Topic C3- Chemical Reactions

Question 1

What is the law of conservation?

Answer 1

• The law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations.
• According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants.

Question 2

How can the relative formula mass be used to show that mass is conserved in a reaction?

Answer 2

1) First work out the total of the relative formula masses of the reactants.
2) Then work out the total of the relative formula masses of the products.
3) Compare the total relative formula masses of the reactants and products.

Mr reactants = Mr products

Question 3

What does a change of mass in a reaction indicate?

Answer 3

There is a gas involved- it means that either gas has been contained or a gas has escaped.

Question 4

Why does an increase in mass happen during a reaction?

Answer 4

• If mass increases, at least one of the reactants is a gas that is found in the air (e.g. oxygen)
• and the products are solids, liquids or aqueous.
• Before the reaction, the gas is present in the air, but it isn’t contained in the reaction vessel
• so you cannot account for its mass.
• When the gas reacts to form part of the product, it becomes contained inside the reaction vessel.
• So the total mass of stuff inside the reaction vessel increases.

Question 5

Why does a decrease in mass happen during a reaction?

Answer 5

• If the mass decreases, it is probably because some (or all) of the reactants are solids, liquids or aqueous
• and at least one of the products is a gas.
• Before the reaction, any solid, liquid or aqueous reactants are contained in the reaction vessel.
• If vessel isn’t enclosed, then the gas that is produced can escape from reaction vessel
• so it is no longer contained in the reaction vessel, and its mass cannot be accounted.
• So the total mass of the components inside the reaction vessel decreases.

Question 6

Name some common 1+ ions:

Answer 6

• Lithium (Li +)
• Sodium (Na +)
• Potassium (K +)

Question 7

Name some common 2+ions:

Answer 7

• Magnesium (Mg 2+)

- Calcium (Ca 2+)

Question 8

Name some common 1- ions:

Answer 8

• Hydroxide (OH -)
• Nitrate (NO3 -)
• Flouride (F -)
• Chloride (Cl -)
• Bromide (Br -)
• Iodide (I -)

Question 9

Name some common 2- ions:

Answer 9

• Carbonate (CO3 2-)
• Sulfate (SO4 2-)
• Oxide (O 2-)
• Sulfide (S 2-)

Question 10

Name the formulas for:

a) water
b) ammonia
c) carbon dioxide
d) hydrogen
e) chlorine
f) oxygen

Answer 10

a) water = H20
b) ammonia = NH3
c) carbon dioxide = C02
d) hydrogen = H2
e) chlorine = Cl2
f) oxygen = O2

Question 11

In terms of state symbols, what is the difference between liquid (l) and aqueous (aq)

Answer 11

• Aqueous means a substance is dissolved in water

- whereas liquid is a pure substance in its melted state

Question 12

What does an ionic equation show?

Answer 12

• In an ionic equation, only the reacting particles (and products they form) are included.
• To write an ionic equation, look at the reactants and products and distinguish what is the exact same on both sides,
• then cancel them out.

Question 13

What do half-equations show?

Answer 13

• They show how the electrons are transferred during reactions.
• The e- in half equations stands for one electron.
• You can only write half equations for reactions where oxidation and reduction occur (redox reaction).

Question 14

In half equations, how can you show an element losing one electron?

Answer 14

X -> X+ + e-

“X” is losing one electron to become an ion.

e.g. Na -> Na+ + e-

Sodium is losing one electron to become a sodium ion.

Question 15

In half equations, how can you show an element gaining an electron?

Answer 15

X+ + e- -> X

“X” is gaining one electron to become a molecule.

e.g. 2H+ + 2e- -> H2

Two hydrogen ions are each gaining one electron to form a hydrogen molecule.

Question 16

What is “the mole”? (In chemistry!)

Answer 16

• A mole is given by Avogadro’s constant and is 6.022x10^23
• Chemists measure the amount of a substance in a unit called ‘the mole’.
• This is a convenient way of counting atoms.
• It allows chemists to make predictions about the masses of different substances that are involved in reactions.
• One mole is the Avogadro number of particles (atoms, molecules, ions or electrons) in a substance.
• One mole of any element has a mass in grams that is equal to its relative atomic mass.

Question 17

What does Avogadro’s constant show?

Answer 17

• When you get 6.022x10^23 number of atoms or molecules, of any element or compound,
• then they weigh exactly the same number of grams as the relative atomic mass, Ar (or relative formula mass, Mr) of the element or compound.

