Revision

• What is the symbol for slightly negative?

• Can non polar molecules have polar bonds?

• What are polar molecules?

• What are the three types of intermolecular forces?Describe each

• Dipole Dipole Interactions, Hydrogen Bonding and Dispersion Forces

Water Cycle

Water Cycle

• The water cycle describes the continuous movement of water on, above, and below the surface of the Earth. Water simply changes state between solid, liquid and gas

• The water we have on earth now has always been here (although some individual water molecules can come and go).

• x

Draw the water cycle on pg 189• Precipitation - water vapour gas becomes a liquid

or solid and falls to earth (rain and snow!)• Evaporation - liquid water becomes a gas phases

as it moves from the ground or bodies of water into the overlying atmosphere

• Transpiration - is the evaporation of water from the aerial parts of plants, especially leaves but also the stem

• Photosynthesis/Respiration• Combustion CHx + O2 H2O + CO2

• Ground Flow• Run off - The variety of ways by which water moves

across the land.

Water and living things - Photosynthesis

What is photosynthesis?• Photosynthesis is the process by which plants, use

the energy from sunlight to produce sugar, which respiration converts into ATP (Adenosine Triphosphate), the "fuel" used by all living things.

Where does photosynthesis occur?• In chloroplasts which are organelles in plant cells

Why does photosynthesis occur in chloroplasts?• Because chloroplasts have chlorophyll which absorbs

light and helps the reactions take place.

Photosynthesis• What are the reactants in photosynthesis?

• Carbon dioxide and water

• What are the products of photosynthesis?

• Glucose and Oxygen

carbon dioxide + water + light energy → glucose + oxygen + water

6 CO2(g) + 12 H2O(l) + photons → C6H12O6(aq) + 6 O2(g) + 6 H2O(l)

Respiration• Respiration occurs in both plant and animal

cells

• All organisms need glucose for energy, plants make glucose and animals eat to get glucose

• But cells cant use glucose so respiration converts glucose and oxygen to ATP (Adenosine Triphosphate) , a type of chemical energy that can be used by the body

Respiration

• Respiration is the process or chemical reaction which converts glucose into an energy form that can be used by

• It occurs in the mitochondriaC6H12O6(aq) + 6 O2(g) → 6 CO2(g) + 12 H2O(l) + energy (ATP)

glucose + oxygen + water carbon dioxide + water + light energy

Other important roles of water in the body

• Water transports soluble wastes around the body.

• Water provides a system to transfer heat. • Water provides a system to cool the body

(Sweat).

Properties of Water

• Water is a very common liquid and that is because it has special chemical properties.

• High mp and bp

Property Explanation

High m.p and b.p

Strong hydrogen bonds between water molecules (intermolecular bonds).

High latent heat values

refers to amount of energy required to change the state of a given substance.

More energy is needed to change the state (melt or boil) due to strong hydrogen bonds.

High Heat capacity

is the capacity of a substance to store heat.

There are strong hydrogen bonds between water molecules – it requires more heat energy to break the intermolecular bonds.

Heat Capacity

• Amount of energy required to raise the temperature of a substance of 1g by 1oC.

• Water has a high heat capacity- it take more energy to heat up 1g of water than 1g of anything else (eg. sand)

• The heat capacity of the oceans is very important in keeping the Earth's temperature relatively stable.

Water in three states

• x

What happens to most liquids solid

• Most liquids have a quite simple behavior when they are cooled they shrink. The liquid contracts as it is cooled

• Why? because the molecules are moving slower they are less able to overcome the attractive intermolecular forces drawing them closer to each other.

What happens to water when it freezes?

• Water expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%.

• There is a strong tendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between two oxygen atoms.

• On freezing, hydrogen bonds of water take on an open crystalline structure and therefore making water less dense.

Liquid

Solid (more space between molecules)

What happens to water when it freezes?• The hydrogen bonding gets stronger as the

temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position).

