Energetics. These processes involve energy exchanges between the reacting system and its surroundings.

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Laurel Wiggins
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1 Energetics Chemical reactions involve: the breaking of bonds between atoms the making of new bonds between atoms These processes involve energy exchanges between the reacting system and its surroundings. Enthalpy is the term used to describe the heat (energy) content of a system. It has the symbol H and is measured in kilojoules. It is only really possible to measure enthalpy changes (delta H, written as ). This can be calculated from measured changes in temperature. Enthalpy change is defined as the heat energy change measured under conditions of constant pressure. In a chemical reaction the enthalpy change is easily calculated by subtracting the (total) enthalpy of the reactants (H r ) from the (total) enthalpy of the products (H p ).

2 Exothermic reactions release heat energy (heat flows from the system to the surroundings). is always negative, so the value always has a minus sign. Endothermic reactions absorb heat energy (heat flows from the surroundings into the system). is always positive. These changes can be shown on enthalpy change diagrams. Standard conditions All chemical reactions have enthalpy changes associated with them. For any reaction, the value of the enthalpy change is dependent on two things: the temperature and pressure the amount of substance used Enthalpy changes are always quoted relative to a standard set of conditions of temperature and pressure. Standard enthalpy changes use the symbol the standard pressure is 100kPa, the substances are in their standard states (this is normally at 298K, but, as measurements can be made at other temperatures the temperature is always given e.g. (298K)). The value (in kilojoules) of is always given per mole of substance.

3 Standard enthalpy change of formation This is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. For example, when producing magnesium oxide the standard enthalpy of formation is: Mg(s) + 0.5O 2 MgO(s) (Note: this is for one mole of magnesium oxide; all state symbols are given in addition to the temperature at which the enthalpy change is measured; the formation of one mole of magnesium oxide releases 602kJ of energy into the surroundings had two moles of magnesium oxide been formed then the amount of energy released would have been 1204kJ.)

4 Standard enthalpy change of combustion This is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions (100 kpa and 298 K). For example the standard enthalpy of combustion of ethane is: C 2 H 6 (g) + 3.5O 2 (g) 2CO 2 (g) + 3H 2 O(l) (Note: this is for one mole of ethane; all state symbols are quoted; when completely burnt in oxygen, one mole of ethane releases 1560kJ of energy into the surroundings.) Other enthalpy changes Many other types of enthalpy change can be measured and the term standard enthalpy change of reaction can be applied to many other reactions. At AS level you will be concerned with formation and combustion reactions.

5 Calculating the Enthalpy of Reaction The value of enthalpy changes accompanying chemical reactions can be determined by the use of a calorimeter. The purpose of a calorimeter is to insulate the reaction system from its surroundings. The reaction is carried out in the calorimeter and the change in temperature of the calorimeter is measured. This enables the enthalpy change for the reaction to be determined. Calculating energy changes There are several types of enthalpy change that can be measured using a calorimeter. These include enthalpy changes of dissolving neutralization reactions between acids and bases formation combustion The heat energy change for any reaction can be calculated using the relationship below: heat = mass of substance x specific heat capacity x temperature change Specific heat capacity (c) is the heat energy required to raise the temperature of 1g of a substance by 1K. For pure water the specific heat capacity is 4.18Jg 1 K 1.

7 Enthalpy of solution A known mass of solid is totally dissolved in a large excess of water whose mass is known. The change in temperature of the water is measured. In an experiment using a simple calorimeter, 8.00g of ammonium nitrate (NH 4 NO 3 ) was dissolved in 50.0g of water. The temperature fell by 10.1 o C. Calculate the enthalpy change for this process. (The specific heat capacity of water is 4.18JK -1 g -1. The heat capacity of the container can be ignored.) Step 1: Step 2: Step 3: Calculate the heat change from the change in temperature using q = mass x specific heat capacity of water x change in temperature q = 50 x 4.18 x 10.1 = J Calculate the mole of ammonium nitrate dissolved moles = Mass / Mr moles = 8 / 80 = 0.10 Calculate the enthalpy change per mole of ammonium nitrate / 0.1 = J mol -1 Step 4: Answers are usually given in kj mol / 1000 = kj mol The answer should be positive as the temperature decreased! If this was a temperature increase then the enthalpy would be negative.

Measuring and Expressing Enthalpy Changes. Copyright Pearson Prentice Hall. Measuring and Expressing Enthalpy Changes. Calorimetry

Measuring and Expressing Enthalpy Changes A burning match releases heat to its surroundings in all directions. How much heat does this exothermic reaction release? You will learn to measure heat flow in