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Energy changes - Chemistry Form 4

Energy Changes in Chemical and Physical Processes

Our bodies need energy in order to perform various tasks such as get out of bed, make breakfast, pedal a bicycle, read a chemistry book and so on. The energy needed to carry out these activities comes from the chemical changes that our bodies induce in the food we eats.

 What is energy, and what different forms does it take? Why do some chemical changes release energy while others absorb it? This topic will attempt to answer such questions and then apply our understanding of energy to some of the important environmental issues that people face today.

Energy is the ability to do work. This energy is stored in food, fossil fuels such as coal and oil as chemical energy. There are many forms of energy which include, chemical, heat, electrical and mechanical energy. Some substances have energy as a result of their particles moving. Such energy is known as kinetic energy (K.E.). Other substances have energy by virtue of their position e.g. coiled spring.

This type of energy is known as potential energy (P.E). The two forms of energy are interconvertible; potential energy can be transformed to kinetic energy. For example, electrical energy can be transformed into light energy and heat energy by passing electricity through a bulb filament. Energy cannot be created or destroyed. Energy changes bring about physical and chemical changes in substances. For example, when other forms of energy are converted to heat energy, there is a change in temperature.

Energy is the ability to do work. This energy is stored in food, fossil fuels such as coal and oil as chemical energy. There are many forms of energy which include, chemical, heat, electrical and mechanical energy. Some substances have energy as a result of their particles moving. Such energy is known as kinetic energy (K.E.). Other substances have energy by virtue of their position e.g. coiled spring.

This type of energy is known as potential energy (P.E). The two forms of energy are interconvertible; potential energy can be transformed to kinetic energy. For example, electrical energy can be transformed into light energy and heat energy by passing electricity through a bulb filament. Energy cannot be created or destroyed. Energy changes bring about physical and chemical changes in substances. For example, when other forms of energy are converted to heat energy, there is a change in temperature.



The following video clip shows the reaction between Sodium hydroxide solution and water. Click to play the video and observe what happens carefully.



From the experiment, we observed that on adding sodium hydroxide pellets in water, the solution formed resulted to an increase in temperature. This means that the reaction generated heat which was lost to the surrounding. Reactions that result to heat loss are said to be exothermic. If you place your palm on the beaker it feels hot. In an exothermic reaction, heat energy is given out and the temperature of the surroundings rise.

The following video clip shows the reaction between water and potassium nitrate.Click to play the video and observe what happens.


From the experiment, we observed that on adding potassium nitrate in water, the solution formed resulted to a decrease in temperature. This means that the reaction absorbed heat from the surroundings. Reactions that result to heat gain are said to be endothermic. If you place your palm on the beaker, it feels cold. In an endothermic reaction, heat energy is absorbed and the temperature of the surrounding fall.

The water put in the beaker is said to be at room temperature. The temperature changes depending on the type of reaction resulting from the substances added. The animation below shows this concept.Click on the letters (a), (b) and (c) to observe what happens.

Enthalpy refers to the heat content of a system and is denoted by letter 'H'.

Enthalpy is not measured directly, but heat lost or gained by the system is considered. This heat lost or gained by the system is referred to as enthalpy change denoted by Delta H (T H).

T H values are usually given for systems or processes occurring at constant temperatures.

In terms of products and reactants.
H = H products - H reactants

In exothermic reactions, Heat of products is less than that of reactants and the value of T H is negative.

In endothermic reactions, Heat of product is greater than that of reactants and the H value is positive.


Energy level diagrams for exothermic. Energy level diagrams are used to show relative enthalpies of the reactants and products. On the x-axis it shows the progress of the reaction from reactants to products. The Y-axis shows the enthalpy (heat content).For the exothermic reaction, the enthalpy H of the products is less than that of the reactants. The enthalpy change ( T H) will be a negative value.The H value is negative and reaction is exothermic.

For the endothermic reaction, the enthalpy, H, of the products is more than that of the reactants. Hence, the enthalpy change (T H) will be a positive value
( t H = + ve).


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In this lesson we will discuss latent heat.


In form 1 we learnt about change of physical states using ice. In the experiment, heat was applied to ice to change it to liquid which later changed to vapour on continuous heating. The graph below illustrate how temperature changes with change in state.



Graph showing temperature changes when ice is heated.

Molar Heat of Fusion

This is the amount of heat needed to convert a given amount of a solid substance into a liquid substance at its melting point.




Graph showing temperature changes when ice is heated.

The region AB represents water in solid state (ice).  Increasing temperature, results into the solid ice melting as illustrated in region BC.  At point C, all the solid ice has melted and further increase in temperature results in change of liquid to gas as illustrated in region DE.  Region BC represents the melting point of solid ice while region DE represents the boiling point.


Molar heat of fusion is the amount of heat energy required to convert one mole of a solid substance into a liquid, at its melting temperature.


