Hess' Law

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

The enthalpy change for the reaction, ∆H_r , is equal to 

• ∆H₁ + ∆H₂
• ∆H₁ - ∆H₂
• -∆H₁ - ∆H₂
• -∆H₁ + ∆H₂

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

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

Tags

NGSS.HS-PS1-4

The standard enthalpy changes of combustion of carbon, hydrogen and methane are shown in the table. 
Which one of the following expressions gives the correct value for the standard enthalpy change of formation of methane in kJ mol^–1?
C(s) + 2H₂(g) → CH₄(g) 

• 394 + (2 × 286) – 891 
• –394 – (2 × 286) + 891 
• 394 + 286 – 891 
• –394 – 286 + 891 

Tags

NGSS.HS-PS1-4

Given the following data: ΔH^○_f[FeO(s)] = –270kJmol^–1
ΔH^○_f [Fe₂O₃(s)] = –820 kJ mol^–1
Select the expression which gives the enthalpy change, in kJ mol^–1, for the reaction:
2FeO(s) + 1⁄2O₂(g) → Fe₂O₃(s) 

• (–820 × 1⁄2) + 270 = –140
• (+820 × 1⁄2) – 270 = +140
• –820 + (270 × 2) = –280 
• +820 – (270 × 2) = +280 

Tags

NGSS.HS-PS1-4

The standard enthalpy changes of formation of iron(II) oxide, FeO(s), and aluminium oxide, Al₂O₃(s), are –266 kJ mol^–1 and –1676 kJ mol^–1 respectively.
What is the enthalpy change under standard conditions for the following reaction?
3FeO(s) + 2Al (s)   ->   3Fe(s) + Al₂O₃(s) 

• +878kJ 
• –878kJ 
• –1942kJ 
• –2474kJ 

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

Use the chart to answer the following question.
Which is correct about energy changes during bond breaking and bond formation? 

• A
• B
• C
• D

Tags

NGSS.HS-PS1-4

C₂H₄(g) + H₂(g)   ->   C₂H₆(g)  ∆H°=-137 kJ mol^-1
Which statement about this information is correct?

• The total energy of the bonds broken in the reactants is greater 
  than the total energy of the bonds 
  formed in the product 
• The bonds broken and the bonds made are of the same strength 
• The total energy of the bonds broken in the reactants is less than the total energy of the bonds formed in the product 
• No conclusion can be made about the sums of the bond enthalpies in the product compared with the reactants 

Tags

NGSS.HS-PS1-4