CHEMICAL CHANGES IN MATTER

INDEX OF THE UNIT :

1. CHEMICAL CHANGES IN MATTER

2. CHEMICAL REACTIONS:

- REACTANTS AND PRODUCTS

- CHEMICAL EQUATIONS

3. THE PROPERTIES OF CHEMICAL REACTIONS

4. THE CHEMICAL RELATIONSHIP AND SOCIETY

- NATURAL AND SYNTHETIC PRODUCTS

- CHEMICAL IMPROVES OUR QUALITY OF LIFE

5. CHEMISTRY AND ENVIRONMENT: ENVIRONMETAL PROBLEMS:

- GLOBAL WARMING, THE GREENHOUSE EFFECT

- THE DESTRUCTION OF THE OZONE LAYER

- ACID RAIN

By the end of this lesson...

... you will find out how to identify chemical and physical changes.

... you will understand what a chemical reaction is.

... you will be able to balance chemical reactions.

... you will discover the importance of Chemistry and chemical products in society.

... you will know what the environmental problems of the atmosphere are.

... you will be able to describe what greenhouse gases are.

... you will be able to describe some of the consequences of scientific and tecnological advances in environment.

1. CHEMICAL CHANGES IN MATTER

Can you remember what we said about natural Sciences? Tell me what was the difference between a material system and a body?

Physics and Chemistry are scientific disciplines that have the purpose of explaining the world in which we live. Both focus their study on portions of matter called: material systems (they are not well-defined, such as atmosphere) and bodies. Matter can undergo two types of changes:

PHYSICAL CHANGES OR CHEMICAL CHANGES?

Brainstorm words for talking about CHANGES

A chemical change produces new substances that did not exist before the change. Chemical changes are those in which the substances at the end differ from those at the beginning.

However, physical changes are those in which after the change, we still have the same substance. A physical change doesn't produce new substances.

Can you remember what phenomenon took place when we put together vinegar and baking soda in the erlenmeyer flask?

To identify whether a chemical change is taking place, we must pay attention to the following phenomena that help us recognise them:

- Gas release (such as when vinegar and sodium bicarbonate come into contact). Bubbling is a clue.

- Colour change (as in rusting)

- Thermal energy exchange (as generate by placing a cold compress on a bruise).

- Energy release as light (a phenomenon observed at the fireflies which can generate light).

New subtances that appear may be found in a different state of aggregation or have a different colour.

Write down as many phenomena as you can about recognising chemical changes.

What can you tell about solutions?

Disolving processes are NOT chemical changes

It  is just a mixture at molecular scale of pre-existing substances.

What is one of the causes of phase changes? Do you know another one?
Does anybody remember if the water molecules change during a phase change?

Phase changes are not chemical changes

ACTIVITY:

Specify in your notebook if a chemical or a physical change ocurrs 

in the following phenomena:

• SOLUTION OF SUGAR IN WATER
• RUST FORMATION ON A PIECE OF IRON
• FRUIT MADURATION
• GASOLINE COMBUSTION IN A CAR ENGINE
• CANDLE MELTING
• TOAST CRÊPES IN A PAN
• BREATHE
• THE BREAK OF A CHALK
• OBTAINING COPPER FROM COPPER OXIDE
• STALACTITES FORMATION
• MAKE A MILKSHAKE AND FRUITS.
• THE BREAKAGE OF A GLASS BOTTLE
• BURN A PIECE OF COAL
• RAINBOW FORMATION
• MIXING BUTTER AND SUGAR
• PERFUME DIFFUSION
• TEARING A PIECE OF PAPER
• HEATING SOAP ON A STOVE
• BURNING PAPER
• MAKING ICE CUBE
• A RUSTED SCREW
• DRYING CLOTHES OUTDOORS
• MIXING WATER AND OIL IN A GLASS
• TURNING GRAPES INTO WINE
• COOKING EGGS
• CLOUD FORMATION

2) Research how to protect windows bars from corrosion.

2. CHEMICAL REACTIONS:

Chemical changes: The chemical reaction

2.1. Reactants and products.

In chemical processes, the starting pure substances (simples or compounds) disappear and new substances are formed. The molecules are broken, the atoms separate and come together in another way forming new molecules. 
A chemical reaction ocurrs when the starting substances, called REACTANTS, transform into other new substances called PRODUCTS. 

A combution reaction ("burning") is a major class of chemical reaction. An example is the combustion of methane. 

This is the representation of propane combustion:

In spanish, Una reacción química

As you can see, in all the combustion reaction:

- Oxygen (O2) is always needed 

- Gases such as carbon dioxide (CO2) and water (H2O) are always released

Therefore, in a chemical reaction a rearrangement of the atoms takes place: THE ATOMS DO NOT CHANGE, THE MOLECULES YES.

