It controls the rate of reaction of a process via motion of atoms. Where there is a movement of atoms down the concentration gradient
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What is the driving force of diffusion?
It is driven by the decrease in ΔG and chemical potential energy.
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Describe inter-diffusion
It is the net drift of transport of a substance from a high concentration to a low concentration.
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What is self-diffusion and where does it occur?
It is the changing of positions of the same type of atom and it occurs in pure metals.
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Give another definition of diffusion in terms of lattice sites
The step-wise migration of atom lattice site to lattice site from high concentration to low concentration.
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What must happen for diffusion to take place?
There must be an empty adjacent site.The atom must have sufficient energy to break the bonds with neighbours.
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What is the fraction of the number of atoms that are capable to diffuse dependent on?
It is dependent on temperature. (The higher the temperature the greater the fraction of atoms)
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Name the two types of crystal lattice diffusion?
1)Vacancy diffusion;2)Interstitial diffusion
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Explain Vacancy diffusion
From a normal lattice position to an adjacent vacant lattice site.It is a function of the number of vacancies and increases with temperature as the #vacancies also increases. It is the mechanism which is used by self-diffusion ad inter-diffusion.
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Explain interstitual diffusion
Involves atoms that migrate from an interstitial position to an empty neighbouring one. Inter-diffusion uses this method(smaller atoms)
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Which is more rapid vacancy or interstitial diffusion?
Interstitial is more rapid than vacancy diffusion since the interstitial atoms are smaller and more mobile- they would be more empty spaces also.
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Give the equation for the rate of mass transfer in terms of M,t,A
J = M/At ] where J:diffusion flux/rate if mass transfer (kg/m^2*s);M:mass diffusing through the perpendicular to a unit cross-sectional per unit time.
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Give the differential form of the equation of diffusion flux
J = 1/A * dM/dt
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Define steady-state diffusion
When the diffusion flux does not change with time.
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What is a concentration profile and what information can we get from it?
Concentration against position graph.Linear. The concentration gradient is the slope of the graph = ΔC/Δx=(C_A-C_B)/(x_A-x_B)
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What is Flick's first law of diffusion?(steady state diffusion)
The diffusion flux is proportional to the concentration gradient. J = -D*(dC/dx) ] where D is the diffusion coefficient (m^2/s)
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What is Flick's second law?
Models what happens in non-steady state systems. ∂C/∂t = D*(∂^2C/∂x^2)
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Other cards in this set
Card 2
Front
What is the driving force of diffusion?
Back
It is driven by the decrease in ΔG and chemical potential energy.
Card 3
Front
Describe inter-diffusion
Back
Card 4
Front
What is self-diffusion and where does it occur?
Back
Card 5
Front
Give another definition of diffusion in terms of lattice sites
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