Energy Changes during Chemical Reactions

Energy Changes during Chemical Reactions

In Senior One, you learnt that heat is usually given out or absorbed in chemical changes and in changes of states.
Chemical changes are often accompanied by energy changes during chemical reactions, and the energy changes happen in various ways. Examples include:
a) Chemical energy converted into heat energy (which is released) when a fossil fuel is burnt
fossil fuel + oxygen carbon dioxide + water + energy
b) Chemical energy converted into light energy when substances, like wood, burn c) Sound energy produced by an explosion
d) Electrical energy converted from chemical energy stored in batteries In this chapter, you will learn about chemical reactions and that in any chemical
reaction, energy — usually in the form of heat — is lost or gained. You will also appreciate the applications of energy changes in real life.
4.1 Chemical Energy
All chemical substances possess chemical energy stored in bonds that connect atoms in a compound,
In this section, learners will deal with energy from chemical reactions in which Old bonds break and new ones form.
Processes in which heat energy is given out to the surroundings are said to be exothermic, while those in which heat energy is absorbed from the surroundings are
said to be endothermic.

Activity 4.1 (a): Exploring the reactions/processes in which energy is given out or absorbed
Work in groups.
What you need:
. Internet

• relevant Chemistry textbooks
  What to do

1. Discuss whether the following processes absorb or give out energy to the
   surroundings.
   a) respiration
   b) photosynthesis
   c) burning charcoal
   d) fermentation
2. Compile your findings in Table 4.7.
   e) dissolving salt in water
   f) melting ice cubes
   g) cooking an egg
   h) evaporating liquid water

3. Give any other four types of reactions in which energy is:
   a) absorbed.
   b) given out.
4. Share your findings to the rest of the class.
   Temperature changes in dissolving substances in water
   In Activity 4.1(a), you found out that when chemical reactions take place, energy is either gained or given out. How did you know that energy is gained or given? Energy can neither be created nor destroyed; rather, it can be transformed from one form to another This is the law of conservation of energy. Energy changes occur in chemical reactions and some physical processes, such as when dissolution of solids in water.
   In Activity 4.1(b), you will investigate what happens when solids are dissolved in water.

Observation and Analysis

1. Which solutes dissolved endothermically?
2. Which solutes dissolved exothermically?
3. Which solute(s) dissolved the most endothemically?
4. Which solute(s) dissolved the most exothermically?

Exercise 4.1

1. What is an exothermic reaction?
2. What are the characteristics of an exothermic reaction?
3. What is an endothermic reaction?
4. What are the characteristics of an endothermic reaction?
   4.2 Importance of Endothermic and Exothermic Reactions
   You have learnt that some reactions make their surroundings warmer while others make them colder.
5. In Activity 4.2, you will find out how both kinds of reactions are important and where they occur in real life.

• Activity 4.2: Finding out the importance of exothermic and endothermic reactions in everyday life
  Classify the processes/ reactions in Table 4.3 as exothermic or endothermic and state the importance of each process/reaction.
  Table 4.3: Importance Of endothermic and exothermic proceses/reactions

4.3 Burning Fuels to Obtain Useful Energy
Energy can be released in chemical reactions in various forms; as light, sound energy. But it is usually released as heat energy. The science of measuring the heat absorbed or evolved during the course of a chemical reaction is called calorimetry. A calorimetry experiment is used to measure the heat energy released from burning fuel.
In the next activity, you will find out the amount of energy released from burning

Activity 4.3(a): Finding out the amount of energy released by
burning ethanol
Work in groups.
What you need:

• spirit burner
  beaker
  thermometer
  . windshield
  What to do
  •copper calorimeter
  tripod stand
  water
  ethanol
  peanut
  oils
  paper
  wood
  1.Measure 200 crn3 of water using a measuring cylinder and pour it into a thin copper calorimeter.
  2.Measure the initial temperature of water in the calorimeter using a thermometer and record it as Tl (in oc).
  3.Add ethanol to a spirit burner, weigh it and record its mass as WI (in g).
  4.Light the spirit burner and let its flame heat the water directly and record the final temperature as T2(in oc).
  5.Weigh the mass of the spirit burner with its contents after the experiment and record it as W2 (in g).

Observation and Analysis

1. Write an expression that can be used to determine the amount of energy released by the burning ethanol.
2. State any assumptions made in the activity.
3. State any source of error in the activity.
4. How can this error be minimized?
   Fair Testing
   When comparing different fuels, it is important to carry out a fair test. Several variables should be kept constant. They include:
   the volume of water used.
   the initial temperature of the water.
   the temperature increase.
   the distance of the flame from the calorimeter.
   More reliable results can be obtained by repeating the experiment many times• The biggest source of error in calorimetry is usually unwanted heat loss to the surroundings. This can be reduced by insulating the sides of the calorimeter and adding a lid.

Assignment 4.1

1. A •staff attendant uses 3 g of gas to boil 400 cm3 of pure water at 220C to prepare morning tea for teachers. If the gas is 85% methane, 5% carbon dioxide, 5% nitrogen; and the rest being hydrogen:
   a) calculate the amount of heat produced.
   b) i) deduce the mass of methane used.
   ii) if she uses the same mass of methane everyday, calculate the
   mass of methane used for one week.
   iii) If a kilogram of the gas costs UGX 11,000, calculate her total
   expenditure on the gas for ninety days.
2. 1 mole of gas burns in excess oxygen to produce 252,000 Joules of energy. When 2 dm3 of the gas were burnt at room temperature, they caused the temperature of 250 cm3 of water to rise from 25 oc to 450C.
   Calculate the amount of heat produced (density of water
   = 1 gcm-3,
   molar gas volume at room temperature = 24 dm3).
   Energy transformation and fuels
   When fuels are burnt, the chemical energy that is released can be used to generate another form of energy, such as heat, light, motion, or electricity.
   A fuel is a substance that provides energy as a result of a chemical change. A fuel
   may provide energy in the form of heat, light, motion, or electricity.

