Latent Heat

Sometimes when heat energy is given to a substance, it does not increase its temperature, rather, it changes the state of the substance; such heat is referred to as latent heat.  There are two types of latent heat:

• Latent heat of fusion
• Latent heat of vaporization

Latent heat of Fusion

Latent heat of fusion: This is the quantity of energy needed to change solid to liquid without any change in temperature.

Latent heat of vaporization

Latent heat of vaporization: This is the amount of energy needed to change liquid to gases (vapor) or steam at constant temperature.

SPECIFIC LATENT HEAT

This is the quantity of heat required to change a unit mass of a substance from one state to another without a change in its temperature.

(a)      SPECIFIC LATENT HEAT OF FUSION (L_f)

This is the quantity of heat required to change a unit mass of a solid to liquid without a change in its temperature.  The specific latent heat of fusion of ice is 33600 J/kg.

                                                                                          …1

(b)     SPECIFIC LATENT HEAT OF VAPORISATION (L_v)

This is the quantity of heat required to convert a unit mass of liquid to vapor (gaseous state) without any change in temperature.  For water the value is 2.26 x 10⁶ J/kg

                                                                                          …2

EXAMPLE

1.       How much heat energy is needed to change 3g of ice at 0⁰C to steam at 100⁰C? (L_f = 336 KJ/kg, C_w = 4200 J/kgk, L_v = 2.26 x 10⁶ J/kg

First stage:

Heat required for the ice to melt at 0⁰C

Second Stage:

Heat required to raise the temperature of the melted ice from 0⁰C to 100⁰C

Third Stage:

Heat required to convert the liquid to steam

The total energy,

CLASSWORK 5

1. Differentiate between specific heat capacity and specific latent heat
2. Calculate the total energy required to evaporate completely 1kg of ice that is initially at – 10⁰C. Given that:

[Specific capacity of Ice = 2.2 x 10³ J/kgk, Specific heat that capacity of water = 4.2 x 10³ J/kgk, Specific latent heat of fusion of Ice = 3.36 x 10⁵ J/kg, Specific latent heat of vaporization = 2.26 x 10⁶ J/kg]

ASSIGNMENT 5

SECTION A

1. Determine the heat required to change 10g of ice at 0^OC to water at 10^OC if specific latent heat of ice is 335J/g and specific heat capacity of water is 4.2J/gk (a) 3.77 x 10³J    (b) 4.00 x 10³J   (c) 4.50 x 10³J (d) 1.33x 10³J (e) 4.9x 10³J
2. Which of the following is the correct SI unit of specific latent heat? (a) Jkgk (b) Jkg^-1k-¹ (c) Jkgk^-1 (d) Jkg^-1 (e) Jkg
3. Latent heat of fusion changes substances from its (a) solid state to liquid state (b) from liquid to solid (c) from liquid to gas
4. The heat required to change 1kg of ice at 0⁰C to water at the same temperature is called (a) specific latent heat of vaporization (b) specific heat capacity (c) specific latent heat of fusion (d) heat capacity (e) specific heat capacity

SECTION B

1. An electric heater immersed in water of mass m, raised the temperature of the water from 40⁰C to 100⁰C in 5.0 minutes. After another 11.25minutes, one-quarter of the water has been converted to steam. Calculate the specific latent heat of vaporization of water. [Specific heat capacity of water = 4200 Jkg^-1K^-1]

Calculate the energy required to vaporize 50g of water initially at 80⁰C. [Specific heat capacity of water = 4.2Jg^-1K^-1; specific latent heat of vaporization of water = 2260 Jg^-1]