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=== Welcome to my learning package on the topic energy change. ===

(c) describe bond breaking as an endothermic process and bond making as an exothermic process (d) explain overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent bonds = = = Content: =

Introduction--
There are many types of reactions. Previously we have learnt or even experimented with several reactions, including but not limited to:


 * 1) Neutralisation
 * 2) Photosynthesis
 * 3) Acid-metal reactions
 * 4) Precipitation

One other very interesting reaction would be the thermite reaction, which is essentially the reaction between iron(III) oxide with aluminum to produce molten iron and aluminum(III) oxide. An interesting video is embedded below for your viewing pleasure:

Equation: Fe 2O3 + 2Al--> Al2O3 + 2Fe

[|Thermite vs Ice!!!] [|Thermite Reaction]

Chemical reactions are certainly intriguing. However, with the huge variety of reactions, how then can we study them more systematically? We can generally spilt them into exothermic and endothermic reactions...

Exothermic Changes

Definition: An exothermic change is one in which heat energy is given out.

This usually happens when bonds are formed. The enthalpy change, H, is negative. Below are some examples of exothermic reactions:

1)Changes of state when bonds are formed-- These include condensation, when water changes from gaseous to liquid state, and  freezing, when water changes from liquid to solid state. In both reactions, heat energy  is given off to the surroundings and the enthalpy change is negative.

Embedded below is an interesting experiment on supercooling water:

media type="youtube" key="xuhUTaFmaX8" height="390" width="480"

2) Dissolving of some anhydrous salts in water:

Due to the fundamental fact that dissolving some salts mean forming bonds between the anhydrous salt and water, bonds are formed and heat is given off in the process. ( The temperature of the solution increases) An example would be the dissolving of anhydrous sodium carbonate(solid) to from sodium carbonate(aqueous).

Equation: Na2CO3(s)-->Na2CO3(aq) + heat

Note: The salt has to be anhydrous. Dissolving salts such as sodium chloride is in fact an endothermic reaction, as bonds between the sodium and chloride ions are broken in the process. Rule of thumb: Bonds must be formed!!!

3) Acid-metal reactions The reaction between acids and metal to produce hydrogen and salt is an exothermic reaction. The energy released during bond forming ( to produce hydrogen and the salt) is greater than the energy absorbed during bond breaking( of the bonds in the acid and the metal). This results is more heat given off to the surroundings relative to the amount of energy absorbed, resulting in negative enthalpy change.



Equation:2HCl(aq) + 2Na(s)--> 2NaCl(aq)+ H2(g)

4) Neutralisation Equation: HCl(aq) + NaOH(aq) --> NaCl( aq) + H2O(l)

When an acid and an alkali react, heat energy is given out. The heat energy given off during bond formation in the products is greater than the energy absorbed during bond breaking in the reactants, resulting in negative enthalpy change. ( Exothermic reaction) An ionic reaction: H+(aq) + OH-(aq)-->H2O(l), eliminates the spectator ions and show that the reaction between the hydrogen ions in the acid and the hydroxide ions in the alkali result in bond formation to form water molecules.

Embedded above is a typical energy diagram of any exothermic reaction. Students are reminded to check for the presence of the following in the circumstance that they are asked to draw an energy diagram.


 * Axes--This is very important as without the labeling of the axes, the entire graph would be meaningless
 * Activation Energy--This is the amount of energy required to 'start' the reaction. More would be covered on activation energy later.
 * Reactants and Products--It is essential for students to label the reactants and the products to properly indicate the enthalpy change as the reaction proceeds.
 * The enthalpy change, H-- the overall change in energy. During an exothermic reaction, enthalpy change is always <0.

Endothermic Reactions

Definition: An endothermic change is one in which heat energy is absorbed.

This usually happens when bonds are broken. The enthalpy change, H, is positive. Below are some examples of endothermic reactions:

1)Changes of state when bonds are broken-- These include evaporation, when water changes from liquid to gaseous state, and  melting, when water changes from solid to liquid state. In both reactions, heat energy  is absorbed from the surroundings and the enthalpy change is positive.

