Enthalpy is a way to track heat energy in chemical reactions at constant pressure, which is the most common condition for reactions in open containers. The heat of reaction is the change in enthalpy, written as delta H, between reactants and products. It matters because the sign and size of delta H tell whether a reaction releases heat to the surroundings or absorbs heat from them.
Energy diagrams make these changes visible by comparing the enthalpy of reactants and products as a reaction proceeds.
In an exothermic reaction, products have lower enthalpy than reactants, so delta H is negative and heat is released. In an endothermic reaction, products have higher enthalpy than reactants, so delta H is positive and heat is absorbed. The peak on an energy diagram represents the transition state, and the energy needed to reach it is the activation energy.
Catalysts lower activation energy but do not change delta H because they do not change the enthalpy of reactants or products.
Key Facts
- delta H = H_products - H_reactants
- Exothermic reaction: delta H < 0, heat is released.
- Endothermic reaction: delta H > 0, heat is absorbed.
- At constant pressure, q_p = delta H.
- Activation energy: E_a = H_transition state - H_reactants.
- A catalyst lowers E_a but does not change delta H.
Vocabulary
- Enthalpy
- Enthalpy is the total heat content of a system at constant pressure, represented by H.
- Heat of reaction
- Heat of reaction is the enthalpy change, delta H, that occurs when reactants form products.
- Exothermic reaction
- An exothermic reaction releases heat to the surroundings and has a negative delta H.
- Endothermic reaction
- An endothermic reaction absorbs heat from the surroundings and has a positive delta H.
- Activation energy
- Activation energy is the minimum energy needed for reactant particles to reach the transition state and react.
Common Mistakes to Avoid
- Reversing the sign of delta H: Products lower than reactants means delta H is negative, not positive, because delta H = H_products - H_reactants.
- Calling heat released positive: In chemistry, heat leaving the reacting system gives delta H < 0 for an exothermic reaction.
- Confusing activation energy with delta H: Activation energy is the climb to the peak of the diagram, while delta H is the difference between products and reactants.
- Thinking catalysts change the heat of reaction: Catalysts change the reaction pathway and lower activation energy, but they do not change the enthalpy of reactants or products.
Practice Questions
- 1 A reaction has H_reactants = 250 kJ/mol and H_products = 90 kJ/mol. Calculate delta H and state whether the reaction is exothermic or endothermic.
- 2 In an energy diagram, the reactants are at 40 kJ/mol, the transition state is at 115 kJ/mol, and the products are at 70 kJ/mol. Find the activation energy and delta H.
- 3 A catalyst is added to an endothermic reaction. Explain what changes on the energy diagram and what stays the same.