• The ice structure is completely hydrogen bonded, and these bonds force the crystalline structure to be very "open“.

On freezing, the hydrogen bonds of water take on an open crystalline structure therefore making water less dense.

Solutions

Solute, Solvent and Solution • Solvent: a liquid or gas that dissolves a solid,

liquid, or gaseous solute, resulting in a solution.

• Solute: is dissolved in another substance, known as a solvent.

• Solution: is a mixture composed of two or more substances a solute and a solvent.

• Solvent + Solute = Solution

A homogeneous mixture is a mixture that has the same appearance and composition throughout the whole thing. They are also referred to as solutions.

Heterogeneous mixture is a substance that is visibly made of different types of things.

To be classified as a solution it must be:

1. Homogenous – solvent and solute distributed evenly throughout solution.

2. Transparent in visible light.

3. Solute particles must be too small to be seen.

What happens when something dissolves

• When one substance dissolves into another, the following processes occur:

1. The particles of the solute are separated from one another.

2. The particles of the solvent are separated from one another.

2. The solvent and solute particles attract each other.

Why do things dissolve?• The properties of solutions are dominated

by the intermolecular forces between solute and solvent.

• A solute will dissolve if the attraction between the solute and solvent particles is strong enough to compete with the solute-solute and solvent-solvent forces of attraction.

• All substances that dissolve in water have ionic or polar covalent bonding.

Like dissolves Like• The newly formed intermolecular forces

between molecules of solute and solvent must be greater than those between just the solute molecules or just the solvent molecules.

• Thus the term "like dissolves like": materials with the same or similar intermolecular forces will mix.

• If the forces are different, the materials will not mix.

Similar Intermolecular Forces• For example, a polar substance such as

glucose (C6H12O6) will dissolve in water. The dipole attractions between molecules of glucose are easily over come by the hydrogen bonds in water.

Dissimilar intermolecular forces

• If the solute and the solvent do not have similar intermolecular forces, the two materials will not interact with each other and a solution will not form.

Example - Hexane and water• The weak dispersion forces

between the non-polar solute (hexane) molecules are not strong enough to over come the powerful hydrogen bonds in water.

• The non-polar hexane molecules and the polar water molecules cannot interact with each other.

• Thus no solution forms. • If they are mixed, they will

separate into different layers fairly quickly

If two things form a solution what can you say about their

intermolecular forces?• x

Miscible and Immiscible• Miscible – when two liquids are placed in the

same container, mix and do not form separate layers.

• Immiscible – when two liquids placed in same container do not mix, but form separate layers

Examples of things that dissolve

1. Polar covalent compounds that can form hydrogen bonds

• The hydrogen bonds between the molecules of the substance and between the water molecules are broken.

• Hydrogen bonds are formed between the water molecules and the molecules of the substance.

2. Polar covalent molecular compounds that ionise

• Polar covalent bonds within the molecules are broken, producing hydrogen ions and anions.

• A covalent bond forms between each + and 𝐻an 𝐻2 molecule, giving 𝑂 𝐻3 + ions. Ion-dipole 𝑂attractions between the newly formed ions and the polar water molecules are formed.

• HCl (l) + H2O(l) H30+(aq) + Cl-(aq)

• Reaction is called an ionisation reaction.

3. Ionic compounds• Ionic bonds within the solid are broken, + and -

ions.

• Hydrogen bonds between water molecules are broken.

• In water the H+ is attracted to the – anion

• The O2- is attracted to the + cation

• This is called an ion- dipole attraction

• Ion-dipole attractions form between the ions and the polar water molecules.

Dissociation reaction• When an ionic compound dissolves in water,

producing separate cations and anions. It is called a dissociation reaction.

• The ions are dissociated from the lattice

Example

NaCl(s) + H2O(aq) Na+(aq) + Cl-(aq)

• Not all ionic compounds are water soluble. • Substances solubility/insolubility can be

determined using a solubility table.