Simple Equilibria and Acid-base reactions

Question 1

What is the LeChateliers principle?

Answer 1

When a system in dynamic equilibrium is subject to a change, the position of the equilibrium will shift to minimise the change.

Question 2

What is dynamic equilibrium?

Answer 2

The equilibrium when the rate of forward reaction is equal to the rate of the reverse reaction, therefore concentrations are constant.

Question 3

How can you alter the position of the equilibrium in a reaction?

Answer 3

*Changing concentration of reactants or products
*Changing the temperature
*Changing the pressure of gases

Question 4

Does a catalyst alter the position of the equilibrium?

Answer 4

Adding a catalyst does not affect the position of the equilibrium, but equilibrium is achieved faster.

Question 5

The equation for concentration in an equilibrium reaction?

Answer 5

Kc = [C]c [D]d / [A]a [B]b

Question 6

If Kc is less than 1?

Answer 6

The reactants concentration is greater than the products.

Question 7

If Kc is 1?

Answer 7

The reactants and products concentration is equal.

Question 8

If Kc is more than 1?

Answer 8

The products concentration is greater than the reactants.

Question 9

What is the units for concentration?

Answer 9

moldm-3

Question 10

How to convert cm into dm?

Answer 10

x10-3.

Question 11

What is an acid?

Answer 11

Donates a proton during a reaction.

Question 12

What is a base?

Answer 12

Accepts a proton during a reaction.

Question 13

What is an alkali?

Answer 13

A base that dissolves in water.

Question 14

What differentiates strong and weak acids?

Answer 14

A strong acid fully disassociates in an aqueous solution whereas a weak acid only partially disassociates.

Question 15

What differentiates concentrated and dilute acids?

Answer 15

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.

Question 16

How to find pH?

Answer 16

pH = -log10[H+]

Question 17

How to find the concentration in pH calculations?

Answer 17

[H+] = 10-pH

Question 18

What universal indicator colour is shown with an acid?

Answer 18

Red, orange and yellow (pH 0-6).

Question 19

What universal indicator colour is shown with an neutral solution?

Answer 19

Green (pH 7).

Question 20

What universal indicator colour is shown with an alkali?

Answer 20

Blue and purple (pH 8-14).

Question 21

What is a standard solution?

Answer 21

A solution whose concentration is accurately known.

Question 22

What are the steps to performing a titration?

Answer 22

1. Pour one solution into a burette using a funnel, remove the funnel and read the burette.
2. Use a pipette to add a measured volume of the other solution into a conical flask and add indicator.
3. Run the acid from the burette to the solution in the conical flask, swirling the flask.
4. Stop when the indicator just changes colour.
5. Read the burette again and subtract you initial from final reading to find your titre.
6. Repeat titration until you have readings with 0.2cm^3 of each other and calculate a mean.

Question 23

What would happen if you increase the acid in the titration?

Answer 23

You would get larger titres, therefore a smaller percentage error.

Question 24

Why would you add indicator to your solution?

Answer 24

To show the end point.

Question 25

Why should you carry out a rough titration first?

Answer 25

To give a rough idea of the end point.

Question 26

Why should you carry out several titrations and determine a mean?

Answer 26

To obtain a more reliable value.

Question 27

Why do mixtures usually need to be acidified to produce acids?

Answer 27

To make sure the salt of the acid is formed.

Question 28

How would you know if a tirtration reaction had completed?

Answer 28

No more effervescence.

Question 29

Describe how to make a standard solution

Answer 29

1. Dissolve mass in small volume of water
2. Add solution to a volumetric flask using a funnel
3. Shake well to mix

Question 30

What is a primary standard?

Answer 30

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.

Question 31

What are the characteristics of a primary standard?

Answer 31

High purity
Stability- low reactivity
Low hygroscopicity
High molar mass- minimise weighing errors

Question 32

How do you prepare a soluble salt by titration?

Answer 32

1. A volume of alkali is measured into a flask and indicator is added.
2. Acid is added from a burette until the indicator changes colour.
3. When the volume of acid needed to neutralise the alkali has been calculated, the process is repeated without the indicator.
4. The solution from the flask is heated to evaporate some of the water, then cooled to form salt crystals.

Question 33

When should you use a back titration method?

Answer 33

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.

Question 34

Describe how to perform a back titration

Answer 34

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.

Question 35

When should you use a double titration method?

Answer 35

When a solution contains a mixture of two bases which are different strengths.

Question 36

What indicators can you use in titrations?

Answer 36

Phenolphthalein (pink to colourless at pH 9)
Methyl orange (yellow to orange at pH 4)

Question 37

Describe how to perform a gravimetric analysis

Answer 37

1. Add excess precipitate agent to a known quantity of an element to ensure all of it reacts
2. Filter the solution under pressure to separate the precipitate and weigh the filter paper before and after
3. Add AgNO3 to filtrate
4. Wash with a few drops of cold solvent to remove impurities
5. Dry to a constant mass