Spectroscopy continued.

·         The Absorption Spectra looks at the frequencies absorbed by an element when light is passed through it

Light  à Gas à Less light

·         The Emission Spectra looks at the light given off when the element is exited

Excited element (heated up) à light is emitted. (Bohr’s theory)

·         Wave theory and particle theory are linked by Planck’s constant.

Bohr’s theory

Bohr stated that electrons start in their normal shell (Ground level or level 1: closest to the nucleus) but the atom still has outer shells (level 2, 3 …etc)

When the electron gains energy, the electron moves + shell, depending on the amount of energy (Which is known as a quantum: the energy required for an electron to jump up to the next level)

When the electron is in a higher level, it’s known to be in an ‘excited state’. However, the electron can’t stay in the excited state after the energy is given off (so it moves back to ground level)

Bohr:  As the electron jumps up a quantum, it absorbs light, which is why we get an absorption spectra. As it jumps back down, it emits the light it absorbed.

Bohr:  As you go towards outer shells, the layers get closer get closer to each another. Which is significant as it justifies why the frequency given off is different as the quantum gets smaller.

The energy of the photon is equal to the quantum. Knowing that, we can calculate frequency.

Both the photon value and the quantum are variables according to the level which the electron is at.

Petrol Components

Petrol is a mixture of:

Alkanes (Aliphatic/saturated) which are often in the three forms of

Straight chains (Typical hydrocarbons such as Ethane)

 Branched chains (Hydrocarbons with extra bonds such as 2-Methyl butane)

 Cyclic chains (Hydrocarbons in a circular structure such as Cyclohexane).

A few Alkenes such as the aromatic Benzene C₆H₆

(Aromatic Hydrocarbons: Hydrocarbons characterized by general alternating double and single bonds between carbons)

Alcohol

Which is increasing in amounts as a component in fuel over the years.

e.g. Ethanol and Ethers

(Ethers are 2 alkenes connected by an Oxygen [such as Ethoxyethane])

Petrol is/needs to be a blend of volatilities, and to burn well, i.e. not produce a lot of carbon monoxide.

Alcohols and ethers are known as oxygenates as they contain Oxygen.

Ethanol can be produced from fermentation, so it’s a biofuel.

Cons: Ethanol and ethers produce less energy per unit. (Low energy density) and that Ethanol is hydroscopic (Absorbs water) which may cause rust in vital components i.e. engine.

Spectroscopy

Recap on related topics:

-    The theory of star formation is a model developed by observing different stars and realising that each is at a different stage.

-          In outer ‘space’ there’s roughly 1 atom present per cm³

-          Dense gas Clouds take place between stars and consists of plasma, nuclei, electrons, ions and dust of other stars that combusted due to Super Novas.

-          In Dense gas clouds, particles have low kinetic energy, which promotes gravitational forces to hold the molecules together. This eventually results in parts of the clouds compressing.

-          Nuclear fusion is a nuclear reaction – it is when nuclei join.

Overview:

Spectroscopy is the study of light and matter.

We use two models to describe light.

1-      The Particle Model

This was deviced by Alvert Einstein. He suggested that light is a stream of photon packages, and that the energy of the photons is relative to the position of the light in the electromagnetic spectrum.

Max Planck suggested that E = h v

Where E is the energy of a photon.

V is the frequency of the light.

h is the “Planck constant” which is equal to 6.63 x 10^-34

2-      The Wave Model

The wave model centres around the idea that light, as a part of the electromagnetic spectrum has a wave with specific characteristics (Wave length and frequency).

n.b. Speed of light (Symbol c) is equal to 3.00 x 10⁸ ms^-1

        Wave length is symbol λ

        Freqncy is symbol v

c =  λ v

Hess's Law!

Hess’s Law

“The overall enthalpy change of a reaction is independent of the route taken.”

Example question:

Given that Δ H ^θ _combustion of water is -286 KJ mol^-1

Given that Δ H ^θ _combustion of Carbon dioxide is -394 KJ mol^-1

Given that Δ H ^θ _combustion of Methanol is -726 KJ mol^-1

Calculate the Δ H ^θ _formation of Methanol.

