Thermodynamics Concepts Quiz

Unit 1: Heat, Energy and Rates

• Heat is the overall thermal energy in a sample, dependent on temperature, the number of particles (mass), and the type of particles.
• A larger object with a lower temperature can contain more heat than a smaller object with a higher temperature.
• Systems can be open, closed, or isolated, affecting energy and matter exchange with surroundings.
• Thermochemistry studies energy changes during physical and chemical changes.
• Heat transfer occurs when a chemical system undergoes a change in energy.
• Exothermic systems release heat to the surroundings.
• Endothermic systems absorb heat from the surroundings.
• Energy change is calculated using q = mcΔT (where q is energy in Joules/kilojoules, m is mass in grams/kilograms, c is specific heat capacity, and ΔT is temperature change).

Energy and Heat

• Temperature is the average kinetic energy of particles.

Measuring Energy Changes

• Calorimeters are devices used to measure energy changes during reactions in isolated compartments.
• Common types include coffee cup and bomb calorimeters.

Energy of Phase Changes

• Phase changes involve transitions between solid, liquid, and gas states, and temperatures remain constant during phase changes.
• The energy absorbed or released during a phase change is calculated using q = m × l (where m is the mass of the substance and l is the latent heat of the substance).

Energy of Phase Changes

• Phase changes are transitions among solid, liquid, and gas states.
• During phase changes, temperature remains constant.
• The energy absorbed or released is due to changing the attractive forces between molecules.
• Latent heat (l) is the energy required for phase changes. The formula is: q = m × l.

Enthalpy

• Enthalpy (H) is the heat content of a system under constant pressure.
• Heat is equal to energy plus the product of pressure and volume (H=E + PV).
• Enthalpy change (ΔH) is calculated as follows: ΔΗ = ΣnH°f products − ΣnH°f reactants.
• Enthalpy of fusion (ΔHfus) is associated with melting/freezing.
• Enthalpy of vaporization (ΔHvap) is associated with vaporization/condensation.
• Enthalpy of combustion (ΔH c ) is associated with fuel combustion.
• Enthalpy of formation (ΔH f ) is associated with producing a substance from its elements.

Heats of Formation

• Thermochemical equations quantify energy changes during reactions, including the energy released or absorbed (e.g., 2H₂(g) + O₂(g) → 2H₂O(l) + 590 kJ).
• The same reaction can have different enthalpy values depending on the physical states of the products and reactants.
• Enthalpy changes for reactions can be calculated using heats of formation (ΔH°f) for products and reactants.

Calculating ∆H Using Bond Energies

• Enthapy changes can be calculated using bond energies. The formula is ΔH= ∑bonds broken - ∑bonds formed
• Bond energies are specific to bonds, often are different for breaking and forming, must multiply the values by the number of the particular bond in the reaction and the coefficient from the balanced chemical reaction
• Bond energy values must be the exact values provided.

Representing Energy Changes Graphically

• Potential energy diagrams are graphic representations of energy changes during a reaction, plots of potential energy versus reaction progress.
• For an endothermic reaction, products have a higher potential energy than reactants, the change in enthalpy is positive (ΔH > 0).
• For exothermic reactions, products have a lower potential energy than reactants, the change in enthalpy is negative (ΔH < 0).
• Activation energy is represented by Ea .

Rates of Reaction

• Reaction rate is the speed at which reactants are consumed or products are formed.
• Reaction rates can be measured by observing changes in concentration, mass, pH, conductivity, color, temperature or pressure.
• Average reaction rate ( r ) = change in concentration / change in time ( Δ [C])/ Δt)

Collision Theory

• Effective collisions have sufficient energy and proper orientation enabling reactions to occur.
• Reaction rates are impacted by collision number, particle energy, and reaction orientation
• Factors affecting reaction rates include concentration, temperature, surface area, presence of catalysts, and the state of the reactants.

Factors Affecting Reaction Rates

• Concentration: Higher concentration increases particle collisions and the frequency of effective collisions
• Temperature: Higher temperature increases kinetic energy, leading to more frequent, and more energetic collisions, and increasing effective collisions.
• Surface area: Larger surface area of reactants increases the area exposed for collisions.
• Catalyst: A catalyst provides an alternative reaction pathway with a lower activation energy, increasing the rate of the reaction.
• State of Reactants: Reactants in the same physical state (homogeneous reactions) react faster than those in different states (heterogeneous reactions).

Rate Law Equations

• Rate laws describe the relationship between reaction rate and reactant concentrations.
• Rate law equations are determined from the rate determining step of a reaction mechanism
• The rate determining step is the slowest step in a reaction mechanism.

Reaction Mechanisms

• Reaction mechanisms are steps showing reactions occur (elementary steps)
• Molecularity is the number of reactant molecules involved in an elementary step.
• Unimolecular steps involve one molecule;
• Bimolecular steps involve two molecules;
• Termolecular steps involve three or more molecules.