Quick Notes

Quick Notes

Chemistry Chapter 3: Thermochemistry
3.3 Application of Exothermic Reaction and Endothermic Reaction in Daily Life
3.3 Application of Exothermic and Endothermic Reactions in Daily Life
CHEMISTRY • Form 5 • Chapter 3: Thermochemistry

3.3Application of Exothermic and Endothermic Reactions in Daily Life

Exothermic and endothermic reactions play crucial roles in our daily lives, from cooking food and treating injuries to powering vehicles. Understanding the heat changes in chemical reactions enables the development of useful products such as self-heating cans, cold packs, and various fuels.

Learning Objectives

  • State a few examples of applications of exothermic and endothermic reactions in daily life.
  • State the meaning of fuel value and its unit.
  • Calculate the fuel value of a fuel using its heat of combustion and molar mass.
  • Analyse fuel value and justify the selection of suitable fuels based on specific factors.

Everyday Applications of Thermochemical Reactions and Factors in Fuel Selection

Compare exothermic and endothermic daily products, then explore the four factors that guide fuel choice.

Exothermic Applications

  • Combustion of fuel for cooking, heating and light.
  • Thermite reaction for welding railway tracks.
  • Hot Can for quick hot food or beverages.
  • Hot packs to warm hands or relieve muscular aches.
  • Air bags that release gas and heat upon collision.

Endothermic Applications

  • Cold packs to treat sports injuries or reduce swelling.
  • Gel pads on the forehead to relieve fever.
  • Self-cooling beverages that chill without refrigeration.
  • Sorbet that makes the tongue feel cold.
  • Sweating, where evaporation absorbs body heat.

Fuel Value

The amount of heat energy released when one gram of fuel is completely burnt in excess oxygen.

Unit: \(\text{kJ g}^{-1}\)

Formula: \(\text{Fuel value} = \dfrac{\text{Heat of combustion}}{\text{Molar mass}}\)

Four Factors in Fuel Selection

  • Fuel value — higher value gives more energy per gram.
  • Cost of fuel — economic viability and price per unit mass.
  • Availability and source — how easily the fuel can be obtained.
  • Environmental effects — soot, greenhouse gases and acid rain.

Brief Explanation

Understand the key ideas behind heat changes and fuel value.

Exothermic reactions release heat to the surroundings, while endothermic reactions absorb heat from the surroundings.

Exothermic in Daily Life

Combustion of fuel is used for cooking, heating and generating light. The thermite reaction welds railway tracks. Hot cans prepare hot food quickly, hot packs warm hands, and air bags release gas and heat during collisions.

Endothermic in Daily Life

Cold packs treat sports injuries, gel pads relieve fever, and self-cooling beverage cans chill drinks without ice. Sorbet makes the tongue feel cold, and when sweat evaporates it absorbs heat from the body, making the body feel cool.

Fuel value is the amount of heat energy released when one gram of fuel is completely burnt in excess oxygen. Its unit is \(\text{kJ g}^{-1}\).
Fuel Fuel value (\(\text{kJ g}^{-1}\))
Wood20
Methane (textbook Table 3.5)27
Charcoal30
Ethanol30
Petrol34
Kerosene37
Natural Gas50
Hydrogen gas143
Worked Example: The heat of combustion of octane, \(\text{C}_8\text{H}_{18}\), is \(5\,500\text{ kJ mol}^{-1}\). Its molar mass is \(8(12) + 18(1) = 114\text{ g mol}^{-1}\). Therefore, fuel value \(= \dfrac{5\,500\text{ kJ mol}^{-1}}{114\text{ g mol}^{-1}} = 48.2\text{ kJ g}^{-1}\).

Factors in Fuel Selection

  • Fuel value: higher fuel value produces more energy per gram.
  • Cost of fuel: economic viability and price per unit mass.
  • Availability and source: how easily the fuel can be obtained and supplied.
  • Environmental effects: pollutants such as soot, greenhouse gases or acid rain.

Vehicle Fuels and RON

Vehicle fuel is classified by Research Octane Number (RON), such as RON 95 or RON 97. Higher RON fuel is more difficult to burn prematurely, while lower RON fuel burns more easily.

Try These Questions First

Answer mentally, then check the explanation.

1

What is meant by the fuel value of a fuel?

Answer: The amount of heat energy released when one gram of fuel is completely burnt in excess oxygen.
2

The heat of combustion of ethanol, \(\text{C}_2\text{H}_5\text{OH}\), is \(1368\text{ kJ mol}^{-1}\) and its molar mass is \(46\text{ g mol}^{-1}\). What is its fuel value?

Answer: \(\text{Fuel value} = \dfrac{1368\text{ kJ mol}^{-1}}{46\text{ g mol}^{-1}} \approx 29.74\text{ kJ g}^{-1}\), rounded to \(30\text{ kJ g}^{-1}\).
3

Why is hydrogen considered an environmentally friendly alternative to petrol?

Answer: Hydrogen has a much higher fuel value (\(143\text{ kJ g}^{-1}\) compared to petrol's \(34\text{ kJ g}^{-1}\)) and its combustion produces only water without releasing soot or harmful carbon emissions.

Common Mistakes

Avoid these errors when working with fuel value and heat changes.

!
Writing the unit of fuel value as \(\text{kJ mol}^{-1}\) instead of \(\text{kJ g}^{-1}\).
!
Including a negative sign in the final answer for fuel value. Fuel value represents the magnitude of heat energy released per gram and is written as a positive value.
!
Classifying sweating as an exothermic process simply because the body cools down. Sweating cools the body because the evaporation of sweat is an endothermic process that absorbs heat from the skin.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

A fuel with the highest fuel value is always the best fuel to use in every situation.

Correct Fact

While fuel value is important, practical selection also depends on fuel cost, availability, safety of handling, and environmental pollution such as soot or greenhouse gases.

Misconception

Cold packs and gel pads create coldness.

Correct Fact

They do not create coldness; they absorb heat energy from the surrounding skin or environment through an endothermic reaction or physical process.

Summary

Key facts to remember.

  • Exothermic applications release heat: combustion for cooking, hot cans, thermite reaction for welding railway tracks, and hot packs.
  • Endothermic applications absorb heat: cold packs, gel pads to relieve fever, self-cooling beverage cans, and evaporation of sweat.
  • Fuel value is the heat energy released when \(1\text{ g}\) of fuel is completely burnt in excess oxygen, measured in \(\text{kJ g}^{-1}\).
  • Formula: \(\text{Fuel value} = \dfrac{\text{Heat of combustion } (\text{kJ mol}^{-1})}{\text{Molar mass } (\text{g mol}^{-1})}\).
  • Four factors dictate fuel choice: fuel value, cost, availability/source, and environmental effects.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following is an application of an endothermic reaction in everyday life?

2

Based on Table 3.5, which fuel has the highest fuel value?

B. Fill in the Blanks

3

The amount of heat energy released when one gram of fuel is completely burnt in excess oxygen is known as the ______.

4

The standard unit used to express the fuel value of a substance is ______.

C. Match the Application with the Description

Drag each item on desktop. On mobile, tap an item and then tap the matching box.

Answer Choices

Thermite reaction Self-cooling beverages Fuel value Higher RON fuel

Description Matching

1Exothermic reaction used for welding railway tracks
2Endothermic system to chill cans without refrigeration
3Heat energy released when one gram of fuel is completely burnt
4Fuel that is more difficult to burn
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Fuel value Exothermic reaction Endothermic reaction Thermite reaction Research Octane Number (RON)