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Rate of Reaction
- the change in concentration of a reactant or product per unit time
- measures: how fast reactants are being used up and how fast products are being made
- Units: mol dm-3 s-1Concentration-time graphs:
- gradient of tangent shows rate of reaction
- when t=0 gradient is the initial rate
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Orders
- the power to which the concentration of the reactant is raised in the rate equations
- Zero order: rate is unaffected by changing the concentration
- First order: rate is proportional to concentration
- Second order: if concentration increases by 2 then the rate increases by 22=4
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Rate constant, k
- the constant that links the rate of reaction with the concentrations of the reactants raised to the power of their orders in the rate equation
- effect of temperature: increasing temperature, speeds up the rate of reaction, increasing the rate constant, k
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Rate equation
- rate=k[A]0[B]1[C]2...
- is also equal to rate=k[B][C]2overall order is the sum of the individual orders
- Units: k=rate/[A]: s-1
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Half-life
- the time taken for the concentration of a reactant to reduce by half
- a first order reaction has a constant half life
- the half life of a first order reaction is independent of the concentration (half life is the same no matter what the concentration is)
- Zero order: half life decreases with time
- Second order: half lif increases with time
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Rate-determining step
- the slowest step in a multi-step reaction
- the order with respect to the reactant tells you how many particles of the reactant are involved in the rate-determining step
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Equiibrium
- Equilibrium constant, Kc=products/reactants
- Units: mol dm-3
- if more product is formed Kc will increase
- Kc is unaffected by concentration or pressure change, or by a catalyst
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Bronsted-Lowry
- Acid: proton donor
- Base: proton acceptor
- Reactions:
- Conjugate acid-base pairs: a pair of two species that transform into each other by gain or loss of a proton
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Strong and weak acids
- Strong acid: an acid that completely dissociates in solution
- Weak acid: an acid that partially dissociates in solution
- Acid dissociation constant, Ka shows the extent of acid dissociation a large Ka means the acid is strong
- pKa=-log10KaKa=10-pKa
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pH and [H+(aq)]
- pH=-log[H+]; [H+]=10-pHKw=[H+][OH-]
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Buffers
- Buffer solution: a system that minimises pH changes on addition of small amounts of an acid or base, can be made up from a weak acid and a salt of the weak acid
- Example: CH3COOH/CH3COONa
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