Consider the change in energy (E). In this experiment, E is the change in heat e

Question

Consider the change in energy (E). In this experiment, E is the change in heat energy. Heat energy can be measured by observing the temperature change of a known amount of water. The formula for this is: Q=mcwT where Q is the change in heat energy of the water, m is the mass of the water, cwis the specific heat of water (4.18 Joules/g° C), and T is the change in the temperature of the water.

Heat energy is required to melt ice because attractive forces (especially hydrogen bonding) between the molecules must be overcome when the molecules undergo a solid liquid phase transition. Since there is no change in temperature during melting, there is no increase in the kinetic energy of the molecules.

Melting corresponds to an increase in potential energy. The formula is: Qf = mLf where Qf is the change in heat energy, m is the mass of ice, and Lf is the heat of fusion of ice.

Suppose a 56.668 g sample of an unknown solid substance absorbed 7.18 kJ as it melted at a constant temperature. What is the heat of fusion of the substance?

Explanation / Answer

we know that

Q = m x Lf

so

Lf = Q / m

given

Q = 7.18 kJ

mass of the sample = 56.668 g

so

Lf = 7.18 kJ / 56.668 g

Lf = 0.1267 kJ/g

so

the latent heat of fusion of the substance is 0.1267 kJ/ g or 126.7 J/g

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