Question 18

What is the formula for the number of moles?

Answer 18

Number of Moles =

mass in g (of element or compound) ÷ Mr (of element ir compound).

Formula triangle:

moles x Mr

Question 19

What is the formula for calculating the number of particles?

Answer 19

The number of particles of a substance can be calculated using:

• the Avogadro constant
• the amount of substance in mole

number of particles = Avogadro constant × amount (mol)

Example:

Calculate the number of water molecules in 0.5 mol of water.

Number of water molecules = 6.022 × 1023 × 0.5

= 3.011 × 1023

It is important to be clear about the particles involved. For example, 3.011 × 1023 water molecules contain 9.033 × 1023 atoms. This is because a water molecule, H2O, contains three atoms.

Question 20

What is the formula for calculating the mass of an atom?

Answer 20

mass of atom (g) = Ar of element ÷ Avogadro’s constant

Example

Calculate the mass of a magnesium atom. (Relative atomic mass, Ar = 24.3)

Mass of Mg atom = 24.3 ÷ 6.022 x 10^23

= 4.04 × 10-23 g

It is important to give the final answer to the lowest number of significant figures used in the calculation. In this example, this is the same number of significant figures as the Ar value.

Question 21

What is the formula for calculating the mass of a molecule?

Answer 21

mass of molecule (g) = Mr of substance ÷ Avogadro’s constant

Example

Calculate the mass of a bromine molecule. (Relative formula mass, Mr = 159.8)

mass of bromine molecule (g) = 159.8 ÷ 6.022 x 10^23

= 2.654 × 10-22 g

This answer is given to 4 significant figures because both numbers used in the calculation are given to 4 significant figures.

Question 22

How can you calculate the mass of a substance in grams its amount in moles and its molar mass:

Answer 22

mass (g) = molar mass (g/mol) × amount (mol)

Example:

Calculate the mass of 0.25 mol of carbon dioxide molecules, CO2. (Relative atomic masses: C = 12.0, O = 16.0)

relative formula mass, Mr = 12.0 + (2 × 16.0) = 44.0

molar mass = 44.0 g/mol

= 11.0 g

The calculation is the same if a substance is a metal or exists as separate atoms, but its Ar is used to work out the molar mass instead of an Mr.

Question 23

What is the formula for calculating the amount of a substance in moles?

Answer 23

Moles from masses
The amount of a substance in moles can be calculated from its mass in grams and its molar mass:

amount (mol) = mass (g) ÷ molar mass (g/mol)

Example

Calculate the amount of carbon atoms in 6.0 g of carbon. (Relative atomic mass: C = 12.0)

molar mass = 12.0 g/mol

amount = 6.0 ÷ 12.0

= 0.50 mol

Question 24

What is a limiting reactant?

Answer 24

• A reaction stops when all of one of the reactants is consumed.
• Any other reactants are said to be in excess.
• The reactant that is consumed in a reaction is the limiting reactant.
• This is because it limits the amount of product that is formed.

Question 25

Why is the amount of product formed in a reaction directly proportional to the amount of limiting reactant?

Answer 25

• If you add more of the limiting reactant
• there will be more reactant particles to take part in the reaction,
• which means more product particles are made
• as long as other reactants are in excess.

Question 26

How can you calculate the mass of product formed from a given mass of a limiting reactant?

Answer 26

1) Write out balanced equation.
2) Work out relative formula masses (Mr) of the reactant.
3) Find out how many moles there are of the substances you know the mass of.
4) Use balanced equation to work out how many moles there’ll be of the other substance.
5) Use the number of moles to calculate the mass.

**You can use this method to find out the mass of a reactant needed to produce a known mass of a product. **

Question 27

How can you balance symbol equations using reacting masses?

Answer 27

• If you know the masses of the reactants and products that took part in a reaction, you can work out balanced symbol equation:
  1) Divide the mass of each substance by its relative formula mass to find the number of moles.
  2) Divide the number if moles of each substance by the smallest number of moles in the reaction.
  3) If needed, multiply all of the numbers by the same amount to make them all whole numbers.
  4) Write the balanced symbol equations for the reaction by putting these numbers in front of the formulas.

**You may need to work out some unknown masses first.

Question 28

How can you calculate the limiting reactant through a balanced equation?

