The Oceans Flashcards
What must the relative strength of the bonds be in order for something to dissolve?
Bonds must be a similar strength. If the energy required to break apart the solute is less than the energy required to form new bonds the solute will dissolve.
What is the lattice enthalpy?
The enthalpy change when one mole of a solid is formed by the coming together of separate gaseosus ions.
What does the lattice enthalpy measure?
The strength of ionic attractions.
Why is the lattice enthalpy always negative?
It is the formation of ionic bonds, so it is an exothermic process and thus is always negative.
When does the lattice enthalpy become more negative?
This occurs when the ionic bonds become stronger, due to the ionic charge increasing or the ionic radii increasing.
What is the enthalpy change of hydration?
The enthalpy change for the formation of a solution of ions from one mole of gaseous ions.
What does the enthalpy change of hydration measure?
The strength of attraction between ions and water molecules.
Why is the enthalpy change of hydration always negative?
It is the formation of ion-dipole bonds, so it is an exothermic process and thus always negative.
When does the enthalpy change of hydration become more negative?
This occurs when ion-dipole bonds become stronger and when the number of water molecules surrounding the ions increases.
What is the enthalpy change of hydration called if polar solvents are used, opposed to water?
The enthalpy change of solvation.
What is the enthalpy change of solution?
The enthalpy change when one mole of solute dissolves to form a dilute solution.
Why do ionic solutes not dissolve in polar solvents?
As the enthalpy change of solvation is so small because there is little attraction between the ions and the solvent.
Why do long polar chains not dissolve in water?
Although hydrogen bonds in water break, very few hydrogen bonds in the molecule break and very few bonds are formed between the water and the solute, so the energy required to break the bonds is more than the energy given out.
What are strong acids?
Powerful hydrogen donors- the donation of hydrogen ions is complete.
What is the strength of strong acids conjugate bases?
Weak.
What do strong bases do?
They fully dissociate in aqueous solutions, thus the concentration of a solution made by dissolving a strong base in water gives the concentration of the hydroxide ions.
How do you calculate the concentration of H+ ions in a strong base?
The ionic product of water is used- as water dissociates slightly.
H+ concentration = Kw / OH- conentration
What are the two assumptions made when working out the hydrogen ion concentration of weak acids?
- The concentration of hydrogen ions is equal to the concentration of the negative ions, as very few hydrogen ions are donated by water.
- Very few molecules dissociate so the concentration of the weak acid in equilibrium is equal to the original concentration.
Why are pKa values used?
Weak acids can have very small Ka values, and in order to compare them the Ka can be converted to the pKa, which is a measure of acid strength.
By which formula can the pKa be calculated?
pKa = -logKa
What is the first step in the reactions of CO2 dissolving in water?
CO2 (g) = CO2 (aq)
What is the overall reaction for CO2 dissolving in water?
CO2 (g) + H2O = 2H+ + CO32-
How do the oceans maintain a relatively constant pH?
They act as buffer solutions.
By what three methods can CO2 levels in the atmosphere be controlled?
- Use alternative methods to produce energy (rather than fossil fuels)
- More economical and efficient use of fuels
- Capture and store CO2
By what four methods can CO2 be captured and stored?
- Convert CO2 to useful products.
- Increase photosynthesis.
- Store gas in natural trenches on the sea floor, where pressure will cause it to liquefy.
- Inject gas onto sea floor (3500m) where it will form a lake of liquid CO2 that will remain undisturbed.
How is CO2 naturally uptaken in the oceans?
Seashells being built from calcium carbonate.
What is a buffer solution?
A solution which can resist changes in pH despite the addition of a small amount of acid or alkali, or dilution.
What two things can a buffer solution consist of?
- A weak acid and one of its salts
2. A weak base and one of its salts
What do buffers contain a large amount of?
A large amount of proton donor and a large amount of proton acceptor, to keep the pH constant.
What occurs if the pH is raised of a buffer solution?
If small amounts of alkali are added the concentration of OH- ions increases. These react with H+ ions, thus causing the position of equilibrium to shift to the right, thus HA produces more H+ ions to replace those lost.
What occurs if the pH is lowered of a buffer solution?
If small amounts of acid are added the concentration of H+ ions increases. These react with the A-, so the position of equilibrium shifts to the left, thus preventing an increase in the concentration of H+ ions.
Which two assumptions are made when rearranging the buffer solution equation?
- All A- ions come from the salt (the weak acid supplies very few)
- Almost all of the HA molecules put into the buffer remain unchanged, so the concentration is unchanged.
Why does diluting a buffer not affect the pH?
The concentration of the acid and the salt are both decreased equally, therefore, their ratio does not change.
What is entropy?
The number of ways of arranging molecules and distributing their quanta of energy.
What occurs if the total entropy change for a process is positive?
It occurs spontaneously.
What occurs if the total entropy change for a process is 0?
The reaction is at equilibrium.
