General Chemistry
The concept of entropy describes the spread of energy or molecular disorder. The total entropy of the universe is always increasing, but the entropy of a system can change with alterations to states of matter, the quantity of reactants, temperature, volume, or pressure. Entropy plays a significant role in influencing whether a reaction proceeds spontaneously. This spontaneity can be determined by calculating a reaction’s change in Gibbs free energy, represented by delta G. If delta G is negative, the reaction occurs spontaneously without additional energy input. However, if delta G is positive, the reaction requires energy input and is not spontaneous. When delta G equals zero, the reaction is in equilibrium, with no net change in the quantity of products or reactants and no change in entropy.
Lesson Outline
<ul> <li>Entropy</li> <ul> <li>Entropy = The "spread" of energy</li> <li>Representation of entropy as the letter S</li> <li>Total entropy of the universe always increasing</li> </ul> <li>Entropy Change</li> <ul> <li>Changes based on states of matter</li> <ul> <li>Gases having the highest entropy</li> <li>Liquids having lower entropy</li> <li>Solids having the lowest entropy</li> </ul> <li>Changes based on quantity of substance: more quantity = more entropy</li> <li>Changes based on temperature: more temperature = more entropy</li> <li>Changes based on volume: more volume = more entropy</li> <ul> <li>Pressure's effect on entropy: more pressure = less entropy, assuming constant temperature/quantity (Boyle's Law)</li> </ul> </ul> <li>Change in Entropy (Delta S) and Reactions</li> <ul> <li>Determining Delta S: entropy of products minus entropy of reactants</li> <li>Positive and negative Delta S</li> <li>Comparing states of matter on either side of a reaction: more moles of gas will generally mean greater entropy</li> </ul> <li>Spontaneity and Gibbs Free Energy</li> <ul> <li>Understanding spontaneous reactions</li> <li>Influence of enthalpy, entropy, and temperature</li> <li>Change in Gibbs free energy during a reaction (Delta G)</li> <ul> <li>Negative Delta G representing spontaneous reactions</li> <li>Positive Delta G representing non-spontaneous reactions</li> <li>Delta G of zero representing equilibrium reactions</li> </ul> </ul> </ul>
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FAQs
Temperature plays a crucial role in determining the entropy and spontaneity of reactions. An increase in temperature typically increases the molecular disorder, which in turn increases entropy. Higher temperatures can also cause reactions to proceed spontaneously even if their enthalpy change is positive, as long as the entropy change is large enough and positive. On the other hand, lower temperatures make spontaneous reactions with only mildly positive enthalpy changes less likely. Overall, the effect of temperature on the spontaneity of a reaction depends on the specific reaction and the changes in both enthalpy and entropy.
Pressure has a direct impact on the entropy and states of matter, as it influences the molecular organization and disorder within a substance. As pressure increases, the molecules are forced closer together, decreasing the available volume and reducing the entropy (molecular disorder). This can cause substances to change from gaseous to liquid or from liquid to solid states. On the other hand, decreasing pressure allows molecules more freedom to move, increasing entropy and leading to changes from solid to liquid or from liquid to gas states. Boyle's Law illustrates the relationship between pressure and volume when temperature is held constant, which can help predict the changes in states of matter based on pressure changes.
Enthalpy and entropy are both thermodynamic properties that help determine the energy and spontaneity of reactions. Enthalpy (ΔH) represents the heat content of a system and generally reflects the bond strengths within molecules. It can be either released (exothermic) or absorbed (endothermic) during a reaction, typically changing with temperature and pressure. Entropy (ΔS), on the other hand, is a measure of molecular disorder or randomness within a system. It tends to increase with temperature, the number of particles, or in reactions that involve a change from a more ordered to a less ordered state (e.g., solid to liquid or gas).
Entropy and equilibrium are intertwined concepts in chemical reactions. At equilibrium, the forward and reverse reactions occur at the same rate, and the composition of the system no longer changes. Entropy plays a key role in reaching equilibrium, as the system attempts to move towards a state of maximum entropy or disorder. When the entropy of a system is at its maximum, the system is at equilibrium, which usually corresponds to a minimum Gibbs free energy. In this state, the forward and reverse reactions are balanced, and no further spontaneous change occurs.
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