C3 Part B

Question 1

What is the numerical form of a “mole”? Give it in standard form

Answer 1

6.023 x 10²³

Question 2

How many atoms of carbon-12 weigh exactly 12g?

Answer 2

6.023 x 10²³

^{(A mole)}

Question 3

Fill in the gaps:

A mole of ………. of any element or compound weighs the same number of ………… as the ……………… (A_r) of the element or compound.

Answer 3

A mole of atoms/molecules of any element or compound weighs the same number of grams as the relative atomic mass (Ar) of the element or compound.

Question 4

Give two possible units that concentration of a substance can be measured in

Answer 4

moles per dm³

grams per dm³

1 litre = 1000 cm³ = 1 dm³

Question 5

Fill in the gaps:

The …….. solute you dissolve in a given volume, the more crowded the solute ……. are and the …… ……….. the solution.

Answer 5

The more solute you dissolve in a given volume, the more crowded the solute molecules are and the more concentrated the solution.

Question 6

What kind of experiment is used to measure how much acid is needed to neutralise an alkali?

Answer 6

Titration

Question 7

Describe the process of a titration

Answer 7

1. Put an alkali in a flask, along with indicator (phenolphthalein or methly orange)
2. Add the acid to the alkali a little bit at a time using a burette, giving the flask a regular swirl
3. When the indicator changes colour, it means all the alkali has ben neutralised
4. Record the amount of acid needed to neutralise the alkali

Question 8

What are the two main titration equations? Draw the equation pyramid if it helps

Answer 8

1. Number of moles = Concentration (moles/dm³) x Volume (dm³)
2. Mass in grams = Number of moles x Relative formula mass

Question 9

Fill in the gaps

An exothermic reaction is one that gives out …….. to the surroundings, usually in the form of ……. and shown by a ……… in temperature.

An ………… reaction in one which takes in energy from the ……….., usually in the form of heat and shown by a …… in temperature.

Answer 9

An exothermic reaction is one that gives out energy to the surroundings, usually in the form of heat and shown by a rise in temperature.

An endothermic reaction in one which takes in energy from the surroundings, usually in the form of heat and shown by a fall in temperature.

Question 10

Energy must be supplied to break existing bonds, so bond breaking is what kind of reaction?

Answer 10

Endothermic

Question 11

Energy is released when new bonds are formed, making it what kind of reaction?

Answer 11

Exothermic

Question 12

Fill in the gaps:

In an endothermic reaction, the energy required to break old bonds is ……… than the energy ……….. when new …….. are formed.

In an ………. reaction, the energy released in bond ……….. is greater than the energy used in ……… old bonds.

Answer 12

In an endothermic reaction, the energy required to break old bonds is greater than the energy released when new bonds are formed.

In an exothermic reaction, the energy released in bond formation is greater than the energy used in breaking old bonds.

Question 13

How can energy transfer be measured in endothermic/exothermic reactions?

Answer 13

By measuring temperature change.

Question 14

What are the issues with the fuels we currently use and have relied on for decades to provide energy?

Answer 14

1. Burning fuels releases CO₂ and contributes to the greenhouse effect, causing a rise in global temperature and thermal expansion of the seas to occur
2. Developing alternative, renewable sources of energy costs losts of money
3. Crude oil is a finite resource
4. The transportation of oil, which can only be found in certain places on earth, costs money and causes harmful emissions to be released.

Question 15

By what process is fuel energy calculated?

Answer 15

Calorimetry

Question 16

Outline the process of calorimetry

Answer 16

1. Put a defined amount of water in a copper can and record its temperature
2. Weigh the spirit burner and the lid that is being used
3. Put the spirit burner underneath the can and light the wick. Heat the water, stirring constantly until the temperature reaches about 50°C.
4. Put out the flame, using the burner lid and measure the dinal temperature of the water.
5. Weigh the spirit burner and the lid again.

(almost exactly the same process can be used to calculate the amount of energy produced by food)

Question 17

What is the equation for working out energy transfer in calorimetry?

Answer 17

Q = mcΔT

Energy transferred (joules) = Mass of water (grams) x Specific heat capacity of water (4.2) x Temperature change (in °C)

Question 18

What is the main difference between the energy level diagrams of endothermic and exothermic reactions?

Answer 18

In exothermic reactions, the products are at a lower energy than the reactions, whereas in endothermic reactions the products are at a higher energy than the reactants.

Question 19

Draw and annotate sketches of energy level diagrams for endothermic and exothermic reactions

Answer 19

Question 20

What is the activation energy of a reaction?

Answer 20

The minimum energy required for reeacting particles to break their bonds

Question 21

How can the activation energy of a reaction be lowered and how is this represented on an energy level diagram?

Answer 21

A catalyst can be addes so that the reaction occurs more easily and more quickly. This will be evident on an energy level diagram by a lower curve.

Note: The overall energy change for the reaction remains the same

Question 22

What are the bond energies applied to:

H-H

Cl-Cl

H-Cl

Answer 22

H-H = 436 kj/mol

Cl-Cl= 242 kj/mol

H-Cl = 431 kj/mol

Question 23

How much energy is required to break one mole of H-H and one mole of Cl-Cl bonds?

(H-H = 436 kj/mol

Cl-Cl= 242 kj/mol)

Answer 23

436 + 242 = 678 kj

Question 24

How much energy does forming two moles of H-Cl release?

