🧪Chemistry⚛️ year 10

Physical change

A change in matter that does not form new substances.

Examples of physical changes

Change in

• Position

• Shape

• Size

• State

Chemical change/reaction

A change in matter that forms one or more new substances.

Examples of chemical changes

• release of light or sound

• Formation of a new gas

• Change in colour

• Disappearance of a solid

• Formation of a new solid

• Change in temperature

Atom

The smallest particle of an element

Molecule

A group of atoms bonded together

Fixed formula, e.g. O2, H2O

Chemical bond

An attractive force that holds two atoms together

Mixture

A combination of substances that can be physically seperated.

Element

Any of the basic substances on the periodic table, such as oxygen (O₂), iron (Fe) and gold (Au).

Compound

Made up of two or more different types of atoms bonded together, such as carbon dioxide (CO₂) and sodium chloride (NaCl).

Chemical equation

Reactants—>Products

e.g. sodium + oxygen → sodium oxide

Reactants

The substances that react with each other

Products

The new substances formed by a reaction

The re-arrangement of atoms

• During a chemical reaction, some of the chemical bonds between atoms are broken and new bonds are formed.

• This re-arrangement of atoms is what produces a new substance.

• The same elements are present after a reaction – they're just arranged in a new way.

Metal atom

1, 2, or 3 electrons in outer shell

Non-metal atom

5, 6 or 7 electrons in outer shell

Lattice

Continuous arrangements of bonded atoms in regular patterns.

• Ratio of elements, e.g. NaCl, Au

Why do elements bond together?

Atoms form chemical bonds to obtain full valence shells.

• By bonding together in chemical reactions, atoms can reach a more stable state.

Ions

Charged particles formed when atoms either lose or gain electrons

Cations

Positively charged ions formed by the loss of electrons

Anions

Negatively charged ions formed by the gain of electrons

Ionic bonds

• The transfer of electrons from one atom to another results in two ions with opposite charges. The attraction between these opposite charges is what makes an ionic bond.

• Occurs between metals and non metals

• Metal cations

• Non-metal anions

Predicting ionic compounds

E.g: Aluminium + Chloride

Aluminium- Metal, charge of 3+

Chloride- Non-metal, charge of 1-

This means you need three negative charges to balance out the positive charge of three.

(Al³⁺)+ (Cl⁻)+ (Cl⁻)+ (Cl⁻)

= AlCl₃

• metals are always written before non-metals

Metallic bonding

• When metals bond with metals, they donate their valence electrons into a common pool.

• This results in metal cations floating on a sea of delocalised electrons

• The positively charged metal ions (cations) are held together by the attraction to this sea of delocalized electrons.

Properties of metals explained by metallic bonding- Electrical Conductivity

Delocalised electrons can move freely through the metal- a flow of electrons is an electric current

Properties of metals explained by metallic bonding- Heat Conductivity

The delocalised electrons carry thermal energy (heat) through the metal quickly and easily.

Properties of metals explained by metallic bonding- Shine

Delocalised electrons move quickly so that light can reflect off all surfaces of the metal.

Covalent bonding

When non-metals bond with non-metals, they share a pair of electrons between atoms.

• Each atom contributes one or more electrons to form a shared pair.

• One pair represents two total electrons

• This sharing allows both atoms to achieve a stable electron configuration (a full outer shell).

How to represent covalent bonding

F-F, H-H, H-O-H

• The number of lines between the element symbols represent the number of bonds

Bonds of common elements

H, F, Cl- single bond ( two electrons are shared)

O- double bond(four electrons are shared)

N- triple bond( six electrons are shared)

C- quadruple bond( 8 electrons are shared)

Law of Conservation of Matter

• In a chemical reaction, atoms are not created or destroyed.

• Total number of atoms of each element remains the same before and after the reaction.

• The bonds of the atoms are just rearranged in different ways, forming new substances.

Unbalanced Equations

• An equation is unbalanced if the number of atoms of each element is not the same on both sides.

Example (Unbalanced Thermite Reaction):
Fe₂O₃ + Al → Fe + Al₂O₃

• The number of Fe and Al atoms do not match.

Balancing equations

An equation is balanced when the number of each type of atom is the same on both sides.

To balance an equation:

• Adjust the numbers in front of chemical formulas (coefficients).

Example (Balanced Thermite Reaction):
Fe₂O₃ + 2Al → 2Fe + Al₂O₃

Subscripts and coefficients in chemical equations

• When balancing an equation, you can’t change the subscript numbers, as this shows how the molecule is naturally formed.

• You can only adjust the coefficients, which change the number of elements or compounds in a chemical equation.

• To find the amount of each type of atom, multiply the coefficient by the subscript

How to balance equations

Composition reactions

• Two or more reactants combine to form a single product.

• General form: A + B → AB

• Examples: N₂+3H₂→2NH₃, 2Mg+O₂→2MgO

multiple reactants, single product

Decomposition reactions

• One reactant breaking down into multiple products

• General form: AB→ A + B

• Examples: 2H₂O→2H₂+O₂, CaCo₃→ CaO+CO₂

Single reactant, multiple products

Displacement reaction

• One element replaces another in a compound

• General form: AB+C→BC+A

• Examples Zn+CuSO₄→ZnSO₄+Cu, Cu + AgSO₄→2Ag+CuSO₄

One metal dissolving, another coming out of solution

Often two metals involved

Neutralisation reaction

• Acids and bases reacting together

• General form: Acid + base→water + salt

• Examples: HCl+ NaOH→H₂O+NaCl, H₂SO₄+Mg(OH)₂→2H₂O + MgSO₄

Acid, base

Precipitation reactions

• Two solutions are mixed and a solid forms

• General form: AB+CD→ AD+CB (one product is solid)

• Examples: Pb(NO₃)+2KI→ PbI₂+2KNO₃

Solid forms from mixing solutions

Three main types of chemical bonding

• metal + metal → metallic bonding

• metal + non-metal → ionic bonding

• non-metal + non-metal → covalent bonding

Collision theory

• The idea that the re-arrangement of atoms requires collisions between the reactant particles.

• It's only when the particles are in contact that new bonds can form to make the products.

What a successful collision involves

Particles colliding with both

• the right orientation, and

• enough energy, or speed, to break their bonds

If both conditions aren't met then the particles will just bounce off each other without reacting.

Rate of a reaction

• How quickly reactants are converted into products.

• The higher the frequency of successful collisions, the higher the reaction rate.

What you need to increase the rate of a chemical reaction:

• surface area

• temperature

• concentration

Surface area

• Surface area is the total outside area of a three dimensional solid.

• As a solid is broken into smaller prices, the surface area increases.

• When the surface area is increased, the chance of reactant particles colliding increases, so there are more reactions and the reaction is faster.

Temperature

• When the temperature is increased, the average kinetic (movement) energy of the particles is increased, so particles move faster.

• More collisions occur, and more particles have enough energy to react. This means there are more reactions and the reaction is faster.

Concentration

• Concentration is how many particles there are per unit volume.

• When the concentration is increased, the chance of reactant particles colliding increases, so there are more reactions and the reaction is faster.