CH0002 - Lecture 5
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What is the activation energy?
The minimum energy that must be input to a chemical system, containing potential reactants, in order for a chemical reaction to occur.
Often described as the energy barrier.
Also described as the transition state.
What is the Arrhenius Equation?
k = A.e-Ea/RT
- k - rate constant
- A - Arrhenius constant
- Ea - activation energy
- R - gas constant
- T - temperature
How can you plot the Arrhenius equation as a straight line graph?
Take natural logarithms of both side.
lnk = lnA - (Ea/R x 1/T)
y = c + mx
Using the Arrhenius equation, plot lnk on the y-axis, and 1/T on the x-axis.
The intercept gives...
The gradient gives...
- Intercept = lnA
- Gradient = -Ea/R
What does a catalyst do?
What doesn't it do?
It lowers the activation energy of a reaction
- A catalyst does not change the position of equilibrium of a reaction but changes the rate at which equilibrium is reached.
- It does this by providing an alternative route with a lower energy barrier.
What are the 3 states of matter?
- Solid: Well defined size and shape
- Liquid: Takes shape of the container in which its placed
- Gas: Another fluid form which can be easily compressed.
What is a Phase diagram?
A diagram which represents the way in which the state of a material depends on temperature and pressure.
What are the 3 important parts of a phase diagram?
The boiling point, the melting point, and the triple point.
What is a triple point?
The pressure and temperature at which a material can be either a solid, liquid or gas.
What are the three main types of intermolecular forces?
- London Forces
- Dipole-Dipole Forces
- Hydrogen bonding
What are London Forces?
- Forces which arise from the attraction between two instantaneous dipoles
- The dipoles arise from fluctuations in the electron distribution in the molecuel
All molecules are drawn together by London Forces
What effect does molar mass have on London Forces?
The strength of London forces increases with molar mass since heavier molecules have more electrons.
What effect does molecular shape have on London forces?
- The strength of London forces also depends on the shape of molecules.
- E.G. Rod shaped molecules can get closer together than spherical molecules.
What are Dipole-Dipole forces?
- Some molecules are polar (The have permanent partial charges)
- These molecules are drawn together by dipole-dipole forces
What is hydrogen bonding?
A hydrogen bond is the electromagnetic attractive interaction between polar molecules, in which a hydrogen atom lies between two small, strongly electronegative atoms with lone pairs of electrons i.e. nitrogen, oxygen, fluorine
What is liquid structure?
The molecules in a liquid are in contact with their neighbors, but they are mobile
What is surface tension?
- The surface of a liquid is smooth because intermolecular forces pull the molecules together and inward.
- There is a net pull inward
- The surface tension of water is much higher than most other liquids
What is capillary action, and why does it happen?
- Capillary action is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity.
- It is caused by the attraction between the liquid and other materials, such as glass.
- The liquid rises becasuse there are stronger forces between the liquid and the molecules in the surface of the capillary.
What is viscosity?
What effect do intermolecular forces have?
- The viscosity of a liquid is its resistance to flow.
- Greater intermolecular forces result in higher viscosity
What is vapour pressure
- Molecules leave the surface of an enclosed liquid continually - Evaporation
- But they can collide and can become trapped again by the surface - Condensation
- The dynamic equilibrium between evaporation and condensation is temperature sensitive - at higher temperatures, more molecules leave the surface.
What is saturated Vapour Pressure
It is the pressure exerted by molecules in the vapour when they are constant, at equilibrium (i.e. there are as many molecules evaporating as condensing)
How does the Saturated Vapour Pressure change with temperature?
What is the boiling point?
What do strong intermolecular forces lead to?
- When the vapour pressure of a liquid matches atmospheric pressure, the liquid vaporises so quickly that bubbles form - this is the boiling point
- Boiling point is defined as the temperature at which a liquid boils when the atmospheric pressure is 1 atm.
- Strong intermolecular forces lead to high boiling points.
What is Raoult's Law?
The vapour pressure of a solvent in the presence of a non-volatile solute is proportional to the mole fraction of the solvent.
Raoult's Law - The vapour pressure of a solvent in the presence of a non-volatile solute is proportional to the mole fraction of the solvent can be expressed mathematically as:
- P = xsolventPpure
- Where P = total vapour pressure
- xsolvent = mole fraction of the solvent
- Ppure = vapour pressure of the pure solvent at a particular temperature
= Moles of solvent / total number of moles
(Total number of moles = Moles of solvent + moles of solute)
Why is Raoult's law limitted?
- Because it will only work for ideal solutions - Ideal solutions are defined as solutions which will obey Raoult's law.
- Most solutions are non-ideal
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