpy changes for two different hydrogenation reactions of C2H2 are:

C2H2+H2---->C2H4 Delta H 1 (there is a degree sign....standard enthalpy of formation??)

*****WAIT A SECOND, IF I USE THE HEAT OF FORMATION VALUES TO CALCULATE THEN WOULD THAT GIVE THE ENTHALPY OF RXN???...which is not relevant to this question,right???
*****But if you change the sign to a neg then how do you know that it is positive originally???I AM VERY VERY PERPLEXED,DR.BOB!!!!!!!!!

I DO UNDERSTAND THAT YOU HAVE TO SWITCH ACCORDING TO HESS'S Law, but I DO NOT UNDERSTAND HOW YOU KNOW THE SIGNS......!!!!!!!!!!!!!!!!!!!!
Why is it pos to neg ??????????
I thought it is the other way around!!!!

BUT DO YOU UNDERSTAND WHAT I DO NOT GET??? IF NOT, I WILL TRY TO CLARIFY IT.

THANK YOU FOR YOUR PATIENCE!!!!

C2H2+2H2---->C2H6 Delta H 2 (there is a degree symbol)

Which expression represents the enthalpy change for the reaction below?

C2H4+H2---->C2H6 Delta H = ?

See your post above.

To determine the enthalpy change for the reaction C2H4 + H2 -> C2H6, we can use Hess's Law. According to Hess's Law, if we can manipulate and combine the given reactions in such a way that the desired reaction is obtained, then the enthalpy change for the desired reaction can be calculated.

In this case, we can use the given reactions:

1. C2H2 + H2 -> C2H4 (ΔH1)
2. C2H2 + 2H2 -> C2H6 (ΔH2)

To obtain the desired reaction, we need to cancel out C2H4 in reaction 1 and H2 in reaction 2. We can do this by multiplying reaction 1 by 2, and reaction 2 by 1/2:

3. 2C2H2 + 2H2 -> 2C2H4 (2 times reaction 1)
4. 1/2C2H2 + H2 -> 1/2C2H6 (1/2 times reaction 2)

Now, we can add reactions 3 and 4 together to obtain the desired reaction:

5. 2C2H2 + 2H2 + 1/2C2H2 + H2 -> 2C2H4 + 1/2C2H6

Simplifying this equation, we get:

6. 5/2C2H2 + 5/2H2 -> 2C2H4 + 1/2C2H6

Now, we can calculate the enthalpy change for the desired reaction by summing the enthalpy changes of reactions 3 and 4:

ΔH = ΔH3 + ΔH4

Since reaction 3 is scaled by a factor of 2, we need to double ΔH1:

ΔH = 2ΔH1 + ΔH2

Therefore, the expression that represents the enthalpy change for the reaction C2H4 + H2 -> C2H6 is:

ΔH = 2(ΔH1) + ΔH2

To determine the enthalpy change for the reaction C2H4 + H2 -> C2H6, you can use Hess's Law. Hess's Law states that the overall enthalpy change of a reaction is the same regardless of the pathway taken.

In this case, you can use the enthalpy changes given for the hydrogenation reactions of C2H2 to C2H4 and C2H2 to C2H6 to calculate the enthalpy change for the reaction C2H4 + H2 -> C2H6.

First, let's consider the reaction C2H2 + H2 -> C2H4. The enthalpy change is represented by Delta H1.

Next, let's consider the reaction C2H2 + 2H2 -> C2H6. The enthalpy change is represented by Delta H2.

To calculate the enthalpy change for the reaction C2H4 + H2 -> C2H6, we need to reverse the first equation and multiply it by a factor to match the stoichiometric coefficients in the desired reaction. In this case, we reverse the first equation and multiply it by 1/2 to get:

C2H4 -> C2H2 + 1/2H2

The enthalpy change for this reversed reaction would be -Delta H1.

Next, we have the reaction:

C2H6 -> C2H2 + 3H2

The enthalpy change for this reaction is -Delta H2.

Now, if we add the reversed first equation and the second equation, we get:

-1/2Delta H1 + -Delta H2

Simplifying this expression gives us the enthalpy change for the desired reaction:

-1/2Delta H1 - Delta H2

Therefore, the expression that represents the enthalpy change for the reaction C2H4 + H2 -> C2H6 is -1/2Delta H1 - Delta H2.