Enthalpy and Heat Transfers PDF

Enthalpy and heat transfers The amount of heat that a system has is known as its enthalpy (H) Exothermic reactions, involve heat being released, indicating a negative change in enthalpy. Endothermic reactions involve heat being absorbed, resulting in a positive change in enthalpy. Standard enthal...

Enthalpy and heat transfers The amount of heat that a system has is known as its enthalpy (H) Exothermic reactions, involve heat being released, indicating a negative change in enthalpy. Endothermic reactions involve heat being absorbed, resulting in a positive change in enthalpy. Standard enthalpy change of reaction: refers to the heat transferred at constant pressure and under standard conditions and states When enthalpy is included beside a chemical reaction, it is known as a thermochemical equation −1 the units of enthalpy are in units of kJ mol The specific heat capacity of a substance is the quantity of energy needed to raise 1 g of a substance by 1 K −1 −1 substance Specific heat capacity (J g K ) Water 4.18 Copper 0.39 Graphite 0.71 Steel 0.47 Calorimetry is a technique used to measure the heat transfer during a physical or chemical process. A calorimeter is the apparatus used to measure temperature changes to calculate the enthalpy for a reaction. The system might be closed but it is not isolated; therefore, it is always ‘connected’ to the earth in some way, making heat loss unavoidable. This heat loss results in smaller changes in temperature than we would expect theoretically. This means that experimentally, we would record a final temperature lower than theoretically possible for exothermic reactions and a final temperature higher than theoretically possible for endothermic reactions. Insulation of calorimeters is one way to reduce heat loss. This can be done by surrounding the calorimeter with insulating materials, increasing the thickness of these materials and minimising contact with the air.