• Energy transformation is a change from one form of energy to another Energy transformation is also called an energy conversion. Changing chemical energy into heat is an example of an energy transformation.

Fuels contain stored energy. The stored energy in fuels can be released by combustion. Combustion is the burning of a substance. The release of energy from fuels is used to make other forms of energy.
When gasoline burns in a car engine, some of the chemical energy in the gasoline is converted into heat. The heat is converted into mechanical energy. The mechanical energy moves the car.
Burning fuel in an electric power plant produces heat. The heat is used to boil water to make steam. The steam turns a turbine. The turbine turns magnets inside a generator. The turning magnets produce an electric current. Each of the steps in this process involves an energy transformation.

Activity 4.3(b): Energy flow through an ecosystem
Work in groups.
What you need:

• relevant Chemistry textbooks
  Internet
  What to do
  Research about energy flow through an ecosystem.
  Read the following passage:
  Energy that supports ecosystems originates from the Sun.
  Primary producers convert the sunlight energy to chemical energy by a process of photosynthesis.
  Energy at producer level is lost through respiration as heat, transpiration and fossilization, where it is stored in fossil fuels, as chemical energy The surplus energy then flows to primary consumers(herbivores) and then to secondary consumers(carnivores) as chemical energy in form of food. Energy at consumer level is lost through excretion and respiration, as well as
  through fossilization, as chemical energy in fossil fuels. Decomposers feed at each trophic level breaking down organic matter releasing heat energy. Energy flow is unidirectional.
  at

Discussion questions

1. Draw a chart to show energy flow through an ecosystem
2. How can energy in fossil be utilized?

4.4 Heat of Reaction and Energy Profiles of Chemical Reactions
During a chemical reaction, there is always an energy change. Energy is given out or taken in. The energy is usually in the form of heat. (But some may be in the form of light and sound). Reactions can be divided into two groups: exothermic and endothermic.
Exothermic reaction: reactants -i products + energy
Endothermic reaction: reactants + energy products
These can be shown using a reaction profile or energy profile.
Thus, a reaction profile is a diagram showing the change in chemical potential energy, referred to as the energy pathway, as a chemical reaction proceeds from reactants to product.
During the chemical reaction, old bonds in the reactants are broken and new ones are formed in the products. The energy in the bonds is called enthalpy (H). Thus, reactants have enthalpy (H reactants) while products have enthalpy (H products). The enthalpy change of a reaction is the difference between the enthalpy of products and the enthalpy of reactants. This difference in enthalpies of reactants
and products is called the heat of reaction, AH.

• Activity 4.4:
  Work in groups.
  What you need: rulers
  What to do
  Exploring the concept of heat of reaction and how to interprete the energy profile diagram pens
  . notebooks

1. Discuss the term ‘heat of reaction’.
2. Under which reaction is the value of the heat negative or positive?
3. Study the energy profile diagrams for different reactions in Figure 4.4:

a) Identify the type of reaction in Figure 4.4.
b)Give reasons to support your answer in (a).
c)Discuss the examples of reactions in A and B.
d)What will be the sign of the value of heat of reaction in the energy profiles
A and B in Figure 4.4?
e)Give reasons to support your answer in (d).
How can you apply the concept of the heat of reaction and the profile diagrams in everyday life?

4. Compile a report of your group findings and present it to the rest of the class. The heat of reaction is the heat change in a chemical reaction at standard conditions between the numbers of moles of reactants shown in the equation for the reaction. The heat of reaction is represented by the symbol AH. The unit of AH is kJmol-1.
   a) The total energy is the same on each side of the arrow, in a reaction. So, in
   exothermic reactions, the products have lower energy than the reactants. This
   is shown on the energy level diagram, as in Figure 4.5.

Exercise 4.2
1.Are the following reactions exothermic or endothermic?
a) Reaction between sodium and water
b) Burning of a candle
c) Frying an egg
2.Draw and explain an energy level diagram for an:
a) endothermic reaction.
b) exothermic reaction.

Explaining energy changes
Making and breaking bonds
In a chemical reaction, bonds must first be broken, then new bonds form. Breaking bonds absorbs energy, while making bonds releases energy, for example, when hydrogen reacts with chlorine in sunshine, to form hydrogen chloride:

The energy absorbed in step 1 is less than the energy given out in step 2. So, this reaction gives out energy, overall. The process is exothermic.
When ammonia is split (with the help Of the catalyst like nickel) into hydrogen and nitrogen: (Here lines are used to show the bonds, and note the triple bond in nitrogen.)

The energy absorbed in step 1 is greater than the energy given out in step 2. Sot the reaction absorbs in energy, overall. The process is endothermic.

Chapter Summary
In this chapter, you have learnt that:
•substances contain chemical energy in their bonds.
chemical changes involve the breakage of the bonds in reactants and formation of new ones in products.
the breakage of bonds absorbs energy while formation of new bonds releases energy.
chemical reactions are either exothermic or endothermic. in exothermic reactions, energy is released to the surroundings. endothermic and exothermic reactions can be shown using a reaction profile.

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