# Entropy and Spontaneity

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• Created by: Lotto65
• Created on: 05-03-17 11:29
What is entropy?
The distribution of available energy among the particles in a system
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What is the symbol for entropy?
S
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The more ways energy can be distributed...
The higher the entropy
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What is another definition of entropy?
The measure of disorder of a system
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In nature, what do systems naturally tend towards...
An increase in entropy
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What three ways can you get an increase in entropy?
Mixing different types of particles; Increasing the number of particles; Increasing the movement of particles
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What is an example of where you mix different types of particles and the entropy increases?
Dissolving sugar in water
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How can you increase the movement of particles?
Heating a liquid or gas
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Where can you get the greatest increase in disorder?
Increase the number of particles in the gaseous state
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What is entropy change?
The change in the disorder of a system
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What does a positive value for entropy change mean?
A system becomes more disordered
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What does a negative value for entropy change mean?
The system becomes more ordered
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What is the standard entropy of a substance?
The entropy change per mole that results from heating a substance from 0K to 298K (standard temperature)
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Can absolute values of entropy be measured?
Yes
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Can absolute values of enthalpy be measured?
No
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How do you calculate the standard entropy change for a reaction?
Standard entropy of products - standard entropy of reactants
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What is the unit of standard entropy?
J/K/ mol
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What is a spontaneous reaction?
A reaction that causes a system to move from a less stable state to a more stable state
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What two factors affect spontaneity?
Entropy change and enthalpy change
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What is a name for when enthalpy and entropy are combined?
Gibbs energy change or 'free energy change'
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What is the equation for calculating Gibbs free energy?
Gibbs energy change (standard conditions) = Enthalpy change (standard conditions) - absolute temperature x entropy change (standard conditions)
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What does it mean if the value for Gibbs free energy is negative?
The reaction is spontaneous
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Why is the value for Gibbs free energy negative when a reaction is spontaneous?
Work has been done
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If a reaction is spontaneous, does that mean it does not need any input of energy?
No - it may need an input
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Will coal naturally burn in air or does it need energy?
It needs energy
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Why will coal not burn on its own in air?
It needs to receive some initial energy to give some molecules activation energy to start the reaction
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What is continually changing in a reaction?
The composition of reactants and products and free energy
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What sort of values for entropy and free energy will be present at equilibrium?
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Will the reaction proceed at equilibrium?
No as the rate of the forward reaction equals the rate of the reverse reaction
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What does the equilibrium composition of an equilibrium mixture depend on?
The value of free energy change
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What is the symbol for the equilibrium constant of a reaction?
Kc
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What does the equilibrium constant for the reaction depend on?
Free energy change
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If enthalpy change is positive and entropy change is positive, what will free energy change be?
Negative at high temperatures and positive at low temperatures
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If enthalpy change is positive and entropy change is negative, what is free energy change? Is it a spontaneous reaction?
Positive so never spontaneous
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If enthalpy change is negative and entropy change is positive, what is free energy change? Is it spontaneous?
Negative so is always spontaneous
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If enthalpy change is negative and entropy change is negative, what is free energy change?
Negative at low temperatures and positive at high temperatures
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Give examples of reactions with a positive entropy change
Mixing two gases; Thermal decomposition
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Give examples of reactions with a negative entropy change?
Gas separation; formation reactions
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The thermal decomposition of ammonium dichromate is spontaneous ...
At any temperature
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The formation of hydrazine from its elements...
Can never be spontaneous
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When is the decomposition of calcium carbonate spontaneous?
At high temperatures
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The hydrogenation of ethene is spontaneous...
Up to a certain temperature
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How can you determine the precise value of free energy change?
Using an energy cycle or the equation
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How do else do you know when a reaction is spontaneous?
When T x free energy change > enthalpy change
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What do you have to pay attention to with free energy change?
Temperature
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## Other cards in this set

### Card 2

#### Front

What is the symbol for entropy?

S

### Card 3

#### Front

The more ways energy can be distributed...

### Card 4

#### Front

What is another definition of entropy?

### Card 5

#### Front

In nature, what do systems naturally tend towards...