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.
Understanding Chemistry: Enthalpy and Heat of Reaction
Chemical reactions rearrange atoms, but the energy change comes from changes in chemical bonds. Breaking a bond needs an input of energy. Forming a bond releases energy.
A reaction gives out heat when bond formation releases more energy than bond breaking requires. It takes in heat when the required bond breaking costs more. This bond view explains why a reaction can involve both energy input and energy release while still having one overall heat change.
It also explains why the same elements can have different enthalpies in different substances. Their atoms are connected in different ways, with different bond strengths.
Chemists measure reaction heat with calorimetry. A simple calorimeter may be an insulated cup holding water and a reacting mixture. The reaction transfers energy to or from the water.
By measuring the water temperature change, students can calculate the heat transferred. The calculation uses mass times specific heat capacity times temperature change. Water is useful because its specific heat capacity is well known.
The reaction heat has the opposite sign to the heat change of the water. If the water warms, the reaction released heat. Real experiments lose some energy to the cup, thermometer, and air, so measured values are often less exact than textbook values.
A balanced chemical equation must include physical states when it represents an enthalpy change. Liquid water, water vapour, and ice have different enthalpies even though each contains the same molecules. Melting, boiling, dissolving, and mixing can therefore involve heat changes without making a new substance.
The amount written in an equation matters too. Enthalpy change is linked to the exact quantities in the balanced equation. If every coefficient is doubled, the heat change doubles.
If an equation is reversed, the direction of energy transfer reverses. These rules are important when using tabulated enthalpy values in calculations.
Many enthalpy problems use Hess's law. This law states that the total enthalpy change depends only on the starting substances and final substances, not on the route taken. Chemists can add several known equations to find the heat change for a reaction that is difficult to measure directly.
They reverse equations, multiply them by whole numbers, then cancel substances that appear on both sides. This works because enthalpy is a state function. Students should carefully track states, coefficients, and signs at every step.
Combustion of fuels, hand warmers, instant cold packs, cooking, and respiration all show energy changes in everyday life. Temperature change alone is not enough to identify the reaction heat, since the amount of material and heat lost to the surroundings affect what is observed.
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.