C5 - quantitative analysis

Question 0

What formula is used to calculate the number moles?

Answer 0

number of moles = mass of chemical/molar mass

Question 1

What is the mass of one mole of a substance?

Answer 1

The mass of 1 mole of a substance is called its molar mass. Molar mass is relative formula mass in grams.

Question 2

What formula is used to calculate the mass of a chemical?

Answer 2

Mass of chemical = number of moles x molar mass

Question 3

What is empirical formula?

Answer 3

Empirical formula is the simplest whole number ratio of each type of atom of a compound.

E.g
Glucose –> C6H12O6 –> CH2O

Question 4

How do you calculate the percentage mass of an element?

Answer 4

% mass of element = total mass of the element in the compound/relative formula mass of compound x 100

Question 5

What is concentration in terms of solute particles?

Answer 5

As concentration increases, the solute particles become more crowded.

Question 6

How do you convert cm cubed to dm cubed and vice versa?

Answer 6

To convert cm cubed to dm cubed, divide by 1000.

To convert dm cubed to cm cubed, multiply by 1000.

Question 7

What’re the equations showing the relationship between the amount of moles, concentration in mol/dm cubed and volume in dm cubed?

Answer 7

Amount in moles = concentration x volume

Concentration = amount in moles/volume

Volume = amount in moles/concentration

Question 8

What is titration reaction?

Answer 8

Where an acid and alkali neutralise:

Acid + alkali –> salt + water

Question 9

Explain a titration curve.

Answer 9

• only alkali is present at the beginning of the experiment so the pH number is high
• as acid is added if starts to neutralise the alkali and the pH drops slowly at first
• the point where all the acids react with all the alkali is called the end point
• at the ends point, pH changes very suddenly

Question 10

What is an advantage of single indicators?

Answer 10

To show an end point, single indicator needs to be used. Mixed indicators like UI give continuous colour changes which are harder to see.

Question 11

What colour does litmus go in alkali and acid?

Answer 11

Litmus
Alkali - blue
Acid - red

Question 12

What colour does phenolphthalein go in acid and alkali?

Answer 12

Phenolphthalein
Alkali - pink
Acid - colourless

Question 13

What colour does screened methyl orange go in acid and alkali?

Answer 13

Screened methyl orange
Acid - pink
Alkali - green

Question 14

What are the equations needed to find the concentration of an acid or an alkali or vice versa?

Answer 14

Concentration = number of moles/volume in dm cubed

Number of moles = concentration x volume in dm cubed

Volume in dm cubed = number of moles/concentration

Question 15

What’re there three ways of measuring the gas produced in a reaction?

Answer 15

• if an upturned bursts or measuring cylinder is used it must be filled with water before it is turned upside down. The volume is read off the scale on the side.
• the scale on a burrette goes the opposite way to that on a measuring cylinder.
• if a balance is used, a loose plug of cotton wool in the neck of the flask prevents a spray. The volume of gas produced can be calculated from the mass of gas produced.

Question 16

How do you calculate the volume of gas?

Answer 16

At room temperature and pressure, 1 mole of a gas takes up 24 dm cubed.

Volume of gas = number of moles x 24 dm cubed

Question 17

Explain direct proportion.

Answer 17

When the limiting reactant is all used up there are no more particles of that substance to collide with the other reactant, so that amount of product formed is directly proportional to the amount of limiting reactant.

Question 18

What is a reaction at equilibrium?

Answer 18

A reaction is at equilibrium when a balance of amounts of reactants and products is reached.

• the rate of forward reaction equals the rate of the backward reaction
• the concentrations of reactants and products do not change

Question 19

What happens when the backward and forward rates balance in a reaction?

Answer 19

When the backward and forward rates balance, the concentrations of reactants and products are never equal.

• if the concentration of reactants is greater than that of the products, the equilibrium position if left.
• if the concentration of the reactants is less than that of the products, the equilibrium position is right.

This only works if it is a closed system, the chemicals do not ‘get out’

• initially, the forward reaction rate is fast, but then it slows as the reactants are used up
• at the same time, the backward reaction rate increases as more is available to react
• eventually the backward reaction is as fast as the forward reaction. Equilibrium has been reached.

Question 20

What can alter the equilibrium?

Answer 20

Changing temperature, pressure or concentration can all alter the position of the equilibrium.

Question 21

How does increased temperature alter the equilibrium?

Answer 21

As the temperature increases in this reaction, less product is made as the equilibrium is moving to the left.

At 40 degrees C there is 52% product.

Question 22

How does increased pressure alter the equilibrium?

Answer 22

As the pressure increases in this reaction, more product is made as the equilibrium is moving to the right.

At 15 atm there is 73% product.

Question 23

the reaction to produce dinitrogen tetroxide is 2NO2 (reversible arrow) N2O4.

This reaction happens in a closed system. How does increases in pressure and pressure affect the equilibrium and why?

