Energy in Chemical Reactions

Energy in Chemical Reactions

Section 9.6 (Timberlake)

Energy Units

The SI unit for energy is the joule (J). You are more familiar with the energy unit, the calorie.

1 calorie = 4.184 Joules

No, you do not have to memorize this.

Heat of Reaction

The heat of reaction is the amount of heat absorbed or released during a reaction that takes place at constant pressure.

The heat of reaction is the difference in energy of products and reactants:

 $\Delta H\ =\ H_{products}-\ H_{reac\tan ts}$  

Heat of Reaction is also called...

...enthalpy change

...or change in enthalpy

Exothermic Reactions

• The energy of the products is LOWER than the energy of the reactants.

• If you're wondering what in the world that means...the energy is stored in the chemical bonds. Breaking bonds and forming bonds absorbs or releases energy.

•  $\Delta H\ is\ a\ negative\ number\ for\ exothermic\ reactions.$  

Endothermic Reactions

• The energy of the products is HIGHER than the energy of the reactants.

• You're probably less familiar with this type of reaction.  If you've ever used a cold pack from a first aid kit, that is an example of an endothermic reaction.

•  $\Delta H\ is\ a\ positive\ number\ for\ endothermic\ reactions.$  

Multiple Select

Which statement(s) are true for an exothermic reaction? Choose all that apply.

• The energy of the products is lower than the energy of the reactants.

• The energy of the products is higher than the energy of the reactants.

• The change in enthalpy is negative.

• The change in enthalpy is positive.

Multiple Select

Which statement(s) are true for an endothermic reaction? Choose all that apply.

• The energy of the products is lower than the energy of the reactants.

• The energy of the products is higher than the energy of the reactants.

• The change in enthalpy is negative.

• The change in enthalpy is positive.

Exothermic Reactions

Here is an example of an exothermic reaction:

 $H_2\left(g\right)+Cl_2\left(g\right)\rightarrow\ 2HCl\left(g\right)\ +\ 185\ kJ$  

Notice that 185 kJ is shown as a product.  That is because heat is a product.
You may also see this written as:

 $H_2\left(g\right)+Cl_2\left(g\right)\rightarrow\ 2HCl\left(g\right)\ \ \ \ \Delta H\ =\ -185kJ$  

Endothermic Reactions

Here is an example of an endothermic reaction:

 $N_2\left(g\right)+O_2\left(g\right)+180\ kJ\ \rightarrow\ 2NO\left(g\right)$  

Notice that 180 kJ is shown as a reactant.  That is because heat is required to convert the reactants to products.
You may also see this written as:

 $N_2\left(g\right)+O_2\left(g\right)\rightarrow\ 2NO\left(g\right)\ \ \ \ \Delta H\ =\ 180kJ$  

Multiple Choice

In the reaction of 1 mol of solid carbon with oxygen gas, the energy of the carbon dioxide gas produced is 393 kJ less than that of the reactants.

Is the reaction exothermic or endothermic?

• exothermic

• endothermic

• not enough information given.

Multiple Choice

In the reaction of 1 mol of solid carbon with oxygen gas, the energy of the carbon dioxide gas produced is 393 kJ less than that of the reactants.

What is the correct balanced chemical equation including the heat of reaction?

• C(s) + O2(g) + 393 kJ

  
  
  

  --> CO2(g)

• C(s) + O2(g) - 393 kJ

  
  
  

  --> CO2(g)

• C(s) + O2(g)

  
  
  

  --> CO2(g) + 393 kJ

• C(s) + O2(g)

  
  
  

  --> CO2(g) - 393 kJ

Multiple Choice

In the reaction of 1 mol of solid carbon with oxygen gas, the energy of the carbon dioxide gas produced is 393 kJ less than that of the reactants.

What is the value, in kilojoules, for the heat of reaction?

•  $\Delta H\ =\ 393\ kJ$  
  

•  $\Delta H\ =\ -393\ kJ$  
  

Calculations of Heat in Reactions

You'll be excited to learn that we can use enthalpy in conversion factors sort of like mole-mole factor.

2H₂O(l) + 572 kJ --> 2H₂(g) + O₂(g)

 $\frac{+572\ kJ}{2\ mol\ H_2O}\ \ or\ \ \frac{+572\ kJ}{2\ mol\ H_2}\ or\ \frac{+572\ kJ}{1\ mol\ O_2}$  


so if 9.00 g of water reacts, we can calculation the heat absorbed as:
 $9.00\ g\ H_2O\ \times\frac{1\ mol\ H_2O}{18.02\ g\ H_2O}\times\frac{+572\ kJ}{2\ mol\ H_2O}=\ +143\ kJ$  

Fill in the Blank

How much heat, in kJ, is released when nitrogen and hydrogen react to form 50.0 g of ammonia, NH3?

 $\Delta H\ =\ -92.2\ kJ$  
Report your answer to three significant figures, and don't forget units.

Hess's Law

We're going to hop on over to regular PowerPoint slides for this last topic...