AP Chemistry 6.1-6.2: Endothermic/Exothermic and Energy Diagrams

Energy

Energy is the capacity to transfer heat or perform work. In chemistry, energy changes commonly accompany chemical reactions and physical changes.

How are potential energy and kinetic energy distinguished?

Potential energy results from an object's position, composition, or condition. Kinetic energy is the energy of motion.

What is an example of chemical potential energy being converted into kinetic energy?

During gasoline combustion, chemical potential energy in the fuel is converted into the kinetic and mechanical energy of expanding gases that move engine pistons.

Law of conservation of energy

Energy cannot be created or destroyed during a chemical or physical change; it can only be transferred or converted from one form to another.

Thermal energy

Thermal energy is the kinetic energy associated with the random motion of atoms and molecules in matter.

Temperature

Temperature is a quantitative measure related to the average kinetic energy of particles. Faster average particle motion corresponds to a higher temperature.

How do thermal energy and temperature differ?

Thermal energy is the total random-motion energy of the particles in a sample, whereas temperature reflects the particles' average kinetic energy. Thermal energy therefore depends on both temperature and the amount of matter.

What happens to temperature when thermal energy changes in a substance without a phase change or chemical reaction?

Adding thermal energy generally increases the temperature, while removing thermal energy decreases it. During a phase change, however, energy can instead change particle arrangement without changing temperature.

Heat ($q$)

Heat is the transfer of thermal energy between objects or substances at different temperatures. Heat spontaneously transfers from the hotter object to the cooler object until thermal equilibrium is reached.

Exothermic process

An exothermic process releases heat from the system to the surroundings, so $q$ for the system is negative. The surroundings generally warm, and the products have lower enthalpy than the reactants.

Endothermic process

An endothermic process absorbs heat from the surroundings into the system, so $q$ for the system is positive. The surroundings generally cool, and the products have higher enthalpy than the reactants.

How can bond-energy changes help explain whether a chemical reaction is endothermic or exothermic?

Breaking bonds requires energy, while forming bonds releases energy. A reaction is exothermic when bond formation releases more energy than bond breaking requires; it is endothermic when bond breaking requires more energy than bond formation releases.

On a chemical energy diagram, what do the axes represent?

The vertical axis represents potential energy, usually in kJ or kJ/mol. The horizontal axis represents the progress of the reaction, not time or a direct measure of reaction rate.

How can an energy diagram distinguish an exothermic reaction from an endothermic reaction?

Compare the energy levels of the products and reactants. In an exothermic reaction, the products are lower in energy than the reactants and $\Delta H<0$. In an endothermic reaction, the products are higher in energy and $\Delta H>0$.

How is the enthalpy change, $\Delta H$, determined from an energy diagram?

The enthalpy change is the difference between product and reactant energy: $\Delta H=H_{\mathrm{products}}-H_{\mathrm{reactants}}$. The sign indicates whether the reaction is endothermic or exothermic.

What is activation energy on an energy diagram?

Activation energy, $E_a$, is the minimum energy required for reactants to reach the transition state or activated complex. For the forward reaction, $E_a=E_{\mathrm{transition\ state}}-E_{\mathrm{reactants}}$.

How is the activation energy for the reverse reaction found from an energy diagram?

For the reverse reaction, subtract the product energy from the transition-state energy: $E_{a,\mathrm{reverse}}=E_{\mathrm{transition\ state}}-E_{\mathrm{products}}$. The forward and reverse activation energies are generally different.

What is the transition state on an energy diagram?

The transition state, or activated complex, is the highest-energy arrangement along the reaction pathway. It corresponds to the peak of the energy diagram and separates reactants from products.

How does a catalyst affect an energy diagram?

A catalyst provides an alternative reaction pathway with a lower activation energy. It does not change the energies of the reactants or products, so it does not change $\Delta H$.