A2 chemistry unit 4

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  • Created by: Elena
  • Created on: 09-05-14 11:14
What is the reaction rate?
The change in the amount of reactants or products per unit time (usually per second) - how fast reactants are converted to products
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Ways to follow the rate of reaction
Gas volume, loss of mass, colour change, clock reaction and electrical conductivity
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How do you follow a reaction using a clock reaction?
For some reactions - sudden colour change when product reaches a certain concentration -ROR worked out by measuring time taken for colour change to happen
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How do you follow a reaction using gas volume?
If gas is given off, you can collect it in a gas syringe & record amount you've got at regular time intervals.
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How do you follow a reaction looking at loss of mass?
If gas is given off, system will lose mass - can measure this at regular intervals with a balance
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How do you follow a reaction using colour change?
Can sometimes track colour change using colorimeter
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How do you follow a reaction on electrical conductivity?
If number of ions changes, so will electrical conductivity
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What do the orders of reactions tell you?
How a reactant's concentration affects the rate
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What is zero order?
If you increase a reactant's concentration by x, the rate stays the same
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What is first order?
If you increase a reactant's concentration by x, the rate increases by x
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What is second order?
If you increase a reactant's concentration by x, the rate increases by x^2
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What is a half-life?
The time taken for half of the reactant to react
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Half-life on a first order conc-time graph
The half-life is constant - always takes the same amount of time for the conc to halve. Half-life is independent of the concentration
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Half-life on a second order conc-time graph
The half-life isn't constant - it increases as the reaction goes on
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What is the rate equation and what are the units?
Rate = K[A]^m[B]^n and the units are moldm^-3s^-1
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What does the rate equation link together?
Reaction rate and reaction concentrations
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what does m & n tell us in a rate equation?
The orders of reaction with respect to reactant A and reactant B. Overall order = M + N
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What is K in the rate equation?
The rate constant - the bigger it is, the faster a reaction. Rate constant is always the same for a certain reaction at particuplar temp - if you increase temp, rate constant rises too. Units vary (don't need to work them out)
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How to work out orders of reaction and rate equations by experimentation
Do a series of experiments monitoring rate of reaction - in each experiment, change conc of only one reactant - plot each experiment on a conc-time graph & calculate initial rate of reaction (t=0 gradient) - analyse results
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What is the rate-determing step?
The slowesr step in a multi-step reaction - decides overall rate
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How to pick out which reactants from chem equation are involved in rate-determining step?
If reactant appears in rate equation, must be affecting rate - reaction must be in rate-determining step. If is doesn't appear in rate equation - won't be involved in rate-determining step
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Important points about rate-determing steps and mechanisms
Doesn't have to be the first step in a mechanism and reaction mechanism can't usually be predicted from just the chemical equation
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What are halogenalkanes hydrolysed by?
Hydroxide ions
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What is nucleophilc substitution?
when a nucleophile attacks another molecule and is swapped for one of the attached groups
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What are the two different mechanisms for nucelophilic substition?
Sn1 and Sn2
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Which halogenalkanes react by the Sn1 mechanism?
Secondary and tertiary halogenalkanes
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Which halogenalkanes react by the Sn2 mechanism?
Primary halogenalkanes
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Sn1 mechanism
Only involve 1 molecule or ion in the rate-determining step. A species leaves at the same time a species joins. Forms a trigonal planar shape and only forms 1 product as species can only attack from one side
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Sn2 mechanism
Involve 2 molecules, 1 molecule and 1 ion or 2 ions in the rate-determining step - Forms a racemic mixture because when 1 species leaves, another species can join either side forming 2 different products
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What is the arrhenius equation?
Used to calculate activation energy- links rate constant (K) with activation energy & temperature (T). K=Ae^-Ea/RT
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What are catalysts used for?
They increase the rate of a reaction by providing an alternative reaction pathway with a lower activation energy. The catalyst is chemically unchanged at the end of the reaction.
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What are homogeneous catalysts?
They are in the same state as the reactants - if reactants are gases, catalyst must be gas too.
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What are heterogeneous catalysts?
They are in a different physical state from the reactants - can be easily separated from products. Also, can be poisoned
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What is a poison (in terms of catalysts)?
It's a substance that clings to the catalyst's surface more strongly than the reactant does, preventing the catalyst from getting involved in the reaction it's meant to be speeding up
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What is a solid heterogeneous catalyst used for and how do you increase their surface area?