Fun Fact: This explains why our sweat glands are more active when there is an overall increase of heat in our body. The evaporation of sweat is an endothermic reaction and hence removes latent heat from our body, resulting in the removal of excess heat!!!

2) Dissolving of some salts in water As previously explained, there are certain cases when dissolving salts in water are exothermic and there are cases when the reaction is endothermic. During the dissolving of ammonium chloride in water, for example, there is a decrease in temperature. This can be explained as energy being absorbed to break the ionic bonds between the ammonium ions and the chloride ions.

Ionic equation: NH4Cl(s) --> NH4+( aq) + Cl-( aq) The below video explains another endothermic reaction, the dissolving of sodium chloride in water.

media type="youtube" key="EBfGcTAJF4o" height="390" width="480"

3) Photosynthesis

Equation: 6CO2 + 6H2O + Energy -->C6H12O6 + 6O2

The above equation shows the process of photosynthesis. This process is endothermic as the energy absorbed during the breaking of bonds in the reactants is greater than the energy released during bond formation in the products, resulting in a positive enthalpy change. Below is a funny song explaining the process of photosynthesis to those unfamiliar with this essential reaction. [|Photosynthesis song!!!!]

4)Decomposition of compounds Decomposition is essentially the breaking of bonds in the compounds. This is an endothermic reaction and explains why the decomposition of compounds require heat to decompose them. An apt example is the decomposition of green copper(II) carbonate into black copper(II) oxide and carbon dioxide upon the provision of heat energy.

CuCO3 (s) --> CuO(s) + CO2(g)



E mbedded above is a typical energy diagram of any endothermic reaction. Students are reminded to check for the presence of the following in the circumstance that they are asked to draw an energy diagram.
 * Axes--This is very important as without the labeling of the axes, the entire graph would be meaningless
 * Activation Energy--This is the amount of energy required to 'start' the reaction. More would be covered on activation energy later.
 * Reactants and Products--It is essential for students to label the reactants and the products to properly indicate the enthalpy change as the reaction proceeds.
 * The enthalpy change, H-- the overall change in energy. During an endothermic reaction, enthalpy change is always >0.

Enthalpy change

Be it an endothermic reaction or an exothermic reaction, the overall energy change throughout the reaction is the heat of reaction or the enthalpy change of the reaction. This energy change is represented with the symbol H and is usually measure in kilojoules(kJ)

The overall enthalpy change can be calculated by adding the energy absorbed during bond breaking in the reactants( positive) and the energy released during bond formation in the products ( negative). An overall positive change signifies that the reaction is endothermic, while a negative change signifies an exothermic reaction.

Embedded below is a simple table of bond energies for common bonds:



Example:

Equation: 2H2+ O2 --> 2H2O

Chemical bonds-

H2-- bonds present= 2 H-H bonds

O2- bonds present= 1 o=o bond

H2O- bonds present= 4 O-H bonds

Hence, during bond breaking, ^H=+ ( 2*436+ 496)= +1368kJ

note: positive sign denotes energy absorbed

During bond formation, ^H= -( 4*463)=-1852kJ

note: negative sign denotes energy released

Hence, overall enthalpy change= +1368-1852=-484kJ

Since overall enthalpy change is <0, reaction is exothermic!!! ( more energy is released relative to absorbed)

__ Activation Energy __

Definition: Activation energy is the minimum energy needed to start a reaction.

All chemical reactions require a minimum amount of energy, or a spark, to start. This happens when particles involved in the reaction collide with sufficient energy and orientation to provide the the activation energy needed to break the bonds and start a reaction. This activation energy is denoted by the 'mountain' in the energy diagram and must be clearly labelled when students are asked to draw it.



Quiz time!!!! Pls complete the following quiz:) Have fun!  [|Know your energy change!!!]