Unit 9 Heat in Chemical Reactions
Background
• Most chemical reactions involve energy – more specifically, changes in energy.
• In a chemical reaction, existing bonds are broken, atoms are rearranged, and new bonds are formed.
• Because bond breaking requires energy and bond formation releases energy, almost all chemical
reactions either absorb or release energy.
• This results in energy flow that is familiar to you as heat.
• Do not confuse heat with temperature.
• Temperature is the degree of hotness or coldness of an object (°C)
• Heat is defined as the energy that is transferred from one object to another due to a difference in
temperature.
The study of the changes in heat in chemical reactions is called thermochemistry.
• Reactions that release heat are called exothermic reactions (heat exiting)
• Reactions that absorb heat are called endothermic reactions (heat coming in)
Exothermic Reactions
The combustion of propane is an exothermic reaction.
1C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) + 2043 kJ
• Heat is produced in this combustion reaction because the energy released as new bonds are formed in
the products is greater than the energy required to break the old bonds in the reactants.
• Since heat is released, it is placed on the product side of the equation
Endothermic Reactions
Water gas, which is a mixture of carbon monoxide and hydrogen, is prepared by passing steam over hot coal
(coal is mostly carbon).
1C(s) + 1H2O(g) + 113 kJ → 1CO(g) + 1H2(g)
• This reaction absorbs heat, so it is endothermic.
• Since heat is needed for the reaction, it is placed on the reactant side of the equation.
Heat and Enthalpy Changes
The heat absorbed or released in a chemical reaction depends on the difference in a quantity called enthalpy.
• Enthalpy is represented by the capital letter H.
• Another expression for enthalpy is heat content.
• The reactants have a certain heat content and the products have a certain heat content.
• When pressure remains constant, the heat absorbed or released during a chemical reaction is equal to
the enthalpy change for the reaction.
Enthalpy Change, ∆H
The enthalpy change for a reaction is written as the enthalpy of the products minus the enthalpy of the
reactants.
The following equation is utilized for solving change in enthalpy problems:
∆H = Hproducts - Hreactants
For exothermic reactions, the heat content of the reactants is greater than the heat content of the
products.
• The ∆H for an exothermic reaction always has a negative sign. (does not mean negative energy)
For endothermic reactions, the heat content of the products is greater than the heat content of the
reactants.
• The ∆H for an endothermic reaction always has a positive sign.
Please note: (not necessarily needed this year, but must understand for college or AP chemistry)
• Standard conditions for thermochemical reactions is normal atmospheric pressure and room
temperature (ie. 1 atmosphere and 25°C)
• Also, the reactants and products are in their standard states. (The standard state for a substance is
defined as its pure form at 1 atmosphere.)
• When a pure element is involved, it must be present in its most stable form at standard pressure.
• In order to determine the change in enthalpy (heat of reaction) for any chemical reaction, you need
to consult a table of standard heats of formation for the substances involved.
Very Important!!!!
• The standard heat of formation for any element is defined to be zero (0 kJ/mol)
Use the following steps for solving change in enthalpy problems.
Step 1 Write the complete balanced equation (this will be given to you this year)
Step 2 Get ∆Hf° from table of info and place under the compounds in the equation
Step 3 Multiply ∆Hf° by moles involved (ie. the coefficients)
Step 4 Add the reactant answers together; add the product answers together
Step 5 Solve the equation
∆H = ∆Hf°products - ∆Hf°reactants
• If the answer is +, then endothermic
• If the answer is -, then exothermic
What should you know for the SOL test?
You should know that you are dealing with an exothermic reaction when you see any of the
following…
1C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g) + 2043 kJ
1C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
∆H = - 2043 kJ
You should know that you are dealing with an endothermic reaction when you see any of the
following.
1C(s) + 1H2O(g) + 113 kJ → 1CO(g) + 1H2(g)
1C(s) + 1H2O(g)
→ 1CO(g) + 1H2(g)
∆H = + 113 kJ
Example:
2C2H6
+
7O2
4CO2
+
6H2O