RC1

TEXT REFERENCE: CHAPTER 10

We have previously looked at two types of changes: physical and chemical.

Chemical changes occur when a new substance is formed. This is also called a chemical reaction. We begin with one or more reactants and we end with one or more products.

Reactants form Products

The new substance or substances will often have different properties to the reactants. They may be a different colour or state. The reactions are not easily reversed (usually) and chemical reactions are often accompanied by large energy changes, which can include heat or light or even sound.

In any chemical reaction, bonds within the reactants are broken and then new bonds are formed in creating the products. Energy is required to break chemical bonds and energy is released when bonds are formed. Hence a chemical reaction is also a transfer of energy.

1.1 investigate a variety of reactions to identify possible indicators of a chemical change

VIDEOS:

• Physical and chemical changes https://www.youtube.com/watch?v=XMLvD-QqI5c&index=24&list=PLeFSFSJ9WqSDKqoVXnj3EsBJckd5oA3ZD 4.27

• RC1 Indicators of chemical reaction https://www.youtube.com/watch?v=gZYMCVJFVPk&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9&index=2 5.44mins

1.2 use modelling to demonstrate:

a) the rearrangement of atoms to form new substances

A chemical change involves breaking chemical bonds and rearranging the atoms in a different order. When we pass electricity through water we can separate it into hydrogen gas and oxygen gas. This is a complete rearrangement of the molecules, hence a chemical change.

Chemical changes are generally not easily reversed, whereas many physical changes are easily reversed. Take ice out of the freezer: it will turn into water. To turn it back into a solid: just put it back in the freezer.

Electrolysis is a chemical change: the decomposition of the compound water into its constituent elements, hydrogen and oxygen. The bubbles are hydrogen gas and oxygen gas and there will be twice as much hydrogen gas as oxygen released.

VIDEOS:

• RC2 Modelling chemical reactions https://www.youtube.com/watch?v=NWaSvk_FdWU&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9&index=7 5.31

1.2 use modelling to demonstrate:

b) the conservation of atoms in a chemical reaction

As a chemical reaction takes place, atoms in the reactants are rearranged to make the products. No atoms are added or taken away. This is called the conservation of atoms. As the atoms are conserved and all atoms have mass, the mass of the reactants will be the same as the mass of the products. This is known as conservation of mass.

You will carry out a practical modelling task. For each reaction, count the number of atoms of each element in the reactants, and in the products.

VIDEOS:

• RC3 Modelling balanced chemical equations https://www.youtube.com/watch?v=5SIzjqs1Xmo&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9&index=6 7.46

• Revision: How to draw Lewis structures: https://www.youtube.com/watch?v=1ZlnzyHahvo&feature=youtu.be

1.3 conduct investigations to predict and identify the products of a range of reactions:

a) synthesis

- define as building a more complex product from elements or smaller reactants

- identify one common application (eg Haber synthesis of ammonia)

- write balanced species neutral equations

A synthesis reaction involves the formation of a compound from its elements or from other, usually smaller, compounds. (Woollett, et al., 2018)

Synthesis is a term which has two separated, but related meanings in Chemistry. An ester is a chemical substance which can be synthesised in the laboratory, from an alcohol and an acid. Esters have a distinctive smell and are used as food flavourings and perfumes. They are complex chemicals made from two reactants. Plastics (polymers) are also synthesised by chemists. These are complex, long chain or branched molecules synthesised from a large number of smaller, monomer units.

However there are more simpler synthesis reactions. The chemical reaction between the elements magnesium and oxygen produces the compound magnesium oxide. This too is a synthesis reaction.

VIDEOS:

• RC4 Synthesis reactions https://www.youtube.com/watch?v=7dquEFoWm5Q&index=1&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9 5.00

1.3 conduct investigations to predict and identify the products of a range of reactions:

b) decomposition

- define reaction type as breaking down a reactant into two or more elements or simpler compounds

- identify one common application (eg discovery of oxygen)

- write balanced species neutral equations

A decomposition reaction involves the breakdown of a single reactant (compound) into two or more products, which may be both elements or a combination of elements and/or compounds.

Energy is needed for decomposition reactions and can be supplied in one of three ways: thermal (heat), electrical (electrolysis), light (photolysis).

