Energy+Changes+in+Chemical+Reactions

When a physical change (change in state) or chemical reaction occurs there is always an energy change involved. The energy change may only be small, but results from the breaking **and** making of bonds within molecules and between particles (atoms, ions or molecules). The energy content of a system is a combination of kinetic and potential energy. This heat content is called **enthalpy,** symbol //H//. It is measured in joules (J) or kilojoules (kJ).
 * ENERGY OF CHANGE - THERMOCHEMISTRY **

When a chemical reaction or change of state occurs, a change in the heat energy of the surroundings is observed. This heat energy change, called the enthalpy change, D r //H, // (where D r //H //= //H//products - //H//reactants), will mean the temperature of the surroundings will either increase or decrease. The change is said to be either **exothermic** or **endothermic.**

In an **exothermic** change the temperature of surroundings increases as heat energy is released. This means that the energy required to break the bonds of the reactants was less than the energy released in forming the new bonds of the products. The chemicals themselves now have less internal energy than they had initially, and D r //H // is negative.
 * Exothermic Change **

Some examples of exothermic processes are  i) combustion reactions such as burning coal or wood, <span style="font-family: Arial,sans-serif; font-size: 11pt;"> ii) reacting Mg metal with dilute HCl <span style="font-family: Arial,sans-serif; font-size: 11pt;"> iii) dissolving sodium hydroxide in water <span style="font-family: Arial,sans-serif; font-size: 11pt;"> iv) cooling steam <span style="font-family: Arial,sans-serif; font-size: 11pt;"> v) freezing water

<span style="font-family: Arial,sans-serif; font-size: 11pt;">Both i) and ii) are chemical reactions, while iii), iv) and v) are all physical changes. Any change from gas to liquid, or liquid to solid will be exothermic as the kinetic energy of the particles is being decreased.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">In an **endothermic** change the temperature of surroundings decreases as heat energy is absorbed. This means that the energy required to break the bonds of the reactants was more than the energy released in forming the new bonds of the products. The chemicals themselves now have more internal energy than they had initially, and D <span style="font-family: Arial,sans-serif; font-size: 11pt;">r //<span style="font-family: Arial,sans-serif; font-size: 11pt;">H //<span style="font-family: Arial,sans-serif; font-size: 11pt;"> is positive.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Endothermic Change **



<span style="font-family: Arial,sans-serif; font-size: 11pt;">Examples of processes which involve an endothermic change are <span style="font-family: Arial,sans-serif; font-size: 11pt;"> i) ice melting <span style="font-family: Arial,sans-serif; font-size: 11pt;"> ii) ammonium nitrate dissolving <span style="font-family: Arial,sans-serif; font-size: 11pt;"> iii) photosynthesis <span style="font-family: Arial,sans-serif; font-size: 11pt;">Note that any phase change from solid to liquid, or liquid to gas will be endothermic as the particles within the system have increased kinetic energy and must absorb this from the surroundings.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">For any reaction, the amount of energy absorbed or released depends only on the actual amount of reactant involved. If the amount reacted is doubled then the magnitude of D <span style="font-family: Arial,sans-serif; font-size: 11pt;">r //<span style="font-family: Arial,sans-serif; font-size: 11pt;">H //<span style="font-family: Arial,sans-serif; font-size: 11pt;"> will double. <span style="font-family: Arial,sans-serif; font-size: 11pt;">In general, the value of D <span style="font-family: Arial,sans-serif; font-size: 11pt;">H given for a chemical reaction is related to a specific balanced equation.
 * <span style="font-family: Arial,sans-serif; font-size: 11pt;">Energy change and the Amount of Reactant **

<span style="font-family: Arial,sans-serif; font-size: 11pt;">For example, consider the reaction <span style="font-family: Arial,sans-serif; font-size: 11pt;"> N2(g) + 3H2(g) ® <span style="font-family: Arial,sans-serif; font-size: 11pt;"> 2NH3(g) D <span style="font-family: Arial,sans-serif; font-size: 11pt;">r //<span style="font-family: Arial,sans-serif; font-size: 11pt;">H //<span style="font-family: Arial,sans-serif; font-size: 11pt;">= -92 kJ mol-1 <span style="font-family: Arial,sans-serif; font-size: 11pt;">This means that when 1 mole of N2 gas reacts with 3 moles of H2 gas to form 2 moles of ammonia <span style="font-family: Arial,sans-serif; font-size: 11pt;">gas, 92 kJ of energy is released i.e. the D <span style="font-family: Arial,sans-serif; font-size: 11pt;">H value is for the molar values shown in the equation. <span style="font-family: Arial,sans-serif; font-size: 11pt;">If instead of reacting 1 mole of N2 we used 2 moles of N2 and 6 moles of H2 then the total energy <span style="font-family: Arial,sans-serif; font-size: 11pt;">energy released would be 2 x 92 kJ or 184 kJ. <span style="font-family: Arial,sans-serif; font-size: 11pt;">Using the relationship m = n x //M// it is possible to calculate the mass of each species which would react to produce the given amount of energy i.e. D <span style="font-family: Arial,sans-serif; font-size: 11pt;">r //<span style="font-family: Arial,sans-serif; font-size: 11pt;">H //<span style="font-family: Arial,sans-serif; font-size: 11pt;">= -92 kJ mol-1 <span style="font-family: Arial,sans-serif; font-size: 11pt;">which means the mass of N2 that would react is (1 mol x 28 g mol-1) = 28 g of N2

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