Answer 23

• if N2O4 is removed as it formed, the equilibrium moves to the right giving a higher yield as more is made to replace it and if more NO2 is introduced, the equilibrium moves to the right.
• the equation shows 2 miles of NO2 produce 1 mile of N2O4, so increasing the pressure moves the equilibrium to the right to reduce the pressure as there are fewer gas particles on the right.
• increasing the temperature moves the reaction in the direction of the endothermic process

These are examples of le Chatelier’s principle, which states if a change is made in a closed system, the equilibrium will shift in a way reduce the effect of the change.

• the equation shows 2 moles of NO2 produce 1 mole of N2O4, so increasing the pressure moves the equilibrium to the right to reduce the pressure as there are fewer gas particles on the right.
• increasing temperature moves the reaction in the direction of the endothermic process.
• these are examples of Le Chatelier’s principle, which states of a change is made in a closed system, the equilibrium will shift in a way to reduce the effect of the change.

Question 24

What is the Contact Process?

Answer 24

In the contact process, the reaction between sulfur dioxide and oxygen is reversible.

sulfur dioxide + oxygen sulfur trioxide
2SO2(g) + O2(g) 2SO3(g)

Question 25

What are the conditions for the contact process?

Answer 25

To obtain the most economic yield, the reaction is carried out at around 450•C at atmospheric pressure using a catalyst of vanadium pentoxide, V2O5.

These conditions achieve the most economic yield because:

• the forward reaction is exothermic, so high temperatures drive the equilibrium to the left, but if the temperature is too low the rate of reaction is too slow. High temperatures increase the ROR. This temperature is a compromise.
• there are three gas molecules on the left of the equation and two on the right, so high pressure increased the yield -it forces the equilibrium further to the right. The equilibrium already lies to the right anyway, so high pressures are not needed.
• catalysts do not affect the position of the equilibrium. They make the reaction go faster.

Question 26

What is a strong acid?

Answer 26

Acids contain H atoms. In water, acid molecules ionise. The H atoms form hydrogen ions, H+

Strong acids ionise completely when they are in water.

strong acid –> hydrogen ions + other ions

• there are lots of H+, so the acid seems v acidic
• lots of collisions occur, so the acid and water react quickly

Question 27

What is a weak acid?

Answer 27

In weak acids, such as ethanoic acid, only of the acid molecules ionise in water.
- a reversible reaction is set up:
weak acid hydrogen ions + other ions

• the reversible reaction forms an equilibrium mixture.
• the equilibrium position is on the left.
• the mixture contains lots of acid molecules but not many H+ ions, so weak acids do not seem to be so acidic.
• fewer collisions occur so the reaction is slow.

Question 28

What does the pH scale show?

Answer 28

The pH scale is used to show the concentration of H+ ions.

high concentration of H+ = low pH number

low concentration of H+ = higher pH number

Question 29

Electrolysis and conductivity

Answer 29

In electrolysis it is the ions that move. Weak acids have fewer H+ ions to move through the liquid than strong acids, so they do not conduct as well as strong acids. However, the same volume of gas will be made if the same amounts of reactants are used. This is because a weak acid will use some of its H+’ions to react and be used up at first, so more molecules of acid will ionise producing more H+ ions. The amount of reactant will determine the amount of product formed as strong or weak acids will produce just as many ions in the end.

Question 30

What does positive ions migrating to the cathode mean for acids?

Answer 30

Positive ions always migrate to the cathode, so during electrolysis all acids produce hydrogen at the cathode.

Question 31

Why is ethanoic acid less conductive that HCl?

Answer 31

There is a lower concentration of H+ ions in ethanoic acid than in hydrochloric acid of the same concentration. There is a lower concentration of H+ ions to carry the charge, so ethanoic acid is less conductive.

Question 32

What does the concentration of an acid tell us?

Answer 32

The concentration of an acid tells us how many moles of acid there are in 1 dm cubed of solution.

Question 33

What does the strength of an acid tell us?

Answer 33

The strength of an acid tells us how much an acid ionises; the degree of ionisation.

Question 34

What do ionic substances form and what happens when this structure is put in water?

Answer 34

Ionic substances form ionic lattices. Te ions are fixed in position within the solid lattice.

• the lattice of ionic substances breaks apart in water and separated ions can move
• in ppt reactions, the reactant ions can move in solution by must collide to react

Question 35

What happens in a ppt reaction involving mixing two different ionic solutions?

Answer 35

In a ppt reaction involving mixing two different ionic solutions, there is an extremely high collision frequency between the ions of the two solutions. The insoluble ppt is formed so fast is appears to be instant.

Question 36

What are the stages for preparing a clean and dry sample of an insoluble salt?

Answer 36

Stage 1: mix: the two solutions to make barium sulfate and sodium chloride

Stage 2: filter: the ppt of barium sulfate, which stays in the filter paper

Stage 3: wash: with distilled water to remove traces of the sodium chloride solution

Stage 4: dry: by leaving the ppt in a warm place for the water to evaporate

Question 37

What are spectator ions?

Answer 37

When lead nitrate and sodium iodide react, only lead ions react with iodide ions as these are precipitated out of the solution. The other ions do not directly take part in the reaction. They are called spectator ions.

Question 38

What does le Chatelier’s principle state?

Answer 38

Le Chatelier’s principle states if a change is made in a closed system, the equilibrium will shift in a way to reduce the effect of the change.