In chemical reactions, heat is either released or absorbed and entropy either increases or decreases. If you know these values, you can determine whether or not a reaction is spontaneous. Copy the following table into your science notebook. Then, fill it out as you read the scenarios below. Listen Enthalpy Change Entropy Change Spontaneous Reaction?
Spontaneous
When entropy is positive and enthalpy is negative, a process is spontaneous. This means the reaction proceeds on its own under standard conditions, giving off energy as the reaction proceeds.
Spontaneous at High Temperatures
When entropy is positive and enthalpy is positive, a process is spontaneous at high temperatures because the products in the system are able to use thermal energy from the surroundings in order to react. Thermal energy increases the kinetic energy of the molecules. An example of this is ice melting. Ice will not melt if the temperature is not high enough.
Spontaneous at Low Temperatures
When entropy is negative and enthalpy is negative, a process is spontaneous at low temperatures. This type of reaction will release energy into the surroundings, and the ability to release this heat is important. If the temperature of the surrounding system is too high, the reactants have to overcome this energy in order to react and release their own heat energy. An example of this is the freezing of water. If the temperature is too high, water will not freeze to form ice.
Not Spontaneous
When entropy is negative and enthalpy is positive, a process is not spontaneous at any temperature. However, the reverse process is spontaneous because that is the energetically favorable direction for the reaction. For example, heat does not flow from a cooler object to a hotter object, but heat will flow from a hotter object to a cooler object.
You can determine whether a reaction is spontaneous and endothermic or exothermic using ΔG calculations.
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