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Why Exothermic Or Endothermic?


If you understand the above section, then you can now identify whether a reaction is exothermic or endothermic. If it gives off one of the above three types of energy then it's exothermic, if it absorbs it, then it's endothermic. The question that still hasn't been answered, though is why? Why are some reactions exothermic while others are endothermic, and why does energy have to be absorbed or released at all?

The answer lies in chemical bonds. Chemical bonds have bond energies associated with them. This bond energy is the amount energy that it takes to break the bonds, and also the amount of the energy that is released when the bonds are formed. Consequently, if the bonds in your reactants have a higher total bond energy than your products, the reaction will be endothermic. If they have a lower total bond energy, it will be exothermic.

The reason for this is the law of conservation of energy, which states that energy cannot be created or destroyed; it can only change forms. In this case, it would mean that whatever energy was used to break the bond will be released if the bond is reformed. For example:

Suppose we have a C-H bond somewhere, and we wanted to break that bond apart into a C and an H. We'd have to put in some amount of energy. Let's call this amount 'x'. Once we put in x energy, by say, adding heat, the C-H bond will break apart. What happened to 'x' though? The conservation of energy law says that 'x' didn't just disappear; it just took on another form, in this case exciting the electrons in C and H. Some of the energy went to the C atom and some went to the H atom. If the C-H bond reformed, then 'x' would be released again. If the C went off and recombined with a different molecule (let's say a Cl), and so did the H (with an F, for instance). Then the energy released from the new pairings would be 'x' plus whatever energy the Cl and F had stored.

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy (ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0). In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

An exothermic reaction is one in which heat is produced as one of the end products.   Examples of exothermic reactions from our daily life are combustion like the burning of a candle, wood, and neutralization reactions. In an endothermic reaction, the opposite happens. In this reaction, heat is absorbed. Or more exactly, heat is required to complete the reaction. Photosynthesis in plants is a chemical endothermic reaction. In this process, the chloroplasts in the leaves absorb the sunlight. Without sunlight or some other similar source of energy, this reaction cannot be completed.

In exothermic reactions the enthalpy change is always negative while in endothermic reactions the enthalpy change is always positive. This is due to the releasing and absorption of heat energy in the reactions, respectively. The end products are stable in exothermic reactions. The end products of endothermic reactions are less stable. This is due to the weak bonds formed.

‘Endo’ means to absorb and so in endothermic reactions, the energy is absorbed from the external surrounding environment. So the surroundings lose energy and as a result

the end product has higher energy level than the reactants. Due to this higher energy bonds, the product is less stable. And most of the endothermic reactions are not spontaneous. ‘Exo’ means to give off and so energy is liberated in exothermic reactions. As a result, the surroundings get heated up. And most exothermic reactions are spontaneous.

When we light a matchstick, it is an exothermic reaction. In this reaction, when we strike the stick, stored energy is released as heat spontaneously. And the flame will have lower energy than the heat produced. The energy being released is previously stored in the matchstick and thus do not require any external energy for the reaction to occur.

When ice melts, it will be due to the heat around. The surrounding environment will have a higher temperature than the ice and this heat energy is absorbed by the ice. The stability of the bonds is reduced and as a result and the ice melts into liquid.

Some exothermic reactions in our lives are the digestion of food in our body, combustion reactions, water condensations, bomb explosions, and adding an alkali metal to water.

Reaction rate
Reaction rate, the speed at which a chemical reaction proceeds. It is often expressed in terms of either the concentration (amount per unit volume) of a product that is formed in a unit of time or the concentration of a reactant that is consumed in a unit of time. Alternatively, it may be defined in terms of the amounts of the reactants consumed or products formed in a unit of time. For example, suppose that the balancedchemical equation for a reaction is of the formA + 3B → 2Z.

The rate could be expressed in the following alternative ways:d[Z]/dt, –d[A]/dt, –d[B]/dtdz/dt, −da/dt, −db/dtwhere t is the time, [A], [B], and [Z] are the concentrations of the substances, and a, b, and z are their amounts. Note that these six expressions are all different from one another but are simply related. Chemical reactions proceed at vastly different speeds depending on the nature of the reacting substances, the type of chemical transformation, the temperature, and other factors. In general, reactions in which atoms or ions (electrically charged particles) combine occur very rapidly, while those in which covalent bonds(bonds in which atoms share electrons) are broken are much slower. For a given reaction, the speed of the reaction will vary with the temperature, thepressure, and the amounts of reactants present. Reactions usually slow down as time goes on because of the depletion of the reactants. In some cases the addition of a substance that is not itself a reactant, called a catalyst, accelerates a reaction. The rate constant, or the specific rate constant, is the proportionality constant in the equation that expresses the relationship between the rate of a chemical reaction and the concentrations of the reacting substances. The measurement and interpretation of reactions constitute the branch of chemistry known as chemical kinetics.

The reaction rate (rate of reaction) or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place. For example, the oxidative rusting of iron under Earth's atmosphere is a slow reaction that can take many years, but the combustion of cellulose in a fire is a reaction that takes place in fractions of a second.

Chemical kinetics is the part of physical chemistry that studies reaction rates. The concepts of chemical kinetics are applied in many disciplines, such as chemical engineeringenzymology and environmental engineering.


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