In a calorimetry experiment, which expression correctly describes the energy change for the water and how to obtain per-mole value?

In a calorimetry experiment, which expression correctly describes the energy change for the water and how to obtain per-mole value?

• A. q = ΔH × n
• B. q = m × c × ΔT for the water; energy change per mole is the heat change divided by moles of reactant.
• C. q = ΔT × m
• D. q = ΔT / (m × c)

Answer: (B)

In calorimetry, the energy change of the water is determined by how much heat it gains or loses, which depends on its mass, its specific heat capacity, and the temperature change. For water, the heat transferred is q = m × c × ΔT, where m is the mass of the water, c is its specific heat capacity (about 4.18 J/g°C), and ΔT is the rise (or fall) in temperature. To express this energy on a per-mole basis, convert the mass to moles and divide the heat by the number of moles: energy per mole = q / n, with n being the moles of water (or of reactant if you’re comparing to the reacting substance). This is why the correct approach uses q = m × c × ΔT for the water, and then divides by the moles to obtain the per-mole value. The other expressions miss either the need for the specific heat, give an inverted form, or mix in ΔH in a way that isn’t appropriate for the water’s heat change in this setup.

In calorimetry, the energy change of the water is determined by how much heat it gains or loses, which depends on its mass, its specific heat capacity, and the temperature change. For water, the heat transferred is q = m × c × ΔT, where m is the mass of the water, c is its specific heat capacity (about 4.18 J/g°C), and ΔT is the rise (or fall) in temperature. To express this energy on a per-mole basis, convert the mass to moles and divide the heat by the number of moles: energy per mole = q / n, with n being the moles of water (or of reactant if you’re comparing to the reacting substance). This is why the correct approach uses q = m × c × ΔT for the water, and then divides by the moles to obtain the per-mole value. The other expressions miss either the need for the specific heat, give an inverted form, or mix in ΔH in a way that isn’t appropriate for the water’s heat change in this setup.