Chemical Reactions

Question 1

What is a chemical reaction?

Answer 1

A chemical change that produces one or more products from two or more reactants.

Question 2

What are reactants and products?

Answer 2

Reactants are the starting materials that create a chemical reaction. Products are the new materials created.

Question 3

Are chemical reactions fast or slow?

Answer 3

They can be either — rusting is slow, but explosions are fast.

Question 4

List some examples of chemical reactions.

Answer 4

Combustion, digestion, respiration, photosynthesis and developing photography film.

Question 5

Will all chemical reactions commence by themselves?

Answer 5

No. Some need a ‘boost’, e.g. light (known as a photochemical reaction, this is why some chemicals are stored in brown bottles) or heat. Photosynthesis is an example — light must be present in order for it to occur.

Question 6

Are chemicals changes permanent or temporary?

Answer 6

Most chemical changes are permanent and irreversible.

Question 7

Give some symptoms of a chemical reaction.

Answer 7

• Colour change
• Smoke/gas/fizzing
• Temperature change
• A change of state of matter
• Noise
• Light
• Distinctive smell
• Disappearance of solid (e.g. Mg in HCl)
etc.

Question 8

What is a physical change?

Answer 8

A reaction that is temporary and reversible (usually), with no new substance being created. The mass will always remain the same — one will always have the same quantity of water as ice (assuming perfect conditions) and 10g of water with 2g of salt will form 12g of salt water.

Question 9

Give some examples of physical changes.

Answer 9

Any change in the state of matter without a new substance (melting/freezing, boiling/condensing, sublimating/desublimating, ionising/de-ionising) will be a physical change. Dissolution is also an example — the solvent (e.g. water) can usually be evaporated from the solute (e.g. salt or orange dilute), reversing the creation of a solution.

Question 10

What happens when magnesium is reacted with oxygen while being heated?

Answer 10

It goes from a grey, shiny metal to one that glows bright white as it forms magnesium oxide. It is a chemical reaction.

Question 11

What happened when calcium carbonate is heated?

Answer 11

CaCO3 is a vibrant green powder. When heated, it thermally decomposes into copper and carbon dioxide, forming a black substance that seems to be boiling — the CO2 ‘lifts’ some of the copper. The other component is CaO, or quicklime.

Question 12

What happens when wax is heated?

Answer 12

It melts before re-solidifying. This is only a physical change.

Question 13

What happens when zinc oxide is heated?

Answer 13

Zinc oxide is white; when heated, it turns yellow before reverting back to white when it cools. Despite the colour change, which is usually a symptom of a chemical reaction, this is only a physical reaction.

Question 14

What happens when iron filings are heated?

Answer 14

Iron filings are grey, powdered iron with a large combined surface area. In oxygen, they burn with orange sparks to create iron oxide.

Question 15

What are the seven kinds of chemical reaction studied in KS3?

Answer 15

Oxidation, reduction, redox, exothermic reaction, endothermic reaction, thermal decomposition, neutralisation.

Question 16

What kind of reaction is this: _ _ _ d _ _ _ o _ ? Give some characteristics of it.

Answer 16

Oxidation. It is the addition of oxygen, e.g. combustion, rusting, oxide creation, respiration.

Question 17

What kind of reaction is this: _ e _ _ c _ _ _ _ ? Give some characteristics of it.

Answer 17

Reduction. The removal of oxygen, e.g. iron extraction from its ore in a blast furnace.

Question 18

What kind of reaction is this: _ _ e _ _ a _ _ _ c _ _ p _ s _ _ _ _ _ ? Give some characteristics of it.

Answer 18

Thermal decomposition — the breaking down of substances using heat. For example, limestone (calcium carbonate) breaks down into CaO and CO2.
The other two kinds of decomposition reaction are electrolytic and photolytic/photochemical.
The opposite of a decomposition reaction is a combination reaction.
Thermal decomposition is always endothermic — it absorbs heat to break the substance down.

Question 19

What kind of reaction is this: _ _ _ o _ ? Give some characteristics of it.

Answer 19

Redox. Oxygen removal and addition in the same reaction. For example, Cu2O and H2 will react to form Cu2 and H2O — a redox reaction, as both substances act as both oxidising agents and get reduced by another oxidising agent.

Question 20

What kind of reaction is this: _ _ _ t _ _ _ m _ _ ? Give some characteristics of it.

Answer 20

Exothermic. Exothermic reactions release heat. Combustion and respiration are examples.

Question 21

What kind of reaction is this: _ n _ _ t _ _ _ _ _ ? Give some characteristics of it.

Answer 21

Endothermic. Endothermic reaction take in heat. Examples include photosynthesis and the thermal decomposition of limestone.

Question 22

What kind of reaction is this: _ _ u _ r_ _ i _ _ t_ _ _ ? Give some characteristics of it.

