Latent Heat vs Specific Heat
When a substance undergoes a phase change, the energy is absorbed or released as heat. Latent heat is the heat that is being absorbed or released from a substance during a phase change. This heat changes do not cause temperature changes as they are absorbed or released. The two forms of latent heat are latent heat of fusion and latent heat of vaporization. Latent heat of fusion takes place during melting or freezing, and latent heat of vaporization takes place during boiling or condensing. The phase change releases heat (exothermic) when converting gas to liquid or liquid to solid. The phase change absorbs energy/ heat (endothermic) when going from solid to liquid or liquid to gas. For example, in the vapor state, water molecules are highly energetic, and there are no intermolecular attraction forces. They move around as single water molecules. Compared to this, liquid state water molecules have low energies. However, some water molecules are capable of escaping to the vapor state if they have high kinetic energy. At normal temperature, there will be equilibrium between the vapor state and liquid state of the water molecules. When heating, at boiling point most of the water molecules will be released to the vapor state. So, when water molecules are evaporating, the hydrogen bonds between the water molecules have to be broken. For this, energy is needed, and this energy is known as the latent heat of vaporization. For water, this phase change occurs at 100 oC (boiling point of water). However, when this phase change occurs at this temperature, heat energy is absorbed by water molecules to break the bonds, but it won’t increase the temperature more.
Specific latent heat means, the amount of heat energy needed to convert a phase completely to another phase of a unit mass of a substance.
Heat capacity is dependent on the amount of substance. Specific heat or specific heat capacity (s) is the heat capacity which is independent of the amount of substances. It can be defined as “the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius (or one Kelvin) at a constant pressure.” The unit of specific heat is Jg-1oC-1. The specific heat of water is very high with the value of 4.186 Jg-1oC-1. This means, to increase the temperature by 1 oC of 1 g of water, 4.186 J heat energy is needed. This high value encounters for the role of water in thermal regulation. To find the heat needed to increase the temperature from t1 to t2 of a certain mass of a substance following equation can be used.
q = m x s x ∆t
q = required heat
m = mass of the substance
∆t = t1-t2
However, above equation does not apply if the reaction involves a phase change. For example, it does not apply when the water is going to the gas phase (at the boiling point) or when the water freezes to form ice (at the melting point). This is because, the heat added or removed during the phase change does not change the temperature.
What is the difference between Latent Heat and Specific Heat?
• Latent heat is the energy absorbed or released when a substance is undergoing a phase change. Specific heat is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius (or one Kelvin) at a constant pressure.
• Specific heat doesn’t apply when a substance is undergoing phase change.
• Specific heat causes temperature change where in latent heat there’s no temperature change involved.
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