VIDEOS:

• RC5 Decomposition reactions https://www.youtube.com/watch?v=O-mhVj1Yz0M&index=3&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9 5.49

1.3 conduct investigations to predict and identify the products of a range of reactions:

c) combustion

- define reaction type as combination with oxygen to form oxides

- identify some common applications (eg petrol engines, carbon monoxide poisoning)

- write balanced species neutral equations

One important reaction involving hydrocarbons is their combustion reactions where they are important fuels. They all burn (when there is sufficient air or oxygen) to produce carbon dioxide and water. eg propane:

C₃H_8(l) + 5O_2(g) → 3CO_2(g) + 4H₂O_(l)

Combustion is an exothermic chemical reaction. There is a net release of energy.

Combustion reactions require the presence of oxygen. Combustion may still occur in a low oxygen environment, however the products of the reaction will be different.

A Bunsen burner shows the differences between complete and incomplete combustion.

1. Collar closed: When the Bunsen burner hole is closed, no oxygen can enter the tube. This means the only fuel-air mixture occurs at the top of the tube. In this limited supply of oxygen, we have incomplete combustion, and black soot (carbon) is one of the products. This occurs at a relatively low temperature. If we assume the only fuel present is methane, then the formula is:

CH_4(g) + O_2(g) → C_(s) + 2H₂O_(g) deltaH = -498 kJ/mol

2. Collar half open: When the Bunsen burner hole is half open, a limited amount of oxygen can enter the tube. This means the fuel-air mixture begins inside the tube. However, this is still a limited supply of oxygen, and we again have incomplete combustion. This time, one of the products is carbon monoxide. This occurs at a hotter temperature than with the collar closed.

2CH_4(g) + 3O_2(g) → 2CO_(g) + 4H₂O_(g) deltaH = -609 kJ/mol

3. When the Bunsen burner hole is open, excess oxygen can enter the tube. This means the fuel-air mixture is rich in oxygen. In this excess supply of oxygen, we have complete combustion, and the products are carbon dioxide and water. This is the most efficient type of combustion and takes place at the highest temperatures.

CH_4(g) + 2O_2(g) → CO_2(g) + 2H₂O_(g) deltaH = -892 kJ/mol

One way of avoiding the carbon and carbon monoxide byproducts is to carry out combustion in a plentiful supply of oxygen.

Volatile, low molecular weight fuels, eg petrol, readily vapourise and will combust when sparked. The products of combustion will depend on the air:fuel ratio. Modern fuel injection systems are electronically controlled to ensure an optimum mix for maximum power and minimum pollution. Catalytic converters further reduce carbon monoxide emissions by converting the CO into CO₂ before it leaves the car’s exhaust.

1 mole of octane produces 5470kJ/mol with complete combustion, or 3253kJ/mol for incomplete combustion.

VIDEOS:

• RC6 Combustion reactions https://www.youtube.com/watch?v=0aBw6aJ-5Ko&index=5&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9 6.23

1.3 conduct investigations to predict and identify the products of a range of reactions:

d) precipitation

- define reaction type as reaction of two solutions forming an insoluble compound

- identify one common application (eg limescale in kettles)

- predict precipitates based on solubility rules

- write balanced species neutral equations

- write balanced ionic equations

A solution is a mixture of ions and water molecules. The ions move freely in the water as they are more attracted to the water molecules than they are to their oppositely charged ions. When two solutions are mixed, the four ionic species may co-exist in solution and remain soluble.

However in some cases, a precipitate may form. A precipitate forms when the attraction between two oppositely charged ions is greater than the attraction between the individual ion and the water molecules.

In this case, the oppositely charged ions will ionically bond to each other and precipitate out of the solution as an insoluble solid.

Ba(NO₃)_2(aq) + CuSO_4(aq) → Cu(NO₃)_2(aq) + BaSO_4(s) (white ppte)

CuCl_2(aq) + 2NaOH_(aq) → 2NaCl_(aq) + Cu(OH)_2(s) (blue ppte)

2KI_(aq) + Pb(NO₃)_2(aq) → 2KNO_3(aq) + PbI_2(s) (yellow ppte)

When small amounts of a precipitate form, it may not settle at the bottom of the solution but remain in suspension. This can make the solution appear cloudy, eg CO₂ can make limewater (calcium hydroxide) milky due to the formation of calcium carbonate as a suspension.