Answer 24

2 x 431 = 862 kj

Question 25

Prove that overall more energy is released than is used to form products using the example of hydrogen and chlorine bonds used to make two moles of HCl

H-H = 436 kj/mol

Cl-Cl= 242 kj/mol

H-Cl = 431 kj/mol

Answer 25

Breaking the bonds:

436 + 242 = 678 kj required

Forming two moles of HCl:

2 x 431 = 862 kj

Energy released:

862 - 678 = 184 kj/mol

Question 26

The reaction between hydrogen and oxygen is exothermic, how is this useful?

Answer 26

Hydrogen and oxygen react to produce water, which isn’t a pollutant. Because the reaction is exothermic, energy from the reaction can be harnessed - either in a combustion engine or a fuel cell.

Question 27

Hydrogen gas can be burnt to power vehicles, what are the pros and cons of this?

Answer 27

Pros: It’s clean energy

Cons: A special, expensive engine is needed. Hydrogen is hard to store safely. Energy is required to make hydrogen from water.

Question 28

What is a fuel cell? When were they developed and why?

Answer 28

A fuel cell is an electrical cell that’s supplied with fuel and oxygen and uses energy from the reaction between them to generate electricity.

Fuel cells were developed in the 1960s as part od a space programme, because they’re more practical than solar cells and safer than nuclear power. (They’re still used on Space Shuttle missions) Fuel cells don’t run down or need recharging, they’ll produce energy as long as fuel is supplied.

Question 29

The car industry is developing fuel cells to replace conventional petrol/diesel engines. What are the two main advantages of fuel cells in cars?

Answer 29

1. They don’t produce any conventional pollutants (no greenhouse gases, no nitrogen, no suldur dioxide, no carbon monoxide - only water and heat)
2. They could help countries become less dependent on crude oil.

Question 30

Why are fuel cells unlikely to mean the end of our dependence on fossil fuels?

Answer 30

1. Hydogen is a gas so takes up a lot of space to store
2. Hydrogen is very explosive
3. Hydrogen fuel is often made from hydrocarbons (from fossil fuels) or by the electrolysis of water (which uses electricity - fossil fuels)

Question 31

Flame tests can be used to identify metal ions. What colour flame do each of these metals create and what are their ionic symbols?

Lithium

Sodium

Potassium

Calcium

Barium

Answer 31

Lithium (Li⁺) - crimson flame

Sodium (Na⁺) - yellow flame

Potassium (K⁺) - lilac flame

Calcium (Ca²⁺) - red flame

Barium (Ba²⁺) - green flame

Question 32

Outline how you might carry out a flame test in a lab

Answer 32

Distill a wire loop by dipping it into hydrochloric acid and rinsing it with distilled waster. Dip the clean wire loop into a sample of the compound you are testing and place it in the clear blue part of a bunsen flame, wait to see what colour the flame turns.

Question 33

Other than using a flame test, how else might you test for metal ions?

Answer 33

Some metal ions form a coloured precipitate with NaOH, so you can add a few drops of sodium hydroxide to the substance you wish to test. If you get a coloured insoluble hydroxide, you can tell which metal was in the compound.

Question 34

When reacted with NaOH, what colour precipitate do each of these metals form and what is the ionic reaction involved?

Calcium

Copper(II)

Iron(II)

Answer 34

Calcium - White:

Ca²⁺_(aq) + 2OH^-_(aq) ⇒ Ca(OH)_2(s)

Copper (II) - Blue:

Cu²⁺_(aq) + 2OH^-_(aq) ⇒ Cu(OH)_2(s)

Iron(II) - Green:

Fe²⁺_(aq)) + 2OH^-_(aq) ⇒ Fe(OH)_2(s)

Question 35

When reacted with NaOH, what colour precipitate do each of these metals form and what is the ionic reaction involved?

Iron (III)

Aluminium

Magnesium

Answer 35

Iron(III) - Brown:

Fe³⁺_(aq) + 3OH^-_(aq) ⇒ Fe(OH)_3(s)

Aluminium - White at first, but then redissolves in excess NaOH to make a colourless solution:

Al³⁺_(aq) + 3OH^-_(aq) ⇒ Al(OH)3(s)

Magnesium - White:

Mg²⁺_(aq) + 2OH^-_(aq) ⇒ Mg(OH)_2(s)

Question 36

To test for carbonates in a substance you have to detect CO₂, what is the method used for this?

Answer 36

1. You must react your substance with a dilute acid, as carbonates will react to produce CO_2.

_​acid + carbonate ⇒ salt + water + carbon dioxide

2. You can test to see if the gas produced is carbon dioxide by bubbing it through limewater, if it is CO₂ then the limewater will go cloudy.

Question 37

How might you test for halide ions in a substance?

Answer 37

To test for chloride, bromide or iodide ions, add dilute nitric acid (HNO₃), followed by silver nitrate (AgNO₃).

Chloride gives a white precipitate of silver chloride.

Bromide gives a cream precipitate of silver chloride.

Iodide gives a yellow precipitate of silver chloride.

Question 38

How would you test for sulfate ions (SO₄^2-) in a substance? (2 main steps)

Answer 38

1. Add dilute hydrochloric acid (HCl) , followed by barium chloride solution (BaCl₂)
2. If a white precipitate of barium sulfate is formed, it means the original compound was a sulfate.