Answer 28

1) Divide the mass of each substance by its Mr or Ar to find how many moles of each substance were reacted.
2) Divide by the smallest number of moles.
3) Compare the ratios between the moles of the products with the balanced chemical equation.

Question 29

What is an exothermic reaction?

Answer 29

An exothermic reaction is one which:

• gives out energy to the surrounding
• usually in the form of heat
• shown by the rise in temperature of the surroundings.

e.g. combustion is always exothermic

Question 30

What is an endothermic reaction?

Answer 30

An endothermic reaction is one which:

• takes in energy from the surroundings
• usually in the form of heat
• shown by a fall in temperature of the surroundings.

e.g. melting ice cubes

Question 31

What are the characteristics of an exothermic reaction profile?

Answer 31

• The products are at a lower energy than the reactants

- The difference in height represents the energy given out in the reaction.

Question 32

What are the characteristics of an endothermic reaction profile?

Answer 32

• The products are at a higher energy than the reactants.

- The difference on height represents the energy taken in during the reaction.

Question 33

What is activation energy?

Answer 33

The activation energy is the:

• minimum amount of energy needed
• for bonds to break
• and a reaction to start.

Question 34

How can you determine the activation energy on a reaction profile?

Answer 34

It is the difference between the reactants and the highest point on the curve.

Question 35

Using a reaction profile, how can you determine if nothing will happen to the reaction?

Answer 35

If the energy input is less than the activation energy there won’t be enough energy to start the reaction- so nothing will happen.

Question 36

What kind of process is bond-breaking?

Answer 36

• It is an endothermic process.
• In endothermic reactions, the energy used to break bonds is greater than the energy released by forming them.
• (Energy must be supplied to break existing bonds)

Question 37

What kind of process is bond-forming?

Answer 37

• It is an exothermic process.
• In exothermic reactions, the energy released by forming bonds is greater than the energy used to break them.
• (Energy is released when new bonds are formed.)

Question 38

What is the formula to calculate the overall energy change in a reaction?

Answer 38

Overall energy change =
energy required - energy released by
to break bonds forming bonds

Question 39

What does a positive energy change indicate?

Answer 39

An endothermic reaction.

Question 40

What does a negative energy change indicate?

Answer 40

An exothermic reaction.

Question 41

What is the pH of a neutral substance?

Answer 41

pH 7

Question 42

What is kind of substance has a pH lower than 7?

What kind of ions do they form in water?

Answer 42

• Acidic substances have a pH of lower than 7.

- They form H+ ions in water.

Question 43

What kind of substance has a pH greater than 7?

What kind of ions do they form in water?

Answer 43

• Alkaline substances have a pH of greater than 7.

- They form OH- ions in water.

Question 44

Explain what the value of pH being inversely proportional to the concentration of hydrogen ions in a solution means?

Answer 44

• As the concentration of hydrogen ions increases, the pH decreases.
• The higher the hydrogen ion concentration, the more acidic something is, so the lower the pH.

Question 45

What is an indicator?

Answer 45

A dye that changes colour depending on whether the solution’s pH is above or below.

Question 46

What is a pH probe and why is it more useful than a universal indicator?

Answer 46

• A pH probe is attached to a pH meter, and is used to measure the pH electronically.
• The probe is placed in the solution that is being measured and the pH is given on a digital display as a numerical value.
• This gives a higher level of accuracy than an indicator.
• It is important you calibrate the pH probe properly (setting it to read pH 7 in a sample of pure water)
• and rinse the probe with deionised water in between reading.

Question 47

What is a neutralisation reaction?

What does this reaction produce?

Answer 47

• A reaction between acids and bases is called neutralisation.
• When an acid neutralises a base (vice versa) the products are neutral (pH of 7).
• At pH 7, the concentration of hydrogen ions equals the concentration of hydroxide ions.
• It produces a salt and water.
• Neutralisation in an aqueous solution can be shown as as ionic equation in terms of H+ and OH- ions:

H+ (aq) + OH- (aq) –> H20 (l)

Question 48

How do titration curves show a change in pH?

Answer 48

• They are used to show where neutralisation happens during a titration.
• There is a vertical point in a curve which is where the solution is neutral.
• This is called the end point of the titration.

Question 49

What does an acid do when they are in water?

Answer 49

• They ionise- produce H+ ions.

- An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton to them.

