Enthalpy and Energy

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

the initial state and the final state and not how the substance achieved that change. It could take five steps to get from enthalpy A to an enthalpy B or just one step; the change in enthalpy will be the same. The enthalpy change will be negative for an exothermic reaction, while the enthalpy change will be positive for an endothermic reaction.

If the change in enthalpy is negative, the reaction will be considered to go downhill energetically speaking; if the change in enthalpy is positive, the energy will go uphill energetically speaking. With this in mind, you need to know that bond breaking in chemistry ALWAYS involves an input of chemical energy (positive enthalpy), while bond-making ALWAYS involves the release of energy (negative enthalpy). Negative enthalpy equals exothermic, while positive enthalpy equals endothermic.

Reversing any chemical reaction will change the sign of the enthalpy. Ice, for example, will absorb heat when it melts and electrostatic interactions are broken, while liquid water must release heat when it freezes. The magnitude of the enthalpy does not change in these types of reactions—only the sign as being plus or minus will change.

The magnitude of the enthalpy changes according to the mass of the substances in the reaction. Larger reactions will produce more heat or will take on more heat, depending on the type of reaction. It can be described as a change in heat energy per mole of substance in a specific reaction. In the reaction:

2 moles of Hydrogen gas (H2) plus one mole of Oxygen gas (O2) goes to 2 moles of H2O (water)

This is an endothermic reaction, which requires 570 kilojoules of energy. Rather than just use the term kilojoule to determine the enthalpy it is written as 570 kilojoules per mole of Oxygen gas or 285 kilojoules of energy per mole of hydrogen gas, using the stoichiometry of the equation in order to determine a particular enthalpy for an equation (thus standardizing the enthalpy).