Reaction Feasibility Flashcards
What equation is the enthalpy change given by for the reaction R —> P?
/\H = Hp - Hr, where Hp and Hr are the enthalpies of the products and reactants.
What is the arbitrary reference point for enthalpy expressions called?
The standard enthalpy of formation.
What is the standard enthalpy of formation defined as?
The enthalpy change involved when one mole of a substance is formed from its elements in their standard states.
What is the standard state of a substance?
Its most stable form at a pressure of 1atm and a specific temperature (usually 298K).
What is the value of the standard enthalpy of formation and the standard free energy of formation of an element in its most stable form?
Zero.
How do you calculate the standard enthalpy change for a reaction from the standard enthalpies of formation?
1) Calculate the total standard enthalpy of formation for all of the products.
2) Calculate the total standard enthalpy of formation for all of the reactants.
3) The difference between the two totals is the standard enthalpy change for the reaction.
* Remember to mulitiply the standard enthalpy of formation of each substance by its stoichometric coefficient.
What is entropy?
Entropy is the amount of disorder within a system and is given the symbol S. The larger the value of entropy, the greater the amount of disorder.
What is the standard entropy of a substance?
The standard entropy of a substance is the entropy of 1 mole of the substance at 1atm and usually a temperature of 298K.
What are the units of entropy?
J K^-1 mol^-1
What entropy values do solid substances tend to have and why?
Solid substances tend to have low entropy values because the particles have fixed positions. The particles can vibrate but not move around, meaning solids are highly ordered.
What entropy values do gaseous substances tend to have and why?
Gasous substances have very high entropy values because the particles have complete freedom to move around, meaning gases are highly disordered.
What entropy values do liquids tend to have and why?
Liquids tend to have entropy values between those of solids and gases as the particles have some freedom to move, but not as much as gases.
What is the third law of thermodynamics?
The third law of thermodynamics states that the entropy of a substance at 0K is zero.
What happens to a solid when the temperature increases from 0K?
The entropy of the solid will steadily increase. When it reaches its melting point, there will be a rapid increase in entropy as the substance changes from solid to liquid. The entropy of the liquid then continous to increase steadily untill it reaches its boiling point where there is an even larger increase in entropy than there was at the melting point. The entropy of the gas then continous to steadily increase.
How can we predict whether a reaction increases or decreases in entropy when the states of the reactants and products are different?
- If the reaction goes from solid —> liquid/gas, there will be an increase in entropy as there is a larger amount of disorder in liquids and gases than there is in solids.
- If the reaction goes from gas —> liquid/solid, there will be a decrease in entropy as there is a smaller amount of disorder in liquids and solids then there is in gases.