Question 50

Give three examples of strong acids:

Answer 50

• Sulfuric acid
• Hydrochloric acid
• Nitric acid

Question 51

Give three examples of weak acids:

Answer 51

• Ethanoic acid
• Citric acid
• Carbonic acid

Question 52

How do strong acids react differently to water than weak acids do?

Answer 52

• Strong acids ionise almost completely in water.
• A large proportion of acid molecules dissociate to release H+ ions.
• They have low pHs (0-2).
• Weak acids do not fully ionise in water.
• Only a small proportion of acid molecules dissociate to release H+ ions.
• Their pHs are around 2-6.

Question 53

What kind of reaction is the ionisation of weak acid?

Answer 53

• The ionisation of weak acid is a reversible reaction,
• which sets up an equilibrium mixture.
• Since only a few of the acid molecules release H+ ions
• the equilibrium lies to the left.

Question 54

What does acid strength tell you?

Answer 54

Acid strength tells you what proportion of the acid molecules ionise in water.

Question 55

What does acid concentration tell you?

Answer 55

• The acid concentration measures how much acid there is in a litre ( 1 dm3) of water.
• Concentration describes the total number of dissolved acid molecules.
• Concentration is how watered-down your water is.
• An acid with a high proportion of acid molecules compared to the volume of water is said to be concentrated.
• An acid with a low proportion of acid molecules compared to the volume of water is said to be dilute.

Question 56

What happens to the pH if the concentration of H+ ions increases by a factor of 10?

Answer 56

The pH decreases by 1.

Question 57

What happens to the pH if the concentration of H+ ions decreases by a factor of 10?

Answer 57

The pH increases by 1.

Question 58

Complete the formula:

Acid + Metal –> …:

Answer 58

Acid +Metal –> Salt + Hydrogen

Question 59

Why isn’t it possible to predict the balanced equation for the reaction of nitric acid with metals?

Answer 59

The reaction of nitric acid with metals produces nitrate salt, but instead of hydrogen gas, the other products are usually a mixture of water, NO and NO2.

Question 60

Complete the formula:

Acid + Metal Carbonate –> …:

Answer 60

Acid + Metal Carbonate –> Salt + Water + Carbon Dioxide

Question 61

Complete the formula:

Acid + Alkali –> …:

Answer 61

Acid + Alkali –> Salt + Water

Question 62

How can you make soluble salts using acid/alkali reactions?

Answer 62

1) Soluble salts can be made by reacting an acid with an alkali.
2) You can’t tell when the reaction has finished- there is no signal when the acid has been neutralised.
3) You cannot add an excess of alkali to the acid and filter out what’s left because the salt is soluble and would be contaminated with excess alkali.
4) Instead, add exactly the right amount of alkali to neutralise the acid. Carry out a titration.
5) Then, carry out the reaction using exactly the right proportions of alkali and acid but with no indicator (you know volumes needed), so salt won’t be contaminated with indicator.
6) The solution that remains when the reaction is complete contains only the salt and water.
7) Slowly evaporate off some of the water and then leave the solution to crystallise. Filter off the solid and dry it.
8) You will be left with a pure, dry salt.

Question 63

How can you make a soluble salt using an acid and insoluble reactant?

Answer 63

1) You can make soluble salts by reacting an acid with an insoluble base.
2) Pick out the right acid, plus a metal or an insoluble base (a metal oxide or metal hydroxide).
3) Add base to acid- the base an acid will react to produce a soluble salt (and water). (*Sometimes you need to heat the reaction to get the acid and base to react.)
4) You will know when all the acid has been neutralised because the excess solid will just sink to the bottom of the flask.
5) Filter off the excess solid to get a solution containing only salt and water.
6) Heat the solution gently to slowly evaporate off some water.
7) Then leave the more concentrated solution to cool and allow salt to crystallise.
8) Filter off the solid and leave it to dry.

Question 64

What is the method to making insoluble salts using the precipitation reaction?

Answer 64

1) Add the correct amount of the first salt in a test tube.
2) Add deionised water to dissolve the salt. Use deionised water to make sure there are no other ions about. Shake it thoroughly.
3) In a separate tube, do the same with the second salt.
4) Pour the two solutions in a small beaker , stir it to mix it together. One of the salts will precipitate out.
5) Put folded piece of filter paper into a filter funnel, and stick the funnel into a conical flask.
6) Pour contents of the beaker into the middle of the filter paper.
7) Rinse the beaker with more deionised water, and pour this into the filter paper- get all the wanted product from the beaker.
8) Rinse contents of the filter paper with deionised water to make sure that all the soluble salt has been washed away.
9) Scrape the newly produced salt onto a fresh piece of filter paper and leave it to dry.

