1 . Compound A decomposes at room temperature in an exothermic reaction while compound B requires heating before it will decompose in an endothermic reaction.

Draw reaction profiles (Energy vs. reaction progress) for both reactions.

Write out a general equation relating the reactants, products, and energy. (i.e. on which side does the energy term go)

Heat Review

Ene

rgy

Ene

rgy

Reaction progress Reaction progress

Compound A Compound B

Exothermic Endothermic

Exothermic: Reactants → Products + kJ

Endothermic: Reactants + kJ → Products

(Energy “exits” or is released)

(Energy “enters” or is required)

 

Heat = 400.0 g x 80.0 oC x 4.18 J/goC 

Heat = 133760 J (gained by H2O)

2. A 500.0 g piece of iron is heated in a flame and dropped into 400.0 g of water at 10.0 oC. The temperature of the water rises to 90.0 oC. How hot wasthe iron when it was first removed from the flame?(Specific heat of iron = 0.473 J/goC)

 Heat gained by H2O = Heat lost by metal 

First, find the heat gained by the water :

  

133760 J = 500.0 g x ∆T x 0.473 J/goC

566 oC = ∆T

 

Heat gained by H2O = Heat lost by metal

   

90.0oC + 566oC = 656oC = Initial temp.

3. The decomposition of 2.50 g of ammonium nitrate to form dinitrogen monoxide and water releases 1.16 kJ of heat. Write the balanced equation including the energy term, and indicate whether the reaction is endothermic or exothermic.NH4NO3 → N2O + 2 H2O

NH4NO3 → N2O + 2 H2O + ? kJ

Exothermic

2.50 g NH4NO3 1 mol = 0.0313 mol 80.0 g

1.16 kJ = 37.1 kJ / mol NH4NO3

0.0313 mol NH4NO3

NH4NO3 → N2O + 2 H2O + ? kJ

NH4NO3 → N2O + 2 H2O + 37.1 kJ