Chemistry/Biology 12 (Nova Scotia Curriculum) JOHNMAR S. DELIGERO Chemistry/Biology 12 (Nova Scotia Curriculum) Sino-Canadian Program Henan Experimental High School Zhengzhou Henan, China http://www.bananateachersworld.wikispaces.com

What happens during a phase change? What happens during a chemical reaction? These changes involve changes in the potential energy of a system, not the kinetic energy.

Enthalpy (H, also known as heat content) is the total energy of a system, some of which is stored as chemical potential energy in the chemical bonds. Chemists use the term enthalpy change (ΔH) to refer to the potential energy change of a system during a process such as a chemical reaction or a physical change. Enthalpy changes are measured at constant pressure. The units of enthalpy change are kJ/mol.

The enthalpy change of a chemical reaction represents the difference between the potential energy of the products and the potential energy of the reactants. In chemical reactions, potential energy changes result from chemical bonds being broken and formed. Chemical bonds are sources of stored energy (potential energy). Breaking a bond is a process that requires energy. Creating a bond is a process that releases energy.

Chemists define the total internal energy of a substance at a constant pressure as its enthalpy, H. Chemists do not work with the absolute enthalpy of the reactants and products in a physical or chemical process. Instead, they study the enthalpy change, that accompanies a process. That is, they study the relative enthalpy of the reactants and products in a system. This is like saying that the distance between your home and your school is 2 km. You do not usually talk about the absolute position of your home and school in terms of their latitude, longitude, and elevation. You talk about their relative position, in relation to each other.

- A chemical bond is caused by the attraction between the electrons and nuclei of two atoms. - Energy is needed to break a chemical bond, just like energy is needed to break a link in a chain. On the other hand, making a chemical bond releases energy. - The strength of a bond depends on how much energy is needed to break the bond. - When a reaction results in a net absorption of energy, it is called an endothermic reaction. - On the other hand, when a reaction results in a net release of energy, it is called an exothermic reaction. - In an exothermic reaction, more energy is released to form bonds than is used to break bonds. Therefore, energy is released.

The enthalpy change of a chemical reaction is known as the enthalpy of reaction, ΔHrxn. The enthalpy of reaction is dependent on conditions such as temperature and pressure. Therefore, chemists often talk about the standard enthalpy of reaction, ΔH0rxn: the enthalpy change of a chemical reaction that occurs at SATP (25°C and 100 kPa). Often, ΔH0rxn is written simply as ΔH0. The “0” symbol is called “nought.” It refers to a property of a substance at a standard state or under standard conditions. You may see the enthalpy of reaction referred to as the heat of reaction in other chemistry books.

Representing Exothermic Reactions There are three different ways to represent the enthalpy change of an exothermic reaction. The simplest way is to use a thermochemical equation: a balanced chemical equation that indicates the amount of heat that is absorbed or released by the reaction it represents. For example, consider the exothermic reaction of one mole of hydrogen gas with half a mole of oxygen gas to produce liquid water. For each mole of hydrogen gas that reacts, 285.8 kJ of heat is produced. Notice that the heat term is included with the products because heat is produced. H2(g) + 1/2O2(g) → H2O(l) + 285.8 kJ 2. You can also indicate the enthalpy of reaction as a separate expression beside the chemical equation. For exothermic reactions, ΔH° is always negative. H2(g) + 1/2O2(g) → H2O(l) ΔH0rxn = −285.8 kJ/mol 3. A third way to represent the enthalpy of reaction is to use an enthalpy diagram.

Representing Endothermic Reactions As for an exothermic reaction, there are three different ways to represent the enthalpy change of an endothermic reaction. 1. You can include the enthalpy of reaction as a heat term in the chemical equation. Because heat is absorbed in an endothermic reaction, the heat term is included on the reactant side of the equation. 117.3 kJ + MgCO3(s) → MgO(s) + CO2(g) 2. You can also indicate the enthalpy of reaction as a separate expression beside the chemical reaction. For endothermic reactions, the enthalpy of reaction is always positive. MgCO3(s) → MgO(s) + CO2(g) ΔH0rxn = 117.3 kJ/mol 3. Finally, you can use a diagram to show the enthalpy of reaction.

117.3 kJ + MgCO3(s) → MgO(s) + CO2(g) The enthalpy change associated with a reaction depends on the amount of reactants involved. For example, the thermochemical equation for the decomposition of magnesium carbonate indicates that 117.3 kJ of energy is absorbed when one mole, or 84.32 g, of magnesium carbonate decomposes. The decomposition of two moles of magnesium carbonate absorbs twice as much energy, or 234.6 kJ. 117.3 kJ + MgCO3(s) → MgO(s) + CO2(g) 234.6 kJ + 2MgCO3(s) → 2MgO(s) + 2CO2(g) Enthalpy of reaction is linearly dependent on the amount of substances that react. That is, if the amount of reactants doubles, the enthalpy change also doubles. In other words, when you multiply the stoichiometric coefficients of a thermochemical equation by any factor, you must multiply the heat term or enthalpy expression by the same factor.

