09/14 Thermochemistry Free Online FlashCards

What is energy in thermochemistry?

Energy is the capacity to transfer heat or perform work. It can be stored as kinetic energy or as potential energy associated with composition, position, or interactions.

What equation expresses the first law of thermodynamics?

The first law is ΔU = q + w, where ΔU is the change in internal energy, q is heat transferred to the system, and w is work done on the system.

How is pressure–volume work calculated?

For pressure–volume work, w = −P_extΔV. Expansion gives w < 0 because the system does work on its surroundings; compression gives w > 0.

How do heat and temperature differ?

Heat is energy transferred because of a temperature difference, whereas temperature measures the average kinetic energy of particles.

What equation relates heat to mass and temperature change?

The relationship is q = mcΔT, where m is mass, c is specific heat capacity, and ΔT = T_final − T_initial.

What is the commonly used specific heat of liquid water?

For liquid water, c_water = 4.184 J g⁻¹ °C⁻¹. This is the heat required to raise 1 g of water by 1 °C.

How are constant-pressure heat and enthalpy related?

At constant pressure with only pressure–volume work, q_p = ΔH. Therefore, an exothermic process has ΔH < 0, while an endothermic process has ΔH > 0.

What is the definition of enthalpy?

Enthalpy is defined as H = U + PV. It is useful because, at constant pressure with only pressure–volume work, its change equals the heat transferred.

What energy balance is used in calorimetry?

In an insulated experiment, q_rxn + q_surroundings = 0, so q_rxn = −q_surroundings. Heat released by the reaction is absorbed by the surroundings.

What characterizes coffee-cup calorimetry?

A coffee-cup calorimeter operates approximately at constant pressure. With negligible calorimeter heat capacity, q_rxn = −m_solution c_solution ΔT.

Why does bomb calorimetry measure ΔU most directly?

A bomb calorimeter operates at constant volume, so ΔV ≈ 0 and pressure–volume work is negligible. The measured heat is therefore related most directly to ΔU.

What does Hess’s law state?

Hess’s law states that the enthalpy change for an overall reaction equals the sum of the enthalpy changes for any steps that add to that reaction.