Latent heat of vapourization is the heat absorbed by a substance on changing from the liquid state into the gaseous state at a constant temperature.

Molar heat of vaporization is the amount of heat energy required to convert one mole of a liquid substance into a gas at its boiling point.

Molar heats of fusion and molar heat of vapourization are used to estimate the strengths of bonds holding the particular together in solids and liquids.
Melting point of substances is high when forces holding particles in solid structure are strong, while the melting point is low when these bonds are weak.

In water for example, its melting point is higher (00C) than that of ethanol (-1170C), while boiling point of water (1000C) is higher than that of ethanol (780C). The inter molecular forces holding water molecules together are stronger than those holding ethanol molecules together . Latent heats of fusion and vaporization of water are thus higher than those for ethanol.


In this lesson we will discuss energy changes in chemical processes.


In physical processes the bonds broken during melting and vapourization are the intermolecular forces of attraction but not the strong covalent bonds between the atoms.
 

In Chemical processes the covalent bonds are broken and new ones are formed. Enthalpy change in chemical processes are determined through experiments.
The value of enthalpy change T H depends on the pressure and temperature in which the reactions are carried out.

250C (298k) and 1 atmosphere (760 mmHg) are reffered to as standard temperature and pressure respectively.

Enthalpy changes measured under these conditions are known as standard enthalpies denoted as T H O

To determine the final quantity of energy released or absorbed during a reaction, we need to know the amount of energy used to start the reaction and the amount of energy released at the end of the reaction. We then compare the two amounts of energies obtained to find out whether the energy used to start the reaction is more or less than the energy obtained.

Bond breaking and bond formation
Molecules have covalent bonds. For the molecules to react the covalent bonds must be broken first and this always requires energy.(endothermic reaction) Reactions leads to formation of new products with new covalent bonds and bond formation is always accompanied by heat loss to the environment hence exothermic reaction. For example in the reaction between hydrogen gas andChlorine gas to form Hydrogen chloride gas the H-H and Cl-Cl covalent bonds are initially broken followed by the formation of H-Cl bonds.

The table below shows the bond energies of different covalent bonds.



Bond energies of different covalent bonds


The following is a worked out example showing the steps followed in calculating the bond energies of the given reactions.

In this lesson we will discuss enthalpy of solution of compounds.




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In this lesson we will discuss enthalpy of solution of compounds.

When a substance like sodium chloride dissolves in water the water molecules interact with the Na ions and Cl ions in the solid. The energy of interaction overcomes the electrostatics forces of attraction between the Na+ and Cl- ions and the ions are separated from each other. Each of the separated ions is then surrounded by several water molecules. The overall energy change may be exothermic or endothermic and is called enthalpy change of solution.

When a substance like sodium chloride  dissolves in water the water molecules interact with the Na+ ions and Cl - ions in the solid. The energy of interaction overcomes the forces of attraction between the Na+ and Cl - ions and the ions are separated from each other. Each of the separated ions is then surrounded by several water molecules. The overall energy change may be exothermic or endothermic and is called enthalpy of solution.

Experiment to determine the molar enthalpy of solution of Sodium hydroxide


Apparatus and Chemicals

5g of sodium Hydroxide
Thermometer
Measuring cylinder
Glass rod
Plastic beaker
Distilled water
Measure 100cm3 of distilled water and transfer it into a plastic beaker.
Measure the temperature of the water and record it in the table.
Add all the 5 g of Sodium Hydroxide in the water and stir.
Note the temperature and record it as final temperature
Work out the temperature change.

The following experiment was carried out to determine the molar enthalpy of solution of Sodium hydroxide. Click to play the video and observe what happens carefully.


Heat of solution the amount of heat evolved or absorbed when one mole of a compound is completely dissolved in water so that further dissolution causes no further temperature change.

Heat of solution is the amount of heat evolved or absorbed when one mole of a compound is completely dissolved in water so that further dissolution causes no further temperature change.

Assuming the sample results obtained from the experiment are shown below. Calculate the molar enthalpy of solution of Sodium hydroxide.
Volume of water =100 cm3
 

Calculate mass of solution given that the density of water is 1g/cm3
Density =Mass/Volume
Mass = density x Volume
1gx100cm3 = 100g
ΔH = MCΔT where M =100g, C= 4.2kJ/kg,
100/1000 x4.2x12
= 5.04 kJ.

5 g of NaOH when dissolved in water released 5.04kJ.
40g (RFM NaOH) when dissolved in water released 40 x 5.04/5 =40.32 kJ/mole
 

To apply the enthalpy of solution we apply the following
ΔH = mass of water x specific heat capacity x temperature change
ΔH = M C ΔT

40 x 5.04/5 = 40.32 kJ/Mol

Example 1

The following is a worked out example showing step by step process on how to calculate the heat of solution using sample results from the video observed.