Answer 22

Neutralisation. An acid and an alkali will react to form a salt and a water. Metals, metal hydroxides, metal oxides and metal carbonates will react differently (metal hydroxides + metal oxides are the same, but metals produce salt and hydrogen while metal carbonates produce salt, water and carbon dioxide, too).

Question 23

What is combustion?

Answer 23

It is burning, which is an oxidation reaction.

Question 24

What is a fuel? and a fossil fuel?

Answer 24

A fuel is a substance that can be used to release useful amounts of energy when it burns. Fossil fuels are created from fossilised organisms. These kinds of fuels are hydrocarbons — substances containing hydrogen and carbon. They release carbon dioxide and water when they burn; examples include coal, crude oil and natural gas. Methane, which is found in natural gas, is a greenhouse gas that releases CO2 and H2O when burned.

Question 25

What is the fire triangle?

Answer 25

It is a triangle made up of three components: fuel, heat and oxygen. These components must affect the other two (hence the triangle) in order for the fire to light. If any one is removed, the fire will go out.

Question 26

What are the three classes of fuels and fires?

Answer 26

Class A, B and C. A is wood, paper, cloth, rubbish, plastics — general household waste. B is flammable liquids e.g. oil or acetone. C is electrical, energised equipment — anything plugged in.

Question 27

What are the three classes of fire extinguisher?

Answer 27

Air-pressurised water, dry powder and CO2. The first can put out class A fires, the second all three, and the third classes B and C.

Question 28

How can fires be put out w/o a fire extinguisher?

Answer 28

Most fires can be put out with water. Some, such as forest fires, may need their nearby fuel supply (i.e. trees) cut, while others that are above 100°C, e.g. a chip-pan fire, will need an alternative method, e.g. a heat/fire-proof blanket or a damp towel.

Question 29

Which chemical tests for water?

Answer 29

Anhydrous copper sulphate, which turns from white to blue if water is present.

Question 30

What is rust?

Answer 30

Rust is a specific kind of chemical reaction.
Iron and steel rust when they come into contact with oxygen and water over a long period of time. It is an oxidation reaction.

Question 31

What is the word equation for rust?

Answer 31

Iron + Oxygen + Water —> Hydrated iron oxide.

Question 32

Which factors can speed up rusting?

Answer 32

Acid rain and salty air.

Question 33

Create an experiment that will prove that water and oxygen are both required for rusting to occur.

Answer 33

Set up three test tubes. In the first (A), place a steel nail in water with air at its head. Seal this tube and leave for a few days. In the second (B), place a steel nail with air and calcium chloride. Seal this tube and leave for a few days. Finally, in tube C, submerge a steel nail in boiled water with a film of oil on the top. Seal and leave for a few days. After a few days, A will have rusted — it has access to steel, oxygen and water. However, B and C will not have done so. Calcium chloride is a drying agent that absorbs all water, while boiled water has no oxygen (it has evaporated) and oil is immiscible with water, thereby preventing air getting in.

Question 34

What are the six kinds of rust prevention?

Answer 34

Paint (large objects), oil/grease film (moving machinery parts), molten tin plating (food cans), chrome plating (bumpers/handlebars), zinc plating/galvanising (gates/machinery), stainless steel — which contains 10-25% chromium or nickel (cutlery/car radiators).

Question 35

What are some advantages for different rust prevention techniques?

Answer 35

Paint is relatively cheap.
Tin is interactive and non-toxic.
Chrome is shiny, decorative and protective.
Zinc is one of the cheapest.

Question 36

What are some disadvantages for rust prevention techniques?

Answer 36

Paint is easily scratched, which allows the water and oxygen access to the iron/steel.
Oil/grease must be renewed every so often.
Tin is the same as paint — easily scratched.
Chrome is one of the most expensive.
Zinc is only temporary and is used for sacrificial protection — it is more reactive and will corrode first.

Question 37

Describe the reaction between citric acid and baking soda.

Answer 37

Citric acid reacting with baking soda is endothermic. Baking soda is sodium hydrogen carbonate. Together, they will create sodium citrate, water and CO2. 20ml of 1.5M + 8g of baking soda. Record citric acid temp every 30s for 2 mins and immediately add baking soda. Stir. They should fizz. Record temp. for every 30s for a further 10 mins in table. The temp. should decrease exponentially and reach the negative numbers.

Question 38

What is a chemical formula?

Answer 38

A formula that shows the elements present in a compound and how many are present.

Question 39

What is the difference between symbol and word equations?

Answer 39

Word equations are composed of the names of elements and compounds, whereas symbol equations consist of chemical formulæ. Symbol equations must balanced.

Question 40

Balance the following chemical equations.

Answer 40

Refer to physical flashcard.