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Quantity of Heat

Background

In our day to day lives, heat is used in different ways. Some of these uses include cooking, ironing, warming water and even warming of rooms among other uses. In this topic, some terms and concepts related to heat will be discussed.

Background

In our day to day lives, heat is used in different ways. Some of these uses include cooking, ironing, warming water and even warming of rooms among other uses. In this topic, some terms and concepts related to heat will be discussed.

Objectives

By the end of the topic, you should be able to

:

  • Define heat capacity and specific heat capacity
  • Determine experimentally heat specific heat capacity of solids and liquids
  • Define specific latent heat of fusion and vaporization
  • Illustrate heat capacity and specific heat capacity
  • Explain the functioning of a refrigerator
  • Solve numerical problems involving quantity of heat.

Introduction

In our form one course we learnt that heat is a form of energy that flows from hotter to colder regions. In this topic, we are going to study the quantity of heat and its effects on substances. The demonstration below shows water being heated in a beaker. Click on the play button to observe what happens.

Observation

The reading on the thermometer rises.

Explanation

Heat from the bunsen burner makes the temperature of the water to rise as recorded by the thermometer.

Conclusion

Heat is a form of energy that flows from one body to another while temperature is the coldness or hotness of a body.

Heat Capacity and Specific Heat Capacity

The demonstration below shows different liquids of equal mass subjected to equal amounts of heat. Click the play button to observe what happens to the readings on the thermometer A, B and C. What do you observe?

Observation

Kerosene had the highest temperature reading followed by alcohol and then water.

Explanation

Kerosene rose to the highest level .This shows that it requires the least heat for a unit rise in temperature while water requires the most amount of heat comparatively.

Conclusion

Different substances require different amounts of heat for a unit rise in temperature. The amount of heat required to raise the temperature of a substance by 1 Kelvin is called heat capacity whose S.I unit is Joules per Kelvin. When a kilogram of a substance is used the amount of heat used to raise its temperature by 1 Kelvin is referred to as its specific heat capacity. Its S.I unit is Joules per kilogram per Kelvin [J/KgK].

Latent heat of fusion

As solids absorb heat, their temperature rises. On reaching a certain temperature known as the melting point, the substance melts and becomes a liquid. The change of state from solid to liquid is known as melting. During melting, the temperature remains constant even though the substance absorbs heat. The energy absorbed during melting is the latent heat of fusion.

title>Untitled Document

Demonstrating latent heat of fusion

To demonstrate latent heat of fusion, play the animation below to see what happens when ice is heated from:

i. -10oC to 0o C

ii. 0oC until all the ice melts

Observations

From: -10oC to 0o C the temperature of ice rises. At 00C, the ice melts with no change in temperature.

Discussion

As ice is heated from -10oC to 00C, it gains heat energy which causes a temperature rise. At 00C, there is no rise in temperature as heating continues. The heat gained at this stage is used to break the intermolecular bonds in ice causing it to melt.

Conclusion

The amount of heat used to break the bonds of a given mass of ice at the melting point is called the latent heat of fusion. The amount of heat used to change the state of a substance from solid to liquid is called latent heat of fusion. For 1Kg of a substance, the amount of heat required to melt it is referred to as the specific latent heat of fusion.

Demonstrating latent heat of vaporization

When a liquid is heated, its temperature rises up to a certain point known as the boiling point. At this temperature, the liquid changes into a gaseous form (vapour). This process is known as vaporization. The heat energy absorbed during vaporization is known as latent heat of vaporization. The demonstration below shows water changing from liquid state to a gaseous state. Click the play button and make your observations.

Observations

From room temperature to 1000C, temperature rises and bubbles are seen rising

At 1000C, the water vaporizes at a constant temperature

Discussion

From room temperature to the boiling point, the heat energy supplied is used to raise the temperature of the water. At the boiling point, the heat supplied does not cause any further change in temperature but changes the state of the water into steam.

Conclusion

The amount of heat used to change the water into steam without a change in temperature is called the latent heat of vaporization of water. The amount of heat required to change the state of a substance from liquid to gaseous state without a change in temperature is the latent heat of vaporization. Specific latent heat of vaporization is the amount of heat required to change 1Kg of a liquid into vapour without a change in temperature.

Cooling curve of naphthalene

When naphthalene is melted and heated above the melting point, then allowed to cool, its temperature changes can best be illustrated by plotting a graph of temperature against time as shown below. Click on the play button to observe the graph.

Description

Section PQ - The temperature drops from 900C to 800C

Section QR- Liquid naphthalene turns from liquid to solid at constant temperature as it loses latent heat of fusion

Section RS - Solid naphthalene cools to surrounding temperature

/head>

Refrigerator

This is a device used to keep substances cold. Play the animation below to observe some examples of different models of refrigerators.

The working of a refrigerator

Play the animation below to see how this device works.

 

 

 

 

Explanation

The working of a refrigerator is such that a volatile liquid is circulated by use of a compressor pump operated by an electric current. The liquid causes cooling by absorbing heat from the fridge contents and uses it as latent heat to vaporize. The vapour is compressed by the pump back to liquid form which is cooled by the cooling fins and the heat conducted/radiated away. In the fridge, convection currents circulate which carry the heat from the compartments to the cooling fins.

 

 

 

A worked Example

The following example shows how you can determine the quantity of heat required to convert ice of mass 100g at -100C to steam at 1000C.(The specific heat capacity of ice is 2.1 x 103 J/kgK, specific heat capacity of water is 4200J/kgK, Specific latent heat of fusion is 3.4 x 105J/kg and specific latent heat of vaporization is 2.26 x 106J/kg). To show the calculation at each stage, click the buttons Q1, Q2, Q3 and Q4

 

 



 Quantity of Heat 

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