The following is a worked out example showing how to calculate the heat of solution of a given amount of Sodium hydroxide dissolved in a specific amount of water..

The following is a worked out example showing how to calculate the heat of solution of a given amount of Ammonium Chloride dissolved in a specific amount of water.


2 g of Sodium Hydroxide is dissolved in 25 cm3 of water. Calculate the amount of heat released given that the temperature change is 50C. Specific heat capacity of water is 4.2kJ/Kg/K
Click on the correct answers.

In this lesson we will discuss Enthalpy of combustion.

A combustion reaction occurs when a substance combines with oxygen. The process is commonly called burning. Most of the substances whose combustion is useful are the organic compounds because they produce heat energy.

Flame

The results obtained were as follows:
Mass of water = 200x1 =200g
Specific heat capacity = 4.2 J/g/K
Initial temperature (T1) = 22C
Final temperature (T2) = 300C
Initial mass f burner (M1) = 101 g
Final mass of burner M2 after burning = 97 g

To calculate the enthalpy of combustion using the above data the following equation is used H = MCΔT
Where M= mass of water, c= specific heat capacity of T = change in temperature.

M = Mass of water
C = specific heat capacity
T = change in temperature.)
Mass f water = 200g
Specific heat capacity = 4.2 J/g/K
Change in temperature = 30-22 =8 c.
Therefore,

H = 200 x 4.2 x8
= -6720J Or

= -6.72 KJ


The following video clip shows the experiment used to heat 200cm3 of water using ethanol.

Click to play the video and observe what happens carefully.


Example : Sample Results

The following results were obtained when a certain amount of water was heated using ethanol. Follow the worked out example carefully.

A beaker containing 250 cm3 of water at 22.50C was heated with a spirit lamp. The final temperature of the water was 36.00C. Calculate the heat enrgy transferred from the flame to the water in the beaker. The density of water is 1 gcm3 and the specific heat capacity is 4.2Jg-1K-1.

The molar enthalpy of combustion of a substance is the energy released when one mole of a substance burns completely in oxygen under standard conditions. It is given the symbol ( Hɵ ).

Example 1

The following is a worked out example showing how to calculate the heat of combustion of ethanol. Follow the worked out example carefully.

Example 2

The example below shows how to calculate the molar enthalpy of combustion of ethanol. Follow the worked out example carefully.

Example 3

The example below shows how to calculate the molar enthalpy of combustion of sulphur. Follow the worked out example carefully.


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In this lesson we will discuss enthalpy of displacement. Displacement reactions are based on the position of metals in the electrochemical series as shown below.


Displacement reactions are those in which a more reactive element displaces less reactive ones from their ionic solutions. For example when Zinc powder is placed in a solution of Copper (II) ions, Zinc metal displaces copper ions from the solution to form Copper solid which is a brown deposit while Zinc ions will be in solution. The following equation summarizes this observation.

Zinc displacing copper ions

Zn (s) + Cu 2+ (aq) Zn2+(aq) + Cu (s)


The following video clip shows how the heat of displacement of Copper is measured.

Click to play the video and observe what happens carefully.



Heat of displacement of Copper

Iron fillings reduce Copper (II) ions in copper (II) sulphate solution to copper metal. The ion fillings are oxidized to Iron (II) ions which dissolve. Excess iron fillings make sure that all the copper (II) ions in the solution are displaced by the ion.

Click on the ENTER button to follow up on a worked example.

Given that in the displacement reaction of copper using Iron, the temperature of the resulting solution rose from 25.0 to 35.0 0C calculate the molar enthalpy of displacement of copper. Assume that the density of solution = 1g/cm3 and specific heat capacity is 4.2 kj/kg/k Solution
Step 1: Calculate the enthalpy change of displacement reaction
 

ΔH =MCΔT
 

Mass of solution = density x Volume
1 x 50 cm3 = 50g
50/1000 x 4.2x10 =2.1kj
 

Step 11: Calculate the number of mole of copper ions displaced.
 

Using the equation
Fe(s)  + CuSO4(aq) FeSO4(aq) + Cu(s)
 

Ionically
Fe (s) + Cu 2+ Fe 2+ (aq) + Cu (s)
No of moles of Copper displaced
50/1000x 0.2 =10/1000 = 0.01mol
Step 111: If 0.1 moles of Copper released 2.1 kJ then 1 mole o Copper displaced released
2.1x1/0.01 = 210 kJ/mol
Molar enthalpy of displacement of copper = -210 kJ/mol
Molar enthalpy of displacement of an element is the heat change when 1 mole of that element is displaced from a solution containing the ions of that element under standard conditions.


In this lesson we will discuss enthalpy of neutralization.

A neutralization is a reaction that occurs when an acid reacts with a base or alkali to form a salt and water as the only products.