Question 65

What is oxidation? in terms of electron transfer

Answer 65

The loss of electrons.

Question 66

What is reduction? in terms of electron transfer

Answer 66

The gain of electrons.

Question 67

What is a redox reaction?

Answer 67

Redox reactions are reactions where both oxidation and reduction are taking place.

Question 68

What is oxidation? in terms of oxygen transfer?

Answer 68

Oxidation is gain of oxygen.

Question 69

What is reduction? in terms of oxygen transfer?

Answer 69

Reduction is loss of oxygen.

Question 70

What does an oxidising agent do?

Answer 70

An oxidising agent accepts electrons and gets reduced

• its gains electrons OR
• loses oxygen

Question 71

What does a reducing agent do?

Answer 71

It donates electrons and gets oxidised.

• loses electrons OR
• gains oxygen

Question 72

What is electrolysis?

Answer 72

1) Electrolysis is the breaking down of a substance using electricity.
2) An electric current is passed though an electrolyte (molten or dissolved ionic compound), causing it to decompose.
3) The positive ions (cations) in the electrolyte will move towards the cathode (negative electrode) and are reduced (gain electrons).
4) The negative ions (anions) in the electrolyte will move towards the anode (positive electrode) are oxidised (lose electrons).
5) As ions gain or lose electrons, they form the uncharged substances and are discharged from the solution.

Question 73

What is an electrochemical cell?

Answer 73

An electrochemical cell is a circuit, made up of the anode and cathode, electrolyte, a power source and the wires that connect the two electrodes.

Question 74

Why can’t an ionic solid be electrolysed?

Answer 74

The ions are in fixed positions and can’t move.

Question 75

Why can molten ionic compounds be electrolysed?

Answer 75

The ions can move freely and conduct electricity.

Question 76

What happens to positive metals ions at the cathode?

Answer 76

They are reduced (gain electrons) to atoms.

Question 77

What happens to negative ions at the anode?

Answer 77

They are oxidised (they lose electrons) to atoms.

Question 78

What is the process of electrolysis of aqueous solutions?

Answer 78

1) In aqueous solutions, as well as the ions from the ionic compound, there will be hydrogen ions (H+) and hydroxide ions (OH-) from the water.
2) At the cathode, if H+ ions and metal ions are present, hydrogen gas will be produced if the metal is more reactive than hydrogen (e.g. sodium).
3) If the metal is less reactive than hydrogen, (e.g. copper or silver), then a solid layer of pure metal will be produced instead.
4) At the anode, if halide ions (Cl-, Br-, I-) are present, molecules of chlorine, bromine or iodine will be formed.
5) If no halide ions are present, then oxygen will be formed from the hydroxide ions.

Question 79

How can you test for chlorine gas?

Answer 79

Chlorine bleaches damp litmus paper, turning it white. (It may turn red at first- this is because a solution of chlorine is acidic)

Question 80

How can you test for hydrogen gas?

Answer 80

Hydrogen makes a “squeaky pop” when burnt with a lighted splint.

Question 81

How do you set up an electrochemical cell?

Answer 81

1) Get two electrodes (inert electrodes platinum or carbon)
2) Clean the surfaces of the electrodes using a piece of emery paper.
3) Do not touch the surfaces of the metals with your hands- you could transfer grease back onto the strips.
4) Place both electrodes into a beaker filled with your electrolyte.
5) Connect the electrodes to a power supply using crocodile clips and wires.

Question 82

What is halide ion?

Answer 82

A halogen atom bearing a negative charge.

Question 83

How does electrolysis with non-inert electrodes work?

Answer 83

1) Non-inert electrodes can decompose into the electrolyte.
2) E.g. you could use copper electrodes in a solution of copper sulfate.
3) To set it up, use the same method as the one above, but use copper electrodes, rather than inert carbon or platinum electrodes.
4) As reaction continues, the mass of the anode will decrease and the mass of the cathode will increase.
5) This is because copper is transferred from the anode to the cathode.
6) If you want to measure how the mass of electrodes has changed during an experiment, dry them first before weighing them- any copper sulfate solution on electrodes may make them appear to have a higher mass than they really do.