Hess's Law

The enthalpies of combustion of C(s), H2(g) and C4H9OH(l) (in kJmol^-1) are as follows  
C(s) + O₂(g)   ->  CO₂(g)                                               ∆H=a
H₂(g) + ½O₂(g)   ->   H₂O(l)                                        ∆H=b
C₄H₉OH(l) + 6O₂(g)   ->   4CO₂(g) + 5H₂O(l)  ∆H=c
What is the enthalpy change for the reaction shown below?
  4C(g) + 5H₂(l) + ½O₂(g)   ->   C₄H₉OH(l)

• c – 4a – 5b
• 2a + 10b - c
• 4a + 5b - c
• 2a + 5b + c

What is the relationship between enthalpies p, q, r and s?

S(s) + H₂(g) -> H₂S(g) ∆H=p

H₂(g) + ½O₂(g) -> H₂O(l) ∆H=q

S(s) + O₂(g) -> SO₂(g) ∆H=r

H₂S(g) + 1½O₂(g) -> H₂O(l) + SO₂(g) ∆H=s

• p = q + r – s

• p = s – q – r

• p = q – r – s

• p = s + r – q

The enthalpy change for the reaction
C(s, graphite) + 1⁄2O₂(g) --> CO(g) cannot be measured directly since some carbon dioxide is always formed in the reaction.
It can be calculated using Hess’s Law and the enthalpy changes of combustion of graphite and of carbon monoxide.
C(s, graphite) + O₂(g) --> CO₂    ΔH=-394 kJmol^–1
CO(g) + 1⁄2O₂(g) --> CO₂  ΔH=-283 kJmol^–1  
The enthalpy change for the reaction of graphite with oxygen to give carbon monoxide is 

• -677 kJmol^–1 
• +111 kJmol^–1 
• -111 kJmol^–1 
• +677 kJmol^–1 

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

(Hess's Law)Calculate the ∆H for the following reaction: 2H₂O₂ →  2H₂O  + 1 O₂ 
You are given these two equations:
2H₂  +  O₂ → 2H₂O            ∆H  =  -572 kJ
H₂  +  O₂  →  H₂O₂            ∆H  =  -188 kJ 

• ∆H  =  -948 kJ 
• ∆H  =  -196 kJ 
• ∆H  =  -384 kJ 
• ∆H  =  -188 kJ 

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

Hess's Law states that...

• all reactions are reversible

• the enthalpy change of a reaction is independent of the route taken

• the direct route results in a greater enthalpy change

• the indirect route results in a lower enthalpy change as energy is lost in the second step

Match Definition to Term:

Heat of Vaporization

• The difference in energy between the solid and liquid states for a given amount of substance

• The difference in energy between the liquid and gas states for a given amount of substance

• The energy associated for a 1 degree temperature change for a given amount of a substance

Match Definition to Term:

Heat of Fusion

• The difference in energy between the solid and liquid states for a given amount of substance

• The difference in energy between the liquid and gas states for a given amount of substance

• The energy associated for a 1 degree temperature change for a given amount of a substance

The Law of Conservation of Energy states:

• Energy can created or destroyed but not transformed 
• Energy cannot be created or destroyed, it can only transformed
• Energy can't be created, destroyed or transformed

Tags

NGSS.HS-PS3-3

NGSS.HS-PS3-4

4. The enthalpies of combustion of C(s), H2(g) and C4H9OH(l) (in kJmol^-1) are as follows

C(s) + O₂(g) -> CO₂(g) ∆H=-393kJ/mol

H₂(g) + ½O₂(g) -> H₂O(l) ∆H=-286kJ/mol

C₄H₉OH(l) + 6O₂(g) -> 4CO₂(g) + 5H₂O(l) ∆H=-1371kJ/mol

What is the enthalpy change for the reaction shown below?

4C(s) + 5H₂(l) + ½O₂(g) -> C₄H₉OH(l)

• +845kJ

• -2275 kJ

• -1631kJ

• -845

When you multiply a chemical equation by 2, what is done to the heat of reaction?

• multiplied by 1/2

• multiplied by 2

• nothing