What you need to know is that, while the reaction takes place inside the chamber, the measurement of the energy or heat produced or taken up by the reaction is of the surrounding water and not the change in temperature of the chamber. The energy of the chamber is equal to the mass multiplied by the change in temperature multiplied by the specific heat of water: 4.18 joules per gram per degree Celsius.
ENTHALPY AND ENERGY Both heat and work are different representations of energy transfer mechanisms. Work transfers energy such as when moving an object from place to place, while heat can transfer from one place or substance to another. The unit of work is Joules, as you remember, and it doesn’t matter what type of energy is involved. Reactions can occur at a constant pressure and reactions can occur at a constant volume. First consider the situation of a constant pressure reaction. The change in energy of a system involving the reaction, called the delta U, is equal to the flow of heat minus the pressure multiplied with the change in volume. If the reaction happens in a closed vessel, t here is no change in volume so the change in energy must equal the heat flow in the reaction. The problem with this is that most reactions are not carried out in completely sealed “constant volume” situations. They happen in open containers at a constant pressure. In such cases, the initial reaction does consider that a change in volume might occur. This introduces a new concept called enthalpy, defined by the initial H. Enthalpy is similar to energy but is not exactly the same because it takes into account the shrinkage and growth of a reaction. Enthalpy is the internal energy plus the pressure multiplied by the volume. The change in enthalpy of a system accounts for the change in internal energy added to the change in pressure-volume of the system. The enthalpy is the heat flow in constant pressure situations. State functions are those functions of a system that depend only on the state of the situation, such as its pressure, temperature, composition, and amount of substance but not on its past history. Internal energy and enthalpy are also state functions of a substance. This means that the internal energy and enthalpy changes depend only on
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