Chemical Energy (College Board AP® Chemistry): Flashcards
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Define heating curve.
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Cards in this collection (22)
Define heating curve.
A graph that shows how the temperature of a substance changes over time as heat is added at a constant rate, including flat regions where phase changes occur at constant temperature.
True or False?
During a phase change, the temperature of a pure substance remains constant because the added heat energy is used to overcome intermolecular forces rather than increase the kinetic energy of the particles.
True.
The flat segments on a heating curve represent phase changes. Heat added during these intervals increases potential energy (breaking intermolecular forces) while average kinetic energy — and therefore temperature — stays constant.
Why is the molar heat of condensation of a substance equal in magnitude but opposite in sign to its molar heat of vaporization?
Condensation is the reverse of vaporization, so the same intermolecular forces that require energy to overcome during vaporization are re-formed during condensation, releasing an equal quantity of energy. The two values are therefore equal in magnitude and opposite in sign.
Define cooling curve.
A graph that shows how the temperature of a substance decreases over time as heat is removed at a constant rate; it is the mirror image of a heating curve, with flat regions at phase changes where temperature is constant.
True or False?
The energy absorbed when a substance melts is greater than the energy released when the same substance freezes.
False.
The energy absorbed during melting equals the energy released during freezing (the complementary process). Melting and freezing are reverse processes, so their enthalpy changes are equal in magnitude and opposite in sign.
The flat, horizontal segments on a heating curve occur because, during a phase change, the added heat energy is used to overcome .......... rather than increase the kinetic energy of the particles.
The flat, horizontal segments on a heating curve occur because, during a phase change, the added heat energy is used to overcome intermolecular forces rather than increase the kinetic energy of the particles.
Define endothermic process.
A process in which the system absorbs heat from its surroundings, resulting in a decrease in the temperature of the surroundings. The enthalpy change (ΔH) is positive.
True or False?
In an exothermic reaction, heat flows from the surroundings into the system, causing the temperature of the surroundings to rise.
False.
In an exothermic reaction, heat flows out of the system into the surroundings. It is the surroundings — not the system — that experience a rise in temperature. The system releases energy.
Why does dissolving ammonium nitrate, NH4NO3, in water cause the solution to feel cold?
The dissolution of NH4NO3 is an endothermic process: the system (dissolving salt) absorbs heat from the surroundings (the water and the container). This removal of thermal energy from the surroundings lowers their temperature, making the container feel cold to the touch.
Define enthalpy of solution (ΔHsolution).
The heat absorbed or released at constant pressure when a given amount of solute dissolves in a given amount of solvent. It is positive for endothermic dissolution and negative for exothermic dissolution.
True or False?
Lattice energy (ΔHlattice) for an ionic compound is always a positive value because separating gaseous ions from the crystal requires energy input.
True.
Lattice energy measures the energy required to completely separate one mole of a solid ionic compound into its gaseous ions. Because energy must be supplied to break the ionic bonds, the process is endothermic and ΔHlattice is always positive.
Define energy diagram.
A graphical representation that shows the relative energy levels of reactants and products in a chemical or physical process, indicating whether the overall energy change is positive (endothermic) or negative (exothermic).
True or False?
In an energy diagram for an exothermic reaction, the products are drawn at a higher energy level than the reactants.
False.
In an exothermic reaction, energy is released to the surroundings, so the products have lower energy than the reactants. The products are drawn at a lower energy level on the diagram.
Why is energy shown as part of the products in the equation for an exothermic reaction, but as part of the reactants in an endothermic reaction?
In an exothermic reaction, energy is released as the reaction proceeds, so it appears on the product side — it is an output of the reaction. In an endothermic reaction, energy must be supplied to drive the reaction, so it appears on the reactant side — it is an input consumed by the reaction.
True or False?
The overall energy change of a reaction shown on an energy diagram equals the energy of the products minus the energy of the reactants.
True.
The energy change ΔE = Eproducts − Ereactants. A negative result indicates an exothermic process (products at lower energy); a positive result indicates an endothermic process (products at higher energy).
In an energy diagram, if the reactants are at a lower energy level than the products, the reaction is classified as .......... .
In an energy diagram, if the reactants are at a lower energy level than the products, the reaction is classified as endothermic.
Define thermal energy.
The total kinetic energy of all the particles that make up a substance, associated with the random motion of atoms and molecules. Thermal energy depends on both the temperature and the mass (number of particles) of the substance.
True or False?
A bathtub of water at 45°C contains more thermal energy than a coffee cup of water at 70°C.
True.
Thermal energy is the total kinetic energy of all particles in a substance, so it depends on mass as well as temperature. Although the coffee cup is hotter (higher average kinetic energy per particle), the bathtub contains far more water molecules, giving it greater total thermal energy.
Why does thermal energy flow from a hot iron at 200°C into water at 25°C, and not in the opposite direction?
Heat flows spontaneously from a body at higher temperature to a body at lower temperature because the particles of the hot iron have greater average kinetic energy. Collisions between iron and water molecules transfer kinetic energy from the more energetic iron particles to the less energetic water molecules, continuing until both reach the same temperature (thermal equilibrium).
Define thermal equilibrium.
The state reached when two bodies in contact have the same temperature, so there is no net flow of thermal energy between them.
True or False?
Temperature and thermal energy measure the same property of a substance.
False.
Temperature measures the average kinetic energy per particle and is independent of the amount of substance. Thermal energy measures the total kinetic energy of all particles and depends on both temperature and mass. Two samples can have the same temperature but very different thermal energies.
Temperature is a measure of the average kinetic energy of particles, while thermal energy is a measure of the .......... kinetic energy of all particles in a substance.
Temperature is a measure of the average kinetic energy of particles, while thermal energy is a measure of the total kinetic energy of all particles in a substance.
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