 __________ CHANGE

 __________ CHANGE

Repaso y actividades on line: Se considera cambio físico.... y químico.... 

Modelo de esferas

Se utilizan para representar una reacción química (al igual que las ecuaciones químicas). Cada clase de átomo se representa con una esfera, de igual tamaño y color. Podemos explicar la ley de la conservación de la masa (Lavoisier), ya que el número de átomos de cada clase permanece constante. En una reacción química hay el mismo número de átomos tanto en los reactivos como en los productos. 

Ejemplos de reacciones químicas

COLLISION THEORY (Teoría de las colisiones)

Collision theory explains how chemical reactions ocurr. This theory states that particles of the reactants hit each other in order to break the preexisting bonds and form all new bonds forming products. To achieve succesful collisions it is necessary that the molecules collide with each other with sufficient energy (activation energy) and with the proper orientation.

Many more successful collisions are brought up by increasing the concentration of the reactant particles or raising the temperature.

2.2. Chemical equations

A chemical equation is a symbolic description of a chemical reaction. In a chemical equation, we write the reactants' chemical formulas on the left and the substances that are obtained (products ) on the right, separated by an arrow.  If there is more than one reactant or product, we write the "+" sign between them.
Sometimes the physical states of the substances (gaseous (g), liquid (l), aqueous (aq), solid (s)) is indicated.

Each chemical formula is preceded by a number (stoichiometric coefficient) which indicates the number of molecules of each substance involved in the reaction.

We have to check that the number of atoms is equal for each chemical compounds in the reactants and products. We need to balance chemical equations if that is not the case.

BALANCING CHEMICAL EQUATIONS

Según el ejemplo anterior, representa con el modelo de las esferas las siguientes reacciones químicas. Nombra los reactivos y los productos que conozcas. Recuerda que las ecuaciones químicas tienen que estar ajustadas.

REACCIONES QUÍMICAS
REACTIVOS	PRODUCTOS
Electrólisis del agua:                      (l)             →               (g)  +         (g)
NH3 (aq) →   N2 (g) +  H2 (g)
Zn (s)   +   HCl (aq)  →     ZnCl2 (s)   +    H2 (g)
NO(g)     +    O2 (g) →     NO2 (g)
2Al + 6HCl (g) → 2AlCl3 (g)  + 3H2 (g)
C3H8 (g)    +     O2  (g)  →    CO2 (g)   +    H2O (l)
Ca (s)   +    O2  (g) →    CaO (s)

 Para seguir aprendiendo más sobre AJUSTES DE REACCIONES QUÍMICAS y MÁS EJERCICIOS

3. THE PROPERTIES OF CHEMICAL REACTIONS

THE LAW OF CONSERVATION OF MASS (ANTOINE LAVOISIER, 1789)

In a chemical reaction, matter is neither created nor destroyed, only transformed. 

So the sum of the masses of the reactants is equal to the sum of the masses of the products of the reaction.

THE LAW OF DEFINITE PROPORTIONS (Proust, 1795):

The masses of the reactants and products maintain a constant proportion during chemical reaction.

THE RATE OF A CHEMICAL CHANGE:

We can establish the rate at which a chemical reaction ocurrs by measuring the time it takes for the products of the reaction to appear or for the reactants to disappear.

Several factors affect the rate of a chemical reaction:

- Temperature: Increasing the temperature increases the rate of a chemical reaction.

- Concentration of the reactants: The greater the concentration of the reactants, the faster the rate of reaction.

4. THE CHEMICAL RELATIONSHIP AND SOCIETY

- NATURAL AND SYNTHETIC PRODUCTS

- CHEMICAL IMPROVES OUR QUALITY OF LIFE

Natural reactions occur naturally in living organisms (E.g. The production of proteins). However, the substances that are necessary for society can also be generated artificially. We can divide the chemical industry in different sub-industries:

a) Phamaceutials. It is one of the most important economics indicator of a country.
b) Petrochemicals. It is essential for obtaining many of our everyday products.
c) Other indutries which are involved in the manufacture of countless products. E.g: Textile industry, Automotive industry, Food industry, etc.

EXERCISES:

1. A wide variety of chemical products that help us in everyday life are obtained through artificial synthesis reactions. Can you give a few examples?

2. Are they natural or artificial products?

a) Mint leaves   b) Synthetic plastics   c) Lycra    d) Cotton    e) Wood

3. What is the difference between Environmetal chemistry and Sustainable chemistry?

4. Analyse the pictures about traditional and sustainable industrial design:

Do you think there are still factories like the one in the first picture?  What problems do they generated? If we haven't change that industry design, what would happened? What are the effects on the environment? What do you think why the concept of SUSTAINABLE DEVELOPMENT is important?

5. CHEMISTRY AND ENVIRONMENT: ENVIRONMETAL PROBLEMS