Bond Energy Calculations

When the solids Ba(OH)₂ and NH₄SCN are mixed, a solution is produced and the temperature drops. Which statement about the energetics of this reaction is correct?

• The reaction is endothermic and ΔH is negative

• The reaction is endothermic and ΔH is positive

• The reaction is exothermic and ΔH is negative

• The reaction is exothermic and ΔH is positive

Tags

NGSS.HS-PS1-4

The following equation shows the formation of magnesium oxide from magnesium metal. Which statement is correct for this reaction?

2Mg(s) + O₂(g) → 2MgO(s) ΔH = -1204 kJ

• 1204 kJ of energy are released for every mole of magnesium reacted

• 602 kJ of energy are absorbed for every mole of magnesium oxide formed

• 602 kJ of energy are released for every mole of oxygen reacted

• 1204 kJ of energy are released for every two moles of magnesium oxide formed

Tags

NGSS.HS-PS1-4

For the reaction 2H₂(g) + O₂(g) → 2H₂O(g) the bond enthalpies (in kJ/mol) are

H-H = x, O=O = y, O-H = z

Which calculation will give the value, in kJ/mol, of ΔH for the reaction?

• 2x + y - 2z

• 4z - 2x - y

• 2x + y - 4z

• 2z - 2x - y

Tags

NGSS.HS-PS1-4

Which is an example of an endothermic physical reaction? 

• a) Ice melting 
• b) Combustion 
• c) Steam condensing 
• d) Photosynthesis

Using the table of average bond energies below the image, the ΔH for the reaction in kJ is.....

• + 160 kJ

• -63 kJ

• - 160 kJ

• -217 kJ

Tags

NGSS.HS-PS1-4

Does breaking bonds require energy, release energy, or not involve energy at all?

• requires energy to break

• releases energy when broken

• No energy involved

Tags

NGSS.HS-PS1-4

What kind of energy change happens in the photosynthesis reaction in plant leaves?

• Endothermic

• Exothermic

• No change

Tags

NGSS.HS-LS1-5

Is this equation endo- or exo-thermic?

PCl3 (s) + Cl2 (g) --> PCl5 (s) + energy

• endothermic

• exothermic

Tags

NGSS.HS-PS1-4

Is this equation endo- or exo-thermic?

2KNO3 (s) + energy --> 2KNO2 (s) + O2

• endothermic

• exothermic

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1

How can we figure out if an overall reaction is endo- or exo-thermic? It is the difference between _______ and __________.

• hot and cold temperatures

• energy to break and energy to form bonds.

• time to form a bond versus time to break a bond

Tags

NGSS.HS-PS1-4