Standard Molar Enthalpy of Formation In a formation reaction, a substance is formed from elements in their standard states. The enthalpy change of a formation reaction is called the standard molar enthalpy of formation, ΔH0f. The standard molar enthalpy of formation is the quantity of energy that is absorbed or released when one mole of a compound is formed directly from its elements in their standard states. When writing a formation equation, always write the elements in their standard states. For example, examine the equation for the formation of water directly from its elements under standard conditions. H2(g) + 1/2O2(g) → H2O() ΔH0f = −285.8 kJ A formation equation should show the formation of exactly one mole of the compound of interest. The following equation shows the formation of benzene, C6H6 under standard conditions. 6C(graphite) + 3H2(g) → C6H6(l) ΔH0f = 49.1 kJ

Standard Molar Enthalpy of Combustion The standard molar enthalpy of combustion, ΔH0comb, is the enthalpy associated with the combustion of 1 mol of a given substance. The change in enthalpy is measured for the products and reactants in their standard states. For example, for methane, ΔH0comb = −965.1 kJ/mol. You can represent the standard molar enthalpy of combustion using a thermochemical equation or using an enthalpy diagram. CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 965.1 kJ Notice that water is shown in the liquid form. Although water is formed as vapour during a combustion reaction, the enthalpy change for a standard molar enthalpy of combustion is measured with the energy change required for products to cool to SATP taken into consideration.

The energy released or absorbed during a chemical reaction depends on the reactants involved. For example, the reaction of hydrogen and oxygen to form water releases heat to the surroundings, while the reaction of magnesium carbonate to form magnesium oxide and carbon dioxide absorbs heat from the surroundings. For each chemical reaction at SATP, there is a specific enthalpy change.

What other factors affect the enthalpy change of a given chemical reaction? Compare the warmth you feel from a burning wooden match and the warmth you feel from a roaring bonfire. Both involve the same chemical reactions (the combustion of wood, primarily cellulose), but the heat released in each case is clearly different. There is a great deal more cellulose and oxygen involved in the reactions in the bonfire compared to the burning matchstick. Therefore, the amount of reactants present plays a role in the energy change. As you know, the symbol for amount is n, and the unit is the mole (mol). The heat released or absorbed by a system during a chemical change can be calculated using the relationship:

Like chemical reactions, changes of state involve changes in the potential energy of a system only. The temperature of the system undergoing the state change remains constant. Because energy is absorbed or released as heat, however, the temperature of the surroundings often changes.

- The energy changes associated with changes of state are important in regulating body temperature. When you sweat, for example, the water absorbs heat from your skin as the water vaporizes. Cats lick their fur, and the vaporizing liquid helps keep them cool. Dogs cool off in a similar way. The evaporating water absorbs heat from their mouths and helps keep them cool.

The energy change associated with a physical change is smaller than the energy change associated with a chemical change. In the case of molecular substances, for example, changes of state involve the breaking of intermolecular forces. Intermolecular forces are generally much weaker than chemical bonds. The energy released or absorbed when intermolecular bonds form or break is much less than the energy released or absorbed when chemical bonds form or break. Enthalpy changes for changes of state between liquid and gas are greater than changes of state between liquid and solid. For example, when a solid melts to form a liquid the attractive forces between particles are not completely broken. The particles remain close together.

When a liquid changes to a gas, however, the attractive forces are completely broken as relatively great distances separate the particles from each other. The difference in potential energy between a liquid and a gas is much greater than the difference in potential energy between a solid and a liquid.

You can represent the enthalpy change that accompanies a change of state—from liquid to solid, for example—just like you represented the enthalpy change of a chemical reaction. You can include a heat term in the equation, or you can use a separate expression of enthalpy change. For example, when one mole of water melts, it absorbs 6.02 kJ of energy. H2O(s) + 6.02 kJ → H2O(l) H2O(s) → H2O(l) ΔH = 6.02 kJ/mol

Normally, however, chemists represent enthalpy changes associated with phase changes using modified ΔH symbols. These symbols are described below. • molar enthalpy of vaporization, ΔHvap : the enthalpy change for the state change of one mole from liquid to gas. • molar enthalpy of condensation, ΔHcond: the enthalpy change for the state change of one mole of a substance from gas to liquid. • molar enthalpy of melting, Δhmelt or enthalpy of fusion, ΔHfus. the enthalpy change for the state change of one mole of a substance from solid to liquid. • molar enthalpy of freezing, ΔHfre : the enthalpy change for the state change of one mole of a substance from liquid to solid.

Vaporization and condensation are opposite processes Vaporization and condensation are opposite processes. Thus, the enthalpy changes for these processes have the same value but opposite signs. For example, 6.02 kJ of heat is needed to vaporize one mole of water. Therefore, 6.02 kJ of heat is released when one mole of water freezes. ΔHvap = −ΔHcond Similarly, melting and freezing are opposite processes. ΔHmelt = −ΔHfre

Another type of physical change that involves a heat transfer is dissolution. When 1 mol of a solute dissolves in a solvent, the enthalpy change that occurs is called the molar enthalpy of solution, ΔHsoln. Dissolution can be either endothermic or exothermic.

Manufacturers take advantage of endothermic dissolution to produce cold packs that athletes can use to treat injuries. One type of cold pack contains water and a salt, such as ammonium nitrate, in separate compartments. When you crush the pack, the membrane that divides the compartments breaks, and the salt dissolves. This dissolution process is endothermic. It absorbs heat for a short time, so the cold pack feels cold. The molar enthalpy of solution, ΔHsoln, of ammonium nitrate is 25.7 kJ/mol. NH4NO3(s) + 25.7 kJ → NH4NO3(aq) Some types of hot packs are constructed in much the same way as the cold packs described above. They have two compartments. One compartment contains a salt, such as calcium chloride. The other compartment contains water. In hot packs, however, the dissolution process is exothermic. The process releases heat to the surroundings. The molar enthalpy of solution, ΔHsoln, of calcium chloride is −82.8 kJ/mol. CaCl2(s) → CaCl2(aq) + 82.8 kJ

You can determine the heat absorbed or released by a state change or dissolution using the equation q = nΔH, just as you did with chemical reactions.