21.1 - 21.5 Flashcards
Define enthalpy of formation
- the enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard a states under standard conditions
2C(s) + 2H2(g) —> C2H6(g)
Define lattice enthalpy of formation
The enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions.
e.g. Na+(g) + Cl-(g) → NaCl(s)
Define the enthalpy change of 1st ionisation
- the enthalpy change when 1 mole of gaseous 1+ ions are made from 1 mole of gaseous atoms
Na(g) —> Na+ (g)
Fill in the blank:
The _____ of the charge on the ion affects the strength of the ionic bond
- size
Fill in the blanks:
The _____ the charge on an ion the ____ the electrostatic attraction between ions and so ____ energy is required to overcome these forces so they have a high _______ and ______ point
1) bigger
2) stronger
3) more
4) boiling
5) melting
Fill in the blank:
The _____ of the ion (ionic radii) affects the strength of the ionic bond
1) size
How does the size of the ion affect the strength of the ionic bond
- the smaller the ion the stronger the electrostatic attraction between ions which mean the smaller ions can pack together more closely and more energy is required to overcome the strong forces
- so there is a higher boiling and melting point
Complete the sentence:
Generally the smaller the ion and the higher the charge the…
- stronger the electrostatic attraction and hence the higher the melting and boiling point ( said to have a high charge density)
Why are born-haber cycles useful
- useful for calculating lattice enthalpies as we can’t calculate them directly from experiments
What do we always have at the bottom of a born-haber cycle
- the ionic solid
What type of process is enthalpy of formation
- EXOTHERMIC
What type of process is enthalpy of atomisation
- ENDOTHERMIC
What type of process is enthalpy of 1st ionisation
- ENDOTHERMIC
What type of process is enthalpy 1st electron affinity
- ENDOTHERMIC/EXOTHERMIC
Doing the term enthalpy change of solution
- the enthalpy change that takes place when 1 mole of a compound is completely dissolved in water under standard conditions.
e.g. NaCl(s) → NaCl(aq)
What 2 criteria do you need for a substance to dissolve
1) substance bonds must break to form free moving ions (endothermic)
2) new bonds formed between the solvent and substance (exothermic)
Why do most ionic compounds dissolve in polar solvents like H2O
- because the delta positive hydrogen is attracted to negative ions and delta negative oxygen is attracted to positive ions
What is the process of hydration of ions
- when the water molecules surround the ion
What do criteria do we need for new bonds to be formed
- new bonds must be the same strength or greater than those broken or the substance is unlikely to dissolve
What do you use to calculate then enthalpy of solution
1) lattice dissociation enthalpy
2) enthalpy of hydration
Define the enthalpy of hydration
- the enthalpy change when 1 mole of aqueous ions is made from 1 mole of gaseous ions
What 2 thing affect the enthalpy change of hydration
1) the charge
2) the size of the ion
How does charge affect the enthalpy change of hydration
- ions with a higher charge attract water molecules more strongly as the electrostatic attraction is stronger
- so more energy is released when the bond is made which means they have a more exothermic enthalpy of hydration
*THE LARGER THE CHARGE THE GREATER THE ENTHALPY OF HYDRATION
How does the size of the ion affect the enthalpy change of hydration
- smaller ions have a higher charge density than larger ions
- they can attract water molecules more strongly hence there is a more exothermic enthalpy of hydration
*THE SMALLER THE ION THE GREATER THE ENTHALPY OF HYDRATION
Define the term entropy(S)
- the measure of disorder in a system
What does higher disorder mean
- the higher the level of entropy
What 2 things affect entropy
1) the state of the compound
2) the number of particles
What state has the lowest level of disorder and entropy and why
- solids as they are arranged neatly into rows
What state has the highest level of disorder and entropy and why
- Gases as they are more disordered
How does the number of particles affect entropy change
- as if the reaction is in the same state but there are more moles are produced so entropy increases as there are more ways energy can be distributed
Complete the sentence:
A reaction can be spontaneous(feasible) even if…
- it is enthalpically unfavourable( endothermic)
What do reactions tend towards
- more disorder so there is an increase in entropy
How is increasingly entropy favourable
- energetically favourable
Complete the sentence:
Some reactions that are enthalpically unfavourable can still…
- spontaneously react if changes in entropy overcome changes in enthalpy
How do you work out the entropy change
^S= S (products) - S (reactants)
What are the units for entrop
JK^-1 mol^-1
What are the standard conditions for entropy
- 100kPa
- 298K
Is entropy positive or negative
- Positive, so it is favourable
What does Gibbs free energy tell us
- if the reaction is feasible (going to happen) or not
What is the symbol for Gibbs free energy
^G
What is the reaction for Gibbs free energy
^G = ^H - T^S
^G= Gibbs free energy (Jmol^-1)
^H= enthalpy change (Jmol^-1)
T= temp (k)
S= entropy change (JK^-1mol^-1)
What is the basic rule for Gibbs free energy
- a reaction is feasible in theory ^G is NEGATIVE or ZERO
How do you calculate ^H
- q=mc^T
What external variable may have an effect on reaction feasibility
- temperature
Draw a feasibility table for feasibility of reactions at different scenarios when ^H and ^S is changed
How do we work out temperature in Gibbs free energy
T= ^H/^S
