Which statement correctly describes enthalpy change and how it can be measured in a calorimeter?

Which statement correctly describes enthalpy change and how it can be measured in a calorimeter?

• A. Enthalpy change is the heat released or absorbed at constant pressure; measured by the temperature change of the water and solution, using q = mcΔT.
• B. Enthalpy change is the rate of a chemical reaction.
• C. Enthalpy change is the energy required to break bonds in reactants and products.
• D. Enthalpy change is the total energy change of the system.

Answer: (A)

Enthalpy change is the heat transferred during a process when pressure is constant. In a calorimeter, the reaction takes place at essentially constant pressure, and the heat released or absorbed by the reaction shows up as a change in the temperature of the water or solution. By measuring how much the water heats up and knowing its mass and specific heat capacity, you calculate the heat using q = m c ΔT. This heat is the enthalpy change at constant pressure (the sign tells you whether heat was released or absorbed). That’s why this description fits best: enthalpy change is heat at constant pressure, measured from the temperature change with q = m c ΔT. The other ideas don’t describe enthalpy the same way: rate of reaction is about how fast, not heat; bond energies relate to breaking/forming bonds but don’t by themselves define the enthalpy change measured in a calorimeter; and total energy change of the system refers to internal energy, not enthalpy at constant pressure.

Enthalpy change is the heat transferred during a process when pressure is constant. In a calorimeter, the reaction takes place at essentially constant pressure, and the heat released or absorbed by the reaction shows up as a change in the temperature of the water or solution. By measuring how much the water heats up and knowing its mass and specific heat capacity, you calculate the heat using q = m c ΔT. This heat is the enthalpy change at constant pressure (the sign tells you whether heat was released or absorbed). That’s why this description fits best: enthalpy change is heat at constant pressure, measured from the temperature change with q = m c ΔT. The other ideas don’t describe enthalpy the same way: rate of reaction is about how fast, not heat; bond energies relate to breaking/forming bonds but don’t by themselves define the enthalpy change measured in a calorimeter; and total energy change of the system refers to internal energy, not enthalpy at constant pressure.