Solid heterogeneous catalysts provide a surface for the reaction to take place on. They're usually in the form of a mesh or fine powder to increase surface area
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What do enthalpy profiles and Boltzmann Distributions show?
Why catalysts work - lowers activation energy = more particles with enough energy to react when they collide so more particles will react in a certain amount of time.
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Why are catalysts important in industrial processes?
They save a fortune because reactions can be carried out at lower temperatures and pressures.
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What does entropy tell you?
How much disorder there is - a measure of the number of ways that particles can be arranged and the number of ways that the energy can be shared between the particles
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Are substances more energetically stable when there's more order or more disorder?
When there's more disorder - so the particles naturally move to try to increase the entropy e.g a gas spontaneously diffusing across a room
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How does physical state affect entropy?
solid particles don't move much so there is hardly any randomness (lowest entropy) but gas particles whizz around so they've got the most random arrangements (highest entropy) & dissolving a solid increases entropy as they can now move freely
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How does the amount of energy a substance has affect entropy?
energy can be measured in quanta (fixed 'packages' of energy). The more energy a quanta has, the more ways they can be arranged so the greater the entropy
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How does the amount of particles affect entropy?
The more particles and moles you've got, the more ways they and their energy can be arranged
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How does temperature affect entropy?
If temp increases, energy of particles increases, so the higher the temp, the more energy quanta a substance has, the more ways these quanta can be distributed (higher entropy)
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What is the standard entropy of a substance?
S-, The entropy of 1 mole of that substance under standard conditions (100kPa (1atm) and 298K). Units are jk^-1mol^-1
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What does the standard entropy of a substance depend on?
It's physical state - solid substances tend to have lower standard entropies than liquids, which are even lower than gases. Complexity - simple atoms or molecules tend to have lower standard entropies than more complicated molecules
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How can entropy increase explain spontaneous endothermic reactions?
Don't usually get spontaneous endothermic reactions as you need to put energy in for endothermic reaction to happen but some happen because of large increase in entropy - product has more particles & also, particles are in a higher entropy state
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How to calculate the total entropy change and what are the units?
total entropy change = entropy change of system + entropy change of surroundings. Units are jk^-1mol^-1
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How to calculate the entropy change of the system?
entropy change of system = entropy change of products - entropy change of reactants
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How to calculate the entropy change of the surroundings?
Entropy change of surroundings = - delta H (energy change in Jmol^-1) / T (temperature in K)
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In order for a spontaneous reaction to occur, what sign (+ or -) must the total entropy change be?
It must be positive as it means the reaction is kinetically favourable
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In an exothermic reaction, what must delta H be (+ or -)
Delta H must be negative in an exothermic reaction so that entropy of surroundings is always positive
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In an endothermic reaction, what must delta H be? (+ or -)
Delta H must be positive in an endothermic reaction so that entropy of the surroundings is always negative.
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If the temperature get's higher, what happens to entropy change of the surroundings?
Entropy change of the surroundings get's smaller and so, it makes a smaller contribution to the total entropy change than it does at lower temperatures
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What is the enthalpy change of solution?
The enthalpy change when 1 mole of solute is dissolved in sufficient solvent that no further enthalpy change occurs on further dilution
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What is the standard lattice enthalpy?
The enthalpy change when 1 mole of a solid ionic compound is formed from gaseous ions under standard conditions
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What is the enthalpy change of hydration?
The enthalpy change when 1 mole of aqueous ions is formed from gaseous ions
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How to find out the enthalpy change of solution?
Use enthalpy cycles - which work on principle of Hess's Law
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What affects lattice enthalpy?
The ionic charge and size - larger the charge = more energy is released when ionic lattice forms - lattice enthalpy is more -ve so lattice enthalpies for compounds with 2+ or 2- ions are more -ve than those with 1+ or 1- ions
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What happens at equilibrium?
The amounts of reactants and products stay the same
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What is dynamic equilibrium?
Amount of reactants and products won't be changing any more so it'll seem like nothing's happening - forward reaction will be going at exactly the same rate as the backward reaction
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What is Kc?
The equilibrium constant - if you know molar conc of each substance at equilibrium, you can work out Kc
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What is the equation for Kc?
[D]^d[E]^e / [A]^a[B]^b (products go on top line & lower case a and b are no. of moles of each substance)
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Does Kc apply to homogeneous catalyst or heterogeneous catalyst?
Only homogeneous catalysts when all products and reactants are in the same phase - if mixture involves solids & gases or solids & liquids, leave out conc of solids but still use Kc. If you have solids & liquids, use Kp instead.
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