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)