VIDEOS:

• RC7 Precipitation reactions https://www.youtube.com/watch?v=uV6oaeuPk6k&index=4&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9 5.20

1.3 conduct investigations to predict and identify the products of a range of reactions:

e) acid-base reactions:

- identify some common applications ( insect bites/stings)

- write balanced species neutral equations

- write balanced ionic equations

A substance which provides H⁺ ions in aqueous solutions (or a compound with hydrogen which ionises water producing hydronium ions) is termed an acid. The excess [H⁺] gives the solution its “acidic” properties. Acids conduct electricity in solution and have a sour taste.

eg. H₂SO_4(aq) → 2H⁺_(aq) + SO₄^2-_(aq)

A substance which produces hydroxide ions in aqueous solutions is called a base. The excess [OH^-] gives the solution its “basic” properties. Bases which are soluble in water are called alkalis. Alkalis have a soapy feel, a bitter tast and conduct electricity in solution.

eg. Ca(OH)_2(aq) → Ca²⁺_(aq) + 2OH^-_(aq)

The solutions reacted to demonstrate neutralisation are usually of a strong acid, such as hydrochloric acid, and a strong base, such as sodium hydroxide. The general equation is:

Acid + base → salt + water

eg HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

The net ionic equation for a neutralisation reaction is:

Whilst we can think of acids as containing H⁺ ions, in fact the H⁺ bonds to a water molecule to form a hydronium ion H₃O⁺.

All neutralisations are exothermic.

When the heat of neutralisation is measured for a range of strong acids and strong bases, the amount of heat released is always approximately 57 kJ per mole of water formed.

A study of the heat energy released for various (strong) acid-base reactions reveal values which are very close to one another, suggesting that the same type of reaction is occurring each time, ie. the production of a salt and water.

POWERPOINT:

Naming acids (Loaded below, or available from https://drive.google.com/open?id=1HUywQJ70Hgwf_7JOFlfLXqNR7nO821cL )

VIDEOS:

• RC8 Acid-base reactions https://www.youtube.com/watch?v=iFXvEAb2dK8&index=11&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9 4.52

1.3 conduct investigations to predict and identify the products of a range of reactions:

f) acid + carbonate reactions

- identify some common applications (baking)

- write balanced species neutral equations

- write balanced ionic equations

Carbonates are a group of ionic salts containing the polyatomic ion CO₃^2-. Common carbonates include calcium carbonate (limestone or marble), sodium carbonate (soda ash), or copper carbonate. Carbonates are basic salts as they can be used to neutralise acids.

When a carbonate reacts with an acid, water and carbon dioxide gas are formed. A salt is also formed. The general equation is:

Acid + carbonate → carbon dioxide + water + a salt

eg HCl_(aq) + Na₂CO_3(aq) →

The same effect is seen with hydrogen carbonates (or bicarbonates), eg bicarbonate of soda (NaHCO₃)

eg HCl_(aq) + NaHCO_3(aq) →

VIDEOS:

• RC9 Acid-carbonate reactions https://www.youtube.com/watch?v=hZY1oUtsSmk&index=10&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9 5.19

VIDEOS:

• Classification of reactions https://www.youtube.com/watch?v=ZWxhjnyUD84 7.10

1.4 investigate the chemical processes that occur when Aboriginal and Torres Strait Islander Peoples detoxify poisonous food items

Chemistry and chemical processes are an old science and the Indigenous Australians have been practicing Chemistry for many thousands of years. The early Australians applied their knowledge of chemistry to the production of food, bush medicines and dyes.

Not all fruits are safe to eat. Aboriginal and Torres Strait Islanders took advantage of their knowledge of chemistry to detoxify foods and make them safe for consumption. One example is the cycad palms in N- and NE- Australia.

The fruit is cut and placed in mesh bags which are then soaked in water for several days. This allows time for toxic chemicals to leach out of the fruit. Insoluble starchy compounds remain after this process which are safe for consumption. Variations on this method include heating the fruit first to reduce the leaching time or even immersion in water to encourage natural fermentation.

Once treated, the seeds can be washed and used as a ground meal for producing a flat bread. (Smith & Davis, 2017)

The Indigenous Australians have also used these techniques to detoxify nuts.

VIDEOS:

• RC10 Detoxifying foods https://www.youtube.com/watch?v=GDSm74Y71Po&index=9&list=PLeFSFSJ9WqSAJfb02g1rwALZbMuRTFaf9 5.44