Chemistry Topic: 7 - Rate of Reactions & Redox Reactions Flashcards
1
Q
Rate Of Reaction Factors & Collision Theory
What is the rate of reaction? How is it calculated? How can it be altered/changed?
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A
- The rate of reaction is a measure of how fast or slow a chemical reaction is.
- It is calculated by how much reactant is used per unit time OR how much product is produced per unit time.
- Changing the conditions a reaction occurs in can alter the rate of the reaction.*
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Q
Physical & Chemical Changes
What is a physical change? What are some examples?
Is it reverseable?
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- Physical changes are changes affecting the form of a chemical substance, but not its chemical composition.
- Physical changes (such as melting or evaporating) do not produce any new chemical substances
- These changes are often easy to reverse
- Making a mixture from 2 or more substances or dissolving a solute in a solvent are examples of physical changes as no new substances are produced and are usually relatively easy to separate
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Q
Physical & Chemical Changes
What are chemical changes? Are they reverseable? What changes accompany chemical changes?
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- Chemical changes (usually referred to as chemical reactions) are reactions where new chemical substances are formed that have very different properties to the reactants.
- There may be signs that a new substance has formed, such as:
- A colour change
- A precipitate being formed
- Bubbles of gas being produced
- Most chemical reactions are very difficult to reverse.
- Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic)
- The majority of chemical reactions are exothermic with only a small number being endothermic.
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Q
Collision Theory
What is the collision theory? What is activation energy? What are successful collisions? How do unsuccessful collisions occur?
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- Collision theory states that in order for a reaction to occur:
- The particles must collide with each other
- The collision must have sufficient energy to cause a reaction i.e. enough energy to break bonds
- The minimum energy that colliding particles must have to react is known as the activation energy
- Collisions which result in a reaction are known as successful collisions
- If they have sufficient energy (i.e. energy greater than the activation energy), they will react, and the collision will be successful
- Not all collisions result in a chemical reaction:
- Most collisions just result in the colliding particles bouncing off each other
- Collisions which do not result in a reaction are known as unsuccessful collisions
- Unsuccessful collisions happen when the colliding species do not have enough energy to break the necessary bonds (i.e. they collide with energy less than the activation energy)
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Q
Collision Theory
What effect does increasing the number of successful collisions have? What does the number of successful collisions depend on?
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- Increasing the number of successful collisions means that a greater proportion of reactant particles collide to form product molecules
- The number of successful collisions depends on:
- The number of particles per unit volume - more particles in a given volume will produce more frequent successful collisions
- The frequency of collisions - a greater number of collisions per second will give a greater number of successful collisions per second
- The kinetic energy of the particles - greater kinetic energy means a greater proportion of collisions will have an energy that exceeds the activation energy and the more frequent the collisions will be as the particles are moving quicker, therefore, more collisions will be successful
- The activation energy - fewer collisions will have an energy that exceeds higher activation energy and fewer collisions will be successful
- These all have an impact on the rate of reaction which is dependent on the number of successful collisions per unit of time
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6
Q
Explaining Rates Using Collision Theory
What is the effect of temperature on the rate of reaction? What is the effect of temperature on collisions?
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- Increasing the temperature will increase the rate of reaction
- This is because the particles will have more kinetic energy than the required activation energy
- Therefore there will be more frequent collisions and a higher proportion of particles have energy greater than the activation energy
- This causes more successful collisions per second, increasing the rate of reaction
- The effect of temperature on collisions is not so straightforward as concentration or surface area; a small increase in temperature causes a large increase in rate
- For aqueous and gaseous systems, a rough rule of thumb is that for every 10 degrees Celsius increase in temperature, the rate of reaction approximately doubles
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Q
Explaining Rates Using Collision Theory
What is the effect of concentration in a reaction?
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- Increasing the concentration of a solution increases the collision rate.
- This is because there will be more reactant particles per unit volume, causing more frequent collisions so there are more successful collisions per second, increasing the rate of reaction.
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8
Q
Explaining Rates Using Collision Theory
What effect does surface area have on rate of reaction? Why?
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- Increasing the surface area of a solid will increase the rate of reaction
- This is because more surface area of the particles will be exposed to the other reactant, producing a higher number of collisions per second
- If you double the surface area you will double the number of collisions per second
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9
Q
Explaining Rates Using Collision Theory
What effect do catalysts have on the rate of reaction? Do they change in any way? What factors are catalysts involved in? What are some examples of catalysts? What amount of catalysts are necessary to have effect on a reaction?