Step 1 – Balance the equation.

C + H₂ >> C₈H₁₈

C + 2 H₂ >> C₈H₁₈

Step 2 – Make the route diagram and label the Δ H’s

Step 3 – realise the route.

In this case, we want Δ H4+ Δ H3 which is equal to Δ H1 + Δ H2

This is because, regardless of what route we take, we end up getting the same product, and therefore the enthalpy change will be the same.

Step 4 – Substitute the values

Δ H1 + Δ H2 = Δ H3 + Δ H4

-394 + (2x-286) = Δ H1 + -726

-966 = Δ H3 – 726

- make the wanted delta H the subject of the formulae, and add the unit. -

Δ H3 = -966 + 726

Δ H3 = -240 Kj Mol ^-1

Thermochemical Definitions

Standard Bond Enthalpy is the energy needed to break a gaseous covalent bond into gaseous component atoms in standard temperature and pressure.

e.g. H-H >> 2H

Standard Enthalpy of Formation is the total enthalpy change when 1 mole of product is formed from its component elements in their standard states.

Standard Enthalpy of Combustion is the total enthalpy change when 1 mole of reactant burns in excess Oxygen.

Standard Enthalpy of Reaction is the total enthalpy change when a specific reaction goes to completion.

* Standard Temperature is 298 Kelvin (25 C)

* Standard Pressure is 1 atmosphere

* Standard State is how you'd find the element at Standard temperature and pressure.

Group S elements

… are group 1 and group 2 on the periodic table.

A level students should identify those as Alkali metals and alkali earth metals, despite their lack of “Metal-like” properties.

Group S are generally reactive, and are relatively soft for metals.

Group 1 elements are less dense than water.

Francium and Radium are radioactive, and Cesium will react very violently with water. (Explosion)

Group S compounds are more useful than the elements.

They’re naturally found as ores.

Mostly, all S elements are reactive with water making Alkali solutions.

e.g. Magnesium + Water >> Magnesium Hydroxide + Hydrogen

The lower down the table, the more reactive the elements, for example Barium is more reactive than Magnesium.

All reactions in group 2 are more violent than group 1.

Group 2 is soluble, but less soluble than Group 1.

Group 2 makes stronger alkali solutions  (11-14 on the pH scale)

The lower down the table, the stronger the alkality. This is useful for neutralising acidic soil, whilst the salt bi product gets trapped in clay.

Thermal Stability: Unreactive elements give reactive compounds. And the further down the S group, the more stable the element.

Hydroxides are more solube as we go down the S group.

+1 ions[Hydroxides] (Chlorides, Bromides, Iodides, Nitrates)

Carbonates are less solube as we go down the S group.

-2 ions [Carbonates] (Sulphates, Oxides)

Nuclear Equations in universal context

Prepare yourself for a boring huge chunk of text...

·         Hydrogen is the most common element in the universe, ever. This is due to its very simple structure.

·         Formation of new element takes place in dense gas clouds. (Model)

·         Dense gas clouds are constantly “swirling and moving”.

·         Generally, atoms in the dense clouds may become plasma.

·         Plasma is a mixture of atoms, ions and electrons.

·         Lighter elements form heavier ones in dense gas clouds via fusion.

·         The temperature of gas clouds can vary between 10 and 100 Kelvin. We know this via replicating the reactions that occur in the clouds.

·         Gas clouds do not have a constant density.

·         There’s a gravitational field in the centre of clouds forming great temperature and pressure at the core.

·         Fusion is the joining of 2 nuclei.

·         When fusion occurs, energy is released, provoking more fusion to occur. However this is not classed as a chain reaction.

·         First reactions in gas clouds often include fusion of Hydrogen forming Helium, releasing gamma.

²H₁ + ³H₁ >> ⁴He₂ + g

·         Fusion can occur between any nuclei.

·         Heavy weight stars have higher temperatures because as they’re heavier, they have a greater gravitational field making them a better element manufacturer.

·         The reason cores are FE is because Iron production doesn’t release any energy as a bi product in order to provoke further fusion to occur.

·         Bigger stars are unstable and may cause SuperNovas.

·         Our sun is a light weighted star.