Unit 5 - Chemistry 2: Chemical reactions and fuels

Question 1

Define catalyst

Answer 1

An agent which increases rate of reaction but remains unchanged

Question 2

What is electrolysis

Answer 2

The chemical effect of electricity on ionic compounds, causing them to break up into simpler substances, usually elements.

Question 3

The products of complete combustion of hydrocarbons are…

Answer 3

The products of complete combustion of hydrocarbons, exemplified by methane, are carbon dioxide and water.

Question 4

Describe neutrality and relative acidity and alkalinity in terms of pH measured using full-range indicator and litmus

Answer 4

*An alkali is a base that dissolves in water

pH levels
Neutral = 7
Acidic 7

An alkali is a chemical which will release OH- ion in water

An acid is a chemical which will release H+ ions in water

Base = anything that will neutralise an acid.

                       Red Litmus	 Blue Litmus Acidic solution	   Stays red	Turns red Neutral solution    Stays red	Stays blue Alkaline solution	  Turns blue	Stays blue

Question 5

Describe the characteristic reactions between acids and metals, bases (including alkalis) and carbonates

Answer 5

Acid + metal oxide -> salt + water
Acid + metal hydroxide -> salt + water
Acid + metal -> salt + hydrogen
Acid + metal carbonate -> salt + carbon dioxide + water
Acid + metal hydrogen carbonate -> salt + carbon dioxide + water
Acid + ammonia solution -> ammonium salt + water

Salt = metal nonmetal
eg hydrochloric + sodium = sodium chloride+water
acid hydroxide

When hydrochloric acid is neutralised, the salt formed is a metal CHLORIDE
When nitric acid is neutralised, the salt formed is a metal NITRATE
When sulphuric acid neutralised, the salt formed is metal SULPHATE.

Question 6

Describe and explain the importance of controlling acidity in the environment (air, water and soil)

Answer 6

Fossil fuels, such as coal, contain sulphur. When they are burnt, the sulphur combines with oxygen. It forms sulphur dioxide. The sulphur dioxide is given off as a gas.
Sulphur dioxide is an unpleasant gas. It damages living things. Humans who breathe in a lot of sulphur dioxide over a long period of time have an increased risk of suffering from colds, bronchitis and asthma. Sulphur dioxide can kill plant leaves. It may completely kill the plant.

Acid in the air may form acid rain once it joins with rain clouds. Acid rain can damage trees by washing important minerals (calcium and magnesium) out of the soil. The acid water also washes out aluminium ions. The aluminium runs into rivers and lakes. Aluminium is very toxic to fish. Acid rain may also damage limestone buildings.

Plants cannot survive in very acidic or alkali soil. When soil acidity changes, the solubility of a number of metal ions also change. Plant growth is really affected by the varying concentration of these metals in solution rather than by the acidity itself. Under acidic conditions, many soil minerals dissolve and increase the concentration of metal ions to toxic levels. Under alkaline conditions, the solubility of minerals decrease to the point that nutrient deficiencies occur. The aim in managing soil pH is not to achieve a particular pH value, but to adjust the acidity to the point where there are no toxic metals in solution and the availability of nutrients is at its maximum.

Limestone is used to treat acidic soil, increasing the pH level.

Question 7

What is the main constituent of natural gas

Answer 7

Methane

Question 8

Describe the effect of concentration, particle size, catalysis and temperature on the speeds of reactions.

Answer 8

Concentration increases - the rate of reaction increases. If it’s more concentrated, there’s a greater chance of reaction because there are more particles in the same volume = greater chance of collision. Eg as more water is added, the thiosulphate solute becomes more dilute. This means the acid molecules become more likely to hit a water molecule not a thiosulphate molecule - so the reaction is slower.

Particle size/surface area increases - rate of reaction increases. Because more particles are exposed -> more reaction

Catalysis - speeds up reaction yet remains unchanged

Temperature increases - rate of reaction increases. Particles move faster because of more kinetic energy = greater chance of collision.

Question 9

Describe the preparation, separation and purification of salts using techniques selected from topic 0 and the reactions mentioned

Answer 9

If the base dissolves in water, you need to add just enough acid to make a neutral solution - check a small sample with universal indicator paper - then evaporate the water. You get larger crystals if you evaporate the water slowly.
Copper oxide, and other transition metal oxides or hydroxides, do not dissolve in water. If the base does not dissolve in water, you need an extra step. You add the base to the acid until no more will dissolve and you have some base left over (called an excess). You filter the mixture to remove the excess base, then evaporate the water in the filtrate to leave the salt behind.

