Simple Equilibria and Acid-base reactions Flashcards
What is the LeChateliers principle?
When a system in dynamic equilibrium is subject to a change, the position of the equilibrium will shift to minimise the change.
What is dynamic equilibrium?
The equilibrium when the rate of forward reaction is equal to the rate of the reverse reaction, therefore concentrations are constant.
How can you alter the position of the equilibrium in a reaction?
*Changing concentration of reactants or products
*Changing the temperature
*Changing the pressure of gases
Does a catalyst alter the position of the equilibrium?
Adding a catalyst does not affect the position of the equilibrium, but equilibrium is achieved faster.
The equation for concentration in an equilibrium reaction?
Kc = [C]c [D]d / [A]a [B]b
If Kc is less than 1?
The reactants concentration is greater than the products.
If Kc is 1?
The reactants and products concentration is equal.
If Kc is more than 1?
The products concentration is greater than the reactants.
What is the units for concentration?
moldm-3
How to convert cm into dm?
x10-3.
What is an acid?
Donates a proton during a reaction.
What is a base?
Accepts a proton during a reaction.
What is an alkali?
A base that dissolves in water.
What differentiates strong and weak acids?
A strong acid fully disassociates in an aqueous solution whereas a weak acid only partially disassociates.
What differentiates concentrated and dilute acids?
A concentrated acid consists of a large quantity of acid and a small quantity of water, whereas a dilute acid contains a large quantity of water and a small quantity of acid.
How to find pH?
pH = -log10[H+]
How to find the concentration in pH calculations?
[H+] = 10-pH
What universal indicator colour is shown with an acid?
Red, orange and yellow (pH 0-6).
What universal indicator colour is shown with an neutral solution?
Green (pH 7).
What universal indicator colour is shown with an alkali?
Blue and purple (pH 8-14).
What is a standard solution?
A solution whose concentration is accurately known.
What are the steps to performing a titration?
- Pour one solution into a burette using a funnel, remove the funnel and read the burette.
- Use a pipette to add a measured volume of the other solution into a conical flask and add indicator.
- Run the acid from the burette to the solution in the conical flask, swirling the flask.
- Stop when the indicator just changes colour.
- Read the burette again and subtract you initial from final reading to find your titre.
- Repeat titration until you have readings with 0.2cm^3 of each other and calculate a mean.
What would happen if you increase the acid in the titration?
You would get larger titres, therefore a smaller percentage error.
Why would you add indicator to your solution?
To show the end point.
Why should you carry out a rough titration first?
To give a rough idea of the end point.
Why should you carry out several titrations and determine a mean?
To obtain a more reliable value.
Why do mixtures usually need to be acidified to produce acids?
To make sure the salt of the acid is formed.
How would you know if a tirtration reaction had completed?
No more effervescence.
Describe how to make a standard solution
- Dissolve mass in small volume of water
- Add solution to a volumetric flask using a funnel
- Shake well to mix
What is a primary standard?
A pure reagent that can be weighed easily and is so pure that its weight is representative of the number of moles of substance contained.
What are the characteristics of a primary standard?
High purity
Stability- low reactivity
Low hygroscopicity
High molar mass- minimise weighing errors
How do you prepare a soluble salt by titration?
- A volume of alkali is measured into a flask and indicator is added.
- Acid is added from a burette until the indicator changes colour.
- When the volume of acid needed to neutralise the alkali has been calculated, the process is repeated without the indicator.
- The solution from the flask is heated to evaporate some of the water, then cooled to form salt crystals.
When should you use a back titration method?
When the reaction between determined substance and titrant is too slow.
When there is a problem with end-point determination.
When the base is an insoluble salt.
Describe how to perform a back titration
A known excess of reagent 1 reacts with an unknown amount of reagent 2. At the end of the reaction, the amount of reagents 1 that remains is found by titration. A simple calculation gives the amount of reagent 1 that has been used and the amount of reagent 2 that has reacted.
When should you use a double titration method?
When a solution contains a mixture of two bases which are different strengths.
What indicators can you use in titrations?
Phenolphthalein (pink to colourless at pH 9)
Methyl orange (yellow to orange at pH 4)
Describe how to perform a gravimetric analysis
- Add excess precipitate agent to a known quantity of an element to ensure all of it reacts
- Filter the solution under pressure to separate the precipitate and weigh the filter paper before and after
- Add AgNO3 to filtrate
- Wash with a few drops of cold solvent to remove impurities
- Dry to a constant mass