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- Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction
- The mass of a catalyst at the beginning and end of a reaction is the same and they do not form part of the equation
- Different processes require different types of catalysts but they all work on the same principle of providing a different pathway for the reaction to occur that has a lower activation energy
- This means a higher proportion of the reactant particles have energy greater than the activation energy and will result in more successful collisions per second
- An important industrial example is iron, which is used to catalyse the Haber Process for the production of ammonia
- Iron beads are used to increase the surface area available for catalysis
- Enzymes are biological catalysts, they work best at specific temperature and pH ranges
- Normally only small amounts of catalysts are needed to have an effect on a reaction
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Q
Redox Reactions
What is a redox reaction? What is oxidation and reduction?
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- Oxidation–reduction reactions, commonly known as redox reactions, are reactions that involve the transfer of electrons from one species to another.
- Oxidation is a reaction in which oxygen is added to an element or a compound
- Reduction is a reaction in which oxygen is removed from an element or compound
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Q
Redox Reactions
Naming using oxidation numbers:
Non-Question Card
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- Transition elements can bond in different ways by forming ions with different charges
- When naming, the charge on the ion is shown by using a Roman numeral after the element’s name
- E.g. iron can form ions with a 2+ charge, called iron(II) ions or a 3+ charge, called iron(III) ions
- The Roman numeral is the oxidation number of the element
- When iron reacts with oxygen to form iron oxide, the formula depends on the oxidation state of the iron ions
- The compound where iron has a 2+ charge has the formula FeO and is called iron(II) oxide
- The compound where iron has a 3+ charge has the formula Fe₂O₃ and is called iron(III) oxide
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12
Q
Redox Reactions
How can redox reactions be defined in terms of electron transfer?
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- Redox reactions can also be defined in terms of electron transfer:
- Oxidation is a reaction in which an element, ion or compound loses electrons
- The oxidation number of the element is increased
- This can be shown in a half equation, e.g. when silver reacts with chlorine, silver is oxidised to silver ions:
Ag → Ag⁺ + e⁻
- Reduction is a reaction in which an element, ion or compound gains electrons
- The oxidation number of the element is decreased
- This can be shown in a half equation, e.g. when oxygen reacts with magnesium, oxygen is reduced to oxide ions:
O₂ + 4e⁻ → 2O₂⁻
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Q
Redox Reactions
Example: Identifying Redox Reactions
Non-Question Card
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- zinc + copper sulphate → zinc sulphate + copper
Zn + CuSO₄ → ZnSO₄ + Cu
- The ions present (with state symbols) in the equation are:
Zn(s) + Cu²⁺(aq) + SO₄²⁻(aq) → Zn²⁺(aq) + SO₄²⁻(aq) + Cu(s) - The spectator ions (those that do not change) are SO₄²⁻ (aq)
- These can be removed and the ionic equation written as:
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
- By analysing the ionic equation, we can split the reaction into two half equations by adding in the electrons to show how the changes in charge have occurred:
Zn(s) → Zn²⁺(aq) + 2e⁻ Cu²⁺(aq) + 2e⁻ → Cu(s) - It then becomes clear that zinc has been oxidised as it has lost electrons
- Copper ions have been reduced as they have gained electrons
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Q
Redox Reactions: Oxidation Number
What is oxidation number? What does it help with? How is it written? Give an example
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- The oxidation number (also called oxidation state) is a number assigned to an atom or ion in a compound which indicates the degree of oxidation (or reduction)
- It shows the number of electrons that an atom has lost, gained or shared in forming a compound
- The oxidation number helps you to keep track of the movement of electrons in a redox process
- It is written as a +/- sign followed by a number (not to be confused with charge which is written by a number followed by a +/- sign)
- E.g. aluminium in a compound usually has the oxidation state +3
- Redox reactions can be identified by the changes in the oxidation number when a reactant goes to a product
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Q
Redox Reactions: Oxidation Numbers
Rules For Assigning Oxidation Numbers
Non-Question Card
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- The oxidation number of any uncombined element is zero.
Examples: H₂, Zn, O₂ - Many atoms or ions have fixed oxidation numbers in compounds.
Examples:- Group 1 elements are always +1
- Group 2 elements are always +2
- Fluorine always -1
- Hydrogen is +1 (except for in metal hydrides like NaH, where it is -1)
- Oxygen is -2 (except for peroxides, where it is -1 and F₂O where it is +2)
- The oxidation number of an element on mono-atomic ion is always same as the charge.
Examples:- Zn²⁺ oxidation number = +2
- Fe³⁺ oxidation number = +3
- Cl- oxidation number = -1
- The sum of the oxidation number in a compound is zero. Examples: NaCl
- Oxidation number of Na = +1
- Oxidation number of Cl = -1
- sum oxidation number = 0
- The sum of oxidation number in ion is equal to the charge on the ion.
Examples: SO₄²⁻- Oxidation number of S = +6
- Oxidation number of 4 O atoms = 4 X (-2)
- Sum oxidation number = -2
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