Acid + metal oxide -> salt + water
Acid + metal hydroxide -> salt + water
Acid + metal -> salt + hydrogen
Acid + metal carbonate -> salt + carbon dioxide + water
Acid + metal hydrogen carbonate -> salt + carbon dioxide + water
Acid + ammonia solution -> ammonium salt + water

Salt = metal nonmetal
eg hydrochloric + sodium = sodium chloride+water
acid hydroxide

Question 10

Describe a practical method for investigating the speed of a reaction involving gas evolution.

Answer 10

Gas evolution = a chemical reaction that produces a gas.

Gas syringes (or measurement of displacement of water by gas in upturned measuring cylinder) can be used to measure the volume of gas produced. Measurement of mass decrease in reaction involving evolution of gas could also be demonstrated.

Measuring the volume of gas involved = gas syringe

Measuring the rate of loss of a gaseous product = cotton wool plug (Marble chips and acid separated by a card. Apparatus placed on a scale and mass is read. To start the reaction, the flask is tilted so the card falls and stuff mixes. A piece of cotton wool is placed at the neck of the flask to allow CO2 to escape. As the gas escapes the mass decreases. Take mass reading over a period of time)

Question 11

Describe and explain the effects of temperature and concentration in terms of collisions between reacting particles.

Answer 11

Collision Theory
All substances are made of atoms or molecules. For a reaction to take place the particles of different substances must collide. The more collisions between particles in a given time, the faster the reaction.

(for a collision to be successful, the molecules must have a minimum amount of energy called ACTIVATION ENERGY)

Temperature: the increased speed of the particles mean that collisions occur more often in a certain time, and it is more likely that a collision will result in the particles reacting.

Concentration: As more water is added, the solute becomes more dilute. This means that acid molecules become more likely to hit a water molecule not a _____ molecule - so the reaction is slower.

Question 12

Use the terms ELECTRODE, ELECTROLYTE, ANODE, CATHODE

Answer 12

Electrode: a conductor through which electricity enters or leaves an object.

Electrolyte: a liquid that contains ions and can be decomposed by electrolysis.

Anode: the positive electrode. Attracts negative ions

Cathode: the negative electrode. Attracts positive ions.

Question 13

Describe electrolysis in terms of the ions present and the reactions at the electrodes.

Answer 13

Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

• Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
• Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.

Question 14

Describe the electrode products, using inert (unreactive) electrodes, in the electrolysis of:

• molten lead (ii) bromide
• aqueous copper chloride

Answer 14

Molten lead (ii) bromide 
PbBr2

Cathode
Pb2+ (l) + 2e- -> Pb (l)

Anode
2Br- (l) -> Br2 (g) + 2e-

• lead metal at the negative electrode
• The lead will form as a liquid at the bottom of the reaction vessel.
• bromine (Br2) at the positive electrode
• The bromine appears as a brown gas at the positive electrode.

Aqueous copper chloride
CuCl2 -> Cu + Cl2

At the cathode Cu is reduced
Cu2+ (aq) + 2e- -> Cu (s)

At the anode Cl is oxidised
2Cl- (aq) -> Cl2 (g) + 2e-

The products of this electrolysis are:

copper metal at the negative electrode

The copper forms as a brown solid on the negative electrode.

chlorine gas (Cl2) at the positive electrode

The chlorine appears as a gas with a characteristic smell at the positive electrode.

Question 15

Predict the products of the electrolysis of a specified binary compound in the molten state

Answer 15

Eg CuSO4 + Zn -> Cu + ZnSO4

In a displacement reaction there is no exchange of oxygen. However copper has been displaced from a solution and zinc has gone into the solution.

• The metal in the solution (Copper, Cu2+) is in the form of an ion, so to form the atom it must gain electrons
• The metal, as the element (zinc) is an atom must lose electrons to form ions in solution.
• One metal has lost electrons, the other has gained them. So there has been an exchange of electrons.
• The metal that goes from the element to a compound is undergoing OXIDATION. Losing electrons so other elements can bond with it to form a compound.
• The one that starts in a compound and forms an element is undergoing REDUCTION. Gaining electrons to be by itself.