Example


The following video clip shows the reaction between hydrochloric acid and Sodium hydroxide. Click to play the video to observe what happen carefully.


The following is a worked out example showing the heat of neutralization between sodium hydroxide and hydrochloric acid. The sample results are represented in the table below.

The following is a worked out example showing the heat of neutralization between 50.0 cm3 of 2M NaOH and

50 cm3 of Sulphuric acid.

The molar enthalpy of neutralization (ΔH) is the energy change when an acid reacts with a base to produce one mole of water.
The enthalpy of neutralization reaction for weak acids/strong base is less exothermic compared to that of strong acid /strong base reaction.


In this lesson we will discuss enthalpy of Hess's law.

The standard heats of formation for many components for example carbon ( IV) oxide and magnesium oxide can be measured directly using a calorimeter. However, there are many compounds for example tetra chloromethane and methane for which the standard heats of formation of these compounds can only be obtained indirectly.

A method which involves knowledge of intermediate steps is followed to obtain the products. The values are then linked in an energy level diagram or energy cycle diagram with the heat of formation of the compound.

The following animation shows the energy level diagram for the formation of carbon (IV) Oxide.


The following animation shows the energy cycle diagram of carbon (IV) Oxide.


Both energy level and energy cycle diagrams for the formation of carbon (IV) oxide show two ways of oxidizing carbon. Route 1 involves burning carbon in excess oxygen to form carbon (IV) oxide. This is the direct way of forming carbon (IV) oxide. Route II involves burning carbon in limited oxygen to form carbon (II) Oxide in oxygen to form carbon (IV) oxide. Carbon (II) Oxide is referred to as an intermediate product.

As shown in the energy level diagram the overall enthalpy change for conversion of Carbon to Carbon (IV) oxide is the same whether it is done in one step (burning carbon in excess air) or in two steps (converting Carbon into Carbon (II) oxide then to carbon (IV) oxide. Hess's Law of constant heat summation states that the enthalpy change of a reaction is the same, regardless of the reaction happening in one step or many steps provided that initial reactants and products are the same.

Given that enthalpy of combustion of carbon ΔH1 = -393kJ/mol and enthalpy of combustion of carbon (II) Oxide ΔH3 = -283kJ/mol. Calculate the enthalpy of formation of carbon (II) Oxide.

In this lesson we will discuss fuels and find out how they pollute the environment.

Fuels are substances that burn in air with the liberation of energy (heat) which can be used in different ways for example to keep warm, to cook, to keep engines running. Fuels may be solids, liquids or gases. The heating value of a fuel refers to the molar enthalpy of the fuel divided by the formula mass of the fuel.

Many of the products of burning fuel are poisonous and pollute the environment. e.g Sulphur (IV) oxide and Nitrogen (IV) oxide which are emitted from the fuels combine with water to form acid rain which damages trees and lakes.

The following are examples of fuels used.

A fuel is a substance that releases energy when burnt. The energy produced in such a combustion process is used either directly as heat or converted to other forms e.g. electrical energy.

Factors to consider when choosing a fuel.
Heating value: The fuels that have a higher heating value e.g. gaseous fuel are more suitable than solid fuels which have less heating value. The following are some of the heating values of some fuels.


2.Ease and rate of combustion

If a fuel is to be used for domestic heating slow burning is preferred. Wood and charcoal are usually chosen because they are cheap and relatively easily available. A fuel that propel a rocket must be able to produce a huge amount of energy in a very short time hence combustion rate should be high.
3.Availability : The fuel to be chosen must be one that is available preferably within the area where it is required
4. Ease of storage and Transportation

Fuels in liquids form require less storage space compared to solids and gases. They are thus easier to transport as well.

Environmental effects- When choosing a fuel to use it is important to take into consideration its impact on the environment. A fuel that is environmentally friendly and non polluting is preferred. It would be hazardous to use petroleum products containing high levels of sulphur or petrol containing dangerous additives such as tetraethyl lead (the anti knock addictive for petrol) which give poisonous lead compounds after burning the engine. Good fuel should be free from toxic products of combustion.

The Carbon (IV) oxide emitted from factories, vehicles, burning wood and charcoal causes global warming which brings about melting of ice caps.

The following illustration shows the effects of global warming.

Carbon (IV) oxide emitted from cooking jikos and due to incomplete combustion of fuels combines with haemoglobin in the blood to form Carbon haemoglobin stable compound which causes suffocation (Prevent absorption of Oxygen)

The Sulphur (IV) oxide emitted from the factories is inhaled by human beings and animals causes respiratory diseases and may lead to death.

The Sulphur (IV) oxide and Nitrogen (IV) oxide emitted from the factories and motor vehicles combine with moisture in the air and falls back as acid rain.

The following are some of the effects of acid rain.


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