Thermodynamics Flashcards
What do chemical reactions take place as a result of?
bond breaking and bond making reactions
What kind of reaction is bond breaking?
endothermic
What kind of reaction is bond making?
exothermic
What is thermochemistry the study of?
the study of energy changes during chemical reactions, measured in kJ mol-1
What are energy changes measured in terms of?
enthalpy changes (or changes in energy contents)
What does a change in energy content result in?
a change in temperature of the surroundings
What do total enthalpy changes always refer to?
the system and the surroundings
What can the system be regarded as?
the part of the world we are investigating e.g. a chemical reaction
What are the surroundings regarded as?
the rest of the world e.g. the air surroundings the reaction vessel, or water the reaction is taking place in.
What is important to remember about the total amount of energy during a chemical reaction?
The total amount of energy is unchanged during a chemical reaction - energy may be exchanged between the materials and the surroundings, but the total amount of energy of the materials and surroundings remains constant
What is the first law of thermodynamics?
energy can neither be created nor destroyed, but can be changed from one form to another
State Hess’s law.
The enthalpy change for a reaction is the same regardless of the route taken
What are the standard conditions for temp and pressure?
298K (25’c) and 1 atmosphere respectively (STP)
What are substances said to be?
in their standard state, i.e. their normal physical states under standard conditions
What concentration are solutions at in their standard state?
1 mol l-1
What are enthalpy changes measured under standard conditions known as? What symbol is used?
a standard enthalpy change delta H’
What is the enthalpy of formation?
the enthalpy change when 1 mole of a substance is formed for its elements under standard conditions.
What is the standard enthalpy of formation of an element?
0
How can the standard enthalpy of a reaction being calculated?
using standard enthalpies of formation
Define entropy.
A term which refers to the chaos or disorder in a system, and measure of chaotic dispersal of energy and matter.
The greater the disorder in a system, the…
greater the entropy
Describe the effect of temperature on entropy.
Entropy increases as temperature increases
What is the standard molar entropy, S*?
the entropy content of one mole of a substance, under standard conditions
What are the units of entropy?
JK-1 mol-1
What does the third law of thermodynamics state?
the entropy of a perfect crystal at 0K is zero
What does the crystal lattice at absolute zero have?
perfect order and therefore no entropy i.e. the particles are not in motion
What happens as a solid is heated?
the particles gain KE and entropy slowly increases
Describe the entropy increase between solid, liquid and gas.
The entropy increase from liquid to gas is greater than that from solid to liquid
What do changes in state involve?
changes in entropy
For any spontaneous process describe what happens to the total entropy of the system and its surroundings.
increases
What is a spontaneous process?
one that can take place on its own irrespective of how long it takes
What are spontaneous exothermic reactions driven by?
the large increase in entropy they generate in the surroundings
How is the entropy of surroundings calculated.
calculated from the standard enthalpy change for the reaction and temperature, T, in Kelvin.
What are spontaneous endothermic reactions driven by?
the large increase entropy they generate in the system i.e. the reaction mixture itself
For a spontaneous endothermic reaction explain what happens to the entropy of the surroundings.
Heat is absorbed from the surroundings - decreases the entropy of the surroundings
What does the spontaneity of a reaction involve?
two thermodynamic quantities, enthalpy and entropy
What are the favourable conditions for a spontaneous reaction?
/\H < 0
/\S > 0
What are the favourable conditions for a non-spontaneous reaction?
/\H > 0
/\S < 0
In order to determine whether a reaction is spontaneous what other thermodynamic quantity is used?
Gibbs Free Energy /\G
What is the unit of /\G?
J mol-1
For a spontaneous reaction, what will /\G always be?
negative
What is the equation to calculate /\G? Include the units
/\G= /\H - T/\S /\G is Gibbs Free Energy in kJ /\H is the enthalpy change in kJ mol-1 T is the temperature in Kelvin (+273) /\S is the entropy change in JK-1 mol-1
For a non-spontaneous reaction, what will /\G always be?
positive
What equation can be used to calculate standard free energy changes?
/\G= {sigma}/\Gproducts - {sigma}/\Greactants
If /\G is negative, what will the reaction proceed at?
any temperature
In terms of the reaction what will happen if /\G is positive?
the reaction will not proceed
What is the temperature at which the reaction becomes feasible?
the CRITICAL TEMPERATURE
At the critical temperature, what is the value of /\G?
0
What equation is used to calculate the temp at which the reaction becomes feasible?
T= /\h//\S
At chemical equilibrium what is the Gibbs free energy value?
At a minimum
If /\G is <0 what does this suggest about the forward reaction?
it is feasible so [products]>[reactants] Equilibrium lies to the RHS and therefore K>1
If /\G is >0 what does this suggest about the reverse reaction?
it is feasible so [reactant]>[products]
Equilibrium lies to the LHS and therefore K<1
If /\G is equal to zero what is the value of K?
1
What does the value of /\G for a reaction not predict?
the rate of the reaction
What may a reaction have that prevents it from taking place?
A reaction may have a negative /\G but have a high activation energy
In a closed system what must the /\Ssystem be?
positive
If /\S is -ve what does this suggest about how disordered the reactants and products are?
[reactants]>[products]
If /\S is +ve what does this suggest about how disordered the reactants and products are?
[products]>[reactants]
