Chapter 10

Energy as Heat A sample can transfer energy to another sample. One of the simplest ways energy is transferred is as heat. Heat is the energy transferred between objects that are at different temperatures Though energy has many different forms, all energy is measured in units called joules (J).

Temperature is a measure of how hot (or cold) something is; specifically, a measure of the average kinetic energy of the particles in an object. When samples of different temperatures are in contact, energy is transferred from the sample that has the higher temperature to the sample that has the lower temperature.

Temperature is an intensive property, which means that the temperature of a sample does not depend on the amount of the sample. Heat is an extensive property, which means that the amount of energy transferred as heat by a sample depends on the amount of the sample.

Changes in energy content can be determined. If 73 J of energy enter a piece of silver and no change in pressure occurs, we know that the enthalpy of the silver has increased by 73 J. Enthalpy, which is represented by the symbol H, is the total energy content of a sample.

Enthalpy is the sum of the internal energy of a system plus the product of the system’s volume multiplied by the pressure that the system exerts on its surroundings.

The particles in a sample are in constant motion. The enthalpy of a sample includes the total kinetic energy of its particles. Both the total and average kinetic energies of a substance’s particles are important to chemistry, because these quantities account for every particle’s kinetic energy.

Molar heat capacity can be used to determine the enthalpy change of a sample. The molar heat capacity of a pure substance is the energy as heat needed to increase the temperature of 1 mol of the substance by 1 K. Molar heat capacity has the symbol C and the unit J/K  mol.

When a substance receives energy in the form of heat, its enthalpy increases and the kinetic energy of the particles that make up the substance increases. The direction in which any particle moves is not related to the direction in which its neighboring particles move. The motions of these particles are random .

Because enthalpy is the total energy of a system, it is an important quantity. The only way to measure energy is through a change. Absolute values cannot be measured. The enthalpy change for one mole of a pure substance is called molar enthalpy change.

When a pure substance is only heated or cooled, the amount of heat involved is the same as the enthalpy change.  H = q for the heating or cooling of substances

A change in enthalpy during a reaction depends on many variables. Temperature is one of the most important variables. To standardize the enthalpies of reactions, data are often presented for reactions in which both reactants and products have the standard thermodynamic temperature of 25.00  C or 298.15 K.

Chemists usually present a thermodynamic value for a chemical reaction by using the chemical equation. This equation shows that when 0.5 mol of H 2 reacts with 0.5 mol of Br 2 to produce 1 mol HBr and all have a temperature of 298.15 K, the enthalpy decreases by 36.4 kJ.

For the H 2 and Br 2 reaction, in which  H is negative, the total energy of the reaction decreases. The energy is released as heat by the system. If the reaction was endothermic, energy in the form of heat would be absorbed by the system and the enthalpy would increase.

The experimental measurement of an enthalpy change for a reaction is called calorimetry. Calorimetry is the measurement of heat-related constants, such as specific heat or latent heat Combustion reactions are always exothermic. The enthalpy changes of combustion reactions are determined using a bomb calorimeter. A calorimeter Is a device used to measure the heat absorbed or released in a chemical or physical change

Inside the pressurized oxygen atmosphere of a bomb calorimeter, most organic matter, including food, fabrics, and plastics, will ignite easily and burn rapidly. Sample sizes are chosen so that there is excess oxygen during the combustion reactions. Under these conditions, the reactions go to completion and produce carbon dioxide, water, and possibly other compounds.

Any two processes that both start with the same reactants in the same state and finish with the same products in the same state will have the same enthalpy change. Hess’s law states that the overall enthalpy change in a reaction is equal to the sum of the enthalpy changes for the individual steps in the process.

When phosphorus is burned in excess chlorine 4 mol of phosphorus pentachloride, PCl 5 , is synthesized. P 4 ( s ) + 10Cl 2 ( g )  4PCl 5 ( g )  H = -1596 kJ Phosphorus pentachloride may also be prepared in a two-step process. Step 1: P 4 ( s ) + 6Cl 2 ( g )  4PCl 3 ( g )  H = -1224 kJ Step 2: PCl 3 ( g ) + Cl 2 ( g )  PCl 5 ( g )  H = -93 kJ

The second reaction must take place four times for each occurrence of the first reaction in the two-step process. This two-step process is more accurately described by the following equations. P 4 ( s ) + 6Cl 2 ( g )  4PCl 3 ( g )  H = -1224 kJ 4PCl 3 ( g ) + 4Cl 2 ( g )  4PCl 5 ( g )  H = 4(-93 kJ) = -372 kJ

So, the total change in enthalpy by the two-step process is as follows: (-1224 kJ) + (-372 kJ) = -1596 kJ This enthalpy change,  H, for the two-step process is the same as the enthalpy change for the direct route of the formation of PCl 5 . This example is in agreement with Hess’s law.

The enthalpy of the formation of CO, when CO 2 and solid carbon are reactants, is found using the equations below. 2C( s ) + O 2 ( g )  2CO( g )  H = -221 kJ C( s ) + O 2 ( g )  CO 2 ( g )  H = -393 kJ You cannot simply add these equations because CO 2 would not be a reactant.

Adding the two equations gives the equation for the formation of CO by using CO 2 and C. 2C( s ) + O 2 ( g )  2CO( g )  H = – 221 kJ CO 2 ( g )  C( s ) + O 2 ( g )  H = 393 kJ 2C( s ) + O 2 ( g ) + CO 2 ( g )  2CO( g ) + C( s ) + O 2 ( g )  H = 172 kJ Oxygen and carbon that appear on both sides of the equation can be canceled. C( s ) + CO2( g )  2CO( g )  H = 172 kJ

The enthalpy change in forming 1 mol of a substance is called the standard enthalpy of formation of the substance,  The values of the standard enthalpies of formation for elements are 0. The following equation is used to determine the enthalpy change of a chemical reaction from the standard enthalpies of formation.  H reaction =  H products -  H reactants

Chapter 10

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Chapter 10