Question 16

PRECIPITATION REACTIONS
Identify usings tests…
-Aqueous cations: copper (ii), iron (ii), iron (iii) and zine
-Anions: carbonate, chloride
-Gases: carbon dioxide, chlorine, hydrogen, oxygen

Answer 16

Aqueous cations:

• Copper (Cu2+). Effect of aq sodium hydroxide = light blue ppt. Insoluble in excess. Effect of aq ammonia = light blue ppt. Soluble in excess, giving a dark blue solution.
• Iron (Fe2+). Effect of aq sodium hydroxide = green ptt. Insoluble in excess. Effect of aq ammonia = green ppt. insoluble in excess.
• Iron (Fe 3+). Effect of aq sodium hydroxide = red-brown ppt. Insoluble in excess. Effect of aq ammonia = red-brown ppt, insoluble in excess.

Anions

• Carbonate. Add dilute acid, then limewater. Results = bubbles, CO2 produced.
• Choride. Acidify with dilute nitric acid, then add aqueous silver nitrate. White ppt

Gases

• Carbon dioxide. Turns limewater milky
• Chlorine. Bleaches damp litmus paper
• Hydrogen. “pops” with a lighted splint.
• Oxygen. Relights a glowing splint.

Question 17

Which fossil fuels produce carbon dioxide on combustion?

Answer 17

Coal, natural gas and petroleum.

Question 18

Describe petroleum and its separation into useful fractions by fractional distillation

Answer 18

• A mixture of different lengths hydrocarbons
• separated by fractional distillation
• sorted according to boiling point
• related to molecule size

Bigger hydrocarbons = harder to break than smaller hydrocarbons as more molecules are bonded/overlapped.

Question 19

Understand the essential principle of fractional distillation in terms of differing boiling points (rangers) of fractions related to molecular size and intermolecular attractive forces

Answer 19

Because they have different boiling points, the substances in crude oil can be separated using fractional distillation. The crude oil is evaporated and its vapours allowed to condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.

At the top of the column = small molecules
-low boiling point
-flows easily
-ignites easily
-very volatile (evaporating easily)
-goes to top of fractionating column
-eg refinery gases (bottled gas)
short carbon chains ie C1 - C4 etc

At the bottom of the column = large molecules
-high boiling point
-not very volatile
-Does not flow easily
-does not ignite easily
eg bitumen for roads and roofs.
long carbon chains ie >25

Question 20

State the use of:

• refinery gas
• gasoline fraction
• diesel oil/gas oil

Answer 20

Refinery gas for heating and cooking
Gasoline fraction for fuel/petrol in cars
Diesel oil/gas oil for fuel in diesel engines. Lorries (Trucks), buses.

Question 21

Describe the properties of alkanes (exemplified by methane)

Answer 21

• Generally unreactive, except in terms of burning
• Main use of hydrocarbons, especially short chain alkanes, is as fuels.
• a family of hydrocarbons that share the same general formula. CnH2n+2
• Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air, which we call burning or combustion.

Question 22

Name, identify and draw the structures of methane, ethane and ethene

Answer 22

Methane:
CH4
      H
       |   
H - C - H
      |
     H

Ethane:
C2H6
       H    H
        |     |
H  -  C - C - H
        |     |
       H    H

Ethene:
C2H4
H          H
   \        /
    C = C
   /        \
 H        H

Question 23

Recognise alkanes and alkenes from their chemical names or from molecular structures

Answer 23

Alkanes = chemical names end in "ane"
Alkenes = chemical names end in "ene"

Alkanes - single bonds
Alkenes - one double bond but each carbon still has 4 bonds.

Question 24

Describe the manufacture of alkenes by cracking

Answer 24

Cracking converts large, saturated hydrocarbons into smaller, unsaturated hydrocarbons which are more useful.

The unsaturated molecules can then be used in polymerisation reactions.
In polymerisation lots of small molecules (monomers) are joined together to make a larger molecule - the polymer.

Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then either:
passed over a hot catalyst, or
mixed with steam and heated to a very high temperature.

These processes break chemical bonds in the molecules, causing thermal decomposition reactions. Cracking produces smaller alkanes and alkenes (another type of hydrocarbon).

http://www.bbc.co.uk/schools/gcsebitesize/science/aqa/substancesfromcrudeoil/alkenesrev1.shtml

Question 25

Distinguish between alkanes and alkenes by the addition reaction of alkenes with bromine

Answer 25

Alkane + bromine = no reaction. Still coloured. Each C has 4 bonds

Alkene + bromine -> colourless
Decolouration of bromine = double bonds
If bromine is added to a hydrocarbon and the mixture goes colourless, then the starting compound was unsaturated.

(Bromine water is a dilute solution of bromine, normally orange-brown in colour. It becomes colourless when shaken with an alkene, but its colour remains the same when it is shaken with alkanes.)