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Entropy
AQA A-level chemistry thermodynamics year 13
| Term | Definition |
|---|---|
| Entropy | The degree of disorder in a system, the more disorder (random) something is, the higher the entropy |
| Spontaneous reactions | Reactions which occur without any external driving force. Some reactions can be reversed if energy is inputted |
| Entropy change in spontaneous reactions | Spontaneous reactions will turn energy and matter from a more ordered state to a more disordered state. Energy is required to reverse to entropy |
| What must be considered when entropy change is happening | Physical and chemical processes naturally tend towards a higher entropy state, this is dependent on the total order for both systems (the change taking place and its surroundings) |
| Third law of thermodynamics | Entropy can only be 0 at absolute 0 (0K). The higher the temperature, the greater the entropy |
| How to tell based off entropy change if a reaction is spontaneous or not | Entropy is measured in JK^-1mol^-1. If entropy change is positive, the reaction is spontaneous. If entropy change is negative, the reaction isn’t spontaneous |
| Gibbs free energy | A thermodynamic potential that combines both enthalpy and entropy to predict whether a chemical reaction is spontaneous under standard conditions |
| Gibbs free energy equation | DeltaG = DeltaH - TDeltaS G = Gibbs free energy H = enthalpy S = entropy T = absolute temperature in kelvin |
| Interpretation of deltaG values | DeltaG < 0 = reaction is spontaneous in the forward direction DeltaG > 0 = reaction is non-spontaneous in the forward direction DeltaG = 0 = system is in equilibrium |
| Gibbs free energy units and how to change entropy to its units | Both enthalpy and Gibbs are measured in KJmol^-1 so entropy value needs to be divided by 1000 and then times by absolute kelvin |
| What happens when heat is added to a system at equilibrium (DeltaG = 0) | The system compensates by increasing the entropy and the substance can change state |
| Values of TDeltaS and DeltaH when DeltaG = 0 | TDeltaS = DeltaH DeltaS = DeltaH/T T = DeltaH/(DeltaS/1000) |
| Gibbs free energy line graph interpretation | X axis = temperature/K Y axis = DeltaG/KJmol-1 Gradient of line = -DeltaS X intercept = temperature range of feasibility Y intercept = Delta H |
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