Ch 9 - Thermochemistry (Expectations)

Define the First Law of Thermodynamics.

The First Law of Thermodynamics states that energy cannot be created or destroyed, only transferred. The change in internal energy of a system is given by ΔE = q + w, where q is heat and w is work.

How does Hess's Law apply to enthalpy changes in chemical reactions?

Hess's Law states that the total enthalpy change for a reaction is the same regardless of the path taken. This means that the enthalpy change is the sum of the enthalpy changes of individual steps in a reaction.

What is the significance of standard heats of formation in calculating ΔH°?

Standard heats of formation, ΔH°_f, are used to calculate the standard enthalpy change of a reaction, ΔH°_rxn, by summing the heats of formation of the products and subtracting the heats of formation of the reactants.

Explain how bond dissociation energies can be used to estimate reaction enthalpies.

Reaction enthalpies can be estimated using bond dissociation energies by calculating the energy required to break bonds in the reactants and the energy released by forming bonds in the products. ΔH ≈ Σ(bond energies of bonds broken) - Σ(bond energies of bonds formed).

What is the relationship between ΔE and ΔH in processes involving expansion work?

In processes involving expansion work, ΔE is typically less than ΔH due to the work done by the system against atmospheric pressure, such as in reactions where the number of gas molecules increases.

How does specific heat capacity differ from molar heat capacity?

Specific heat capacity (C_s) is the amount of heat required to raise the temperature of 1g of a substance by 1°C, whereas molar heat capacity (C_m) is the heat required to raise the temperature of 1 mole of a substance by 1°C.

Describe how a calorimeter can be used to determine the enthalpy change of a reaction.

A calorimeter, such as a coffee cup calorimeter, measures heat transfer during a reaction at constant pressure. By recording the temperature change, mass, and specific heat capacity of the system, the heat change q can be calculated, which equals the enthalpy change ΔH.

What is the role of enthalpy as a state function?

As a state function, enthalpy (H) depends only on the initial and final states of a system, not on the path taken. This makes it useful for calculating changes in heat content in reactions regardless of the reaction pathway.

Differentiate between exothermic and endothermic reactions based on ΔH.

Exothermic reactions release heat and have a negative ΔH, while endothermic reactions absorb heat and have a positive ΔH.

Analyze the effect of temperature change on the direction of heat transfer in a system.

If the temperature of a system increases, it is an exothermic process where heat is released to the surroundings. Conversely, if the temperature decreases, it is an endothermic process where the system absorbs heat from the surroundings.

In the context of real gases, when is work considered positive or negative?

In thermodynamics, work done on the system is considered positive, such as when a gas is compressed and its volume decreases. Work done by the system is negative, such as when a gas expands and its volume increases.

How do phase changes affect the internal energy and enthalpy of a system?

During a phase change at constant pressure, the enthalpy changes but the internal energy may include work associated with volume changes, affecting ΔH and ΔE differently based on expansion or contraction.

Explain the use of a balanced chemical equation in determining ΔH of a reaction.

The balanced chemical equation provides the stoichiometry required to relate ΔH to the moles of reactants and products, facilitating the calculation of enthalpy changes for reactions.

Define standard formation reaction and provide an example.

A standard formation reaction is the formation of 1 mole of a compound from its constituent elements in their standard states at 25°C and 1 atm. Example: C(s) + O_2(g) → CO_2(g).

How can you differentiate between bond breaking and forming in terms of energy change?

Bond breaking is an endothermic process requiring energy input, while bond forming is an exothermic process releasing energy. The overall energy change in a reaction depends on the balance between these two processes.