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Chapter 6: Chemical reactions

Physical change - is a change in the physical state of a substance that not involves its chemical change.

  • Physical changes (such as melting or evaporating) do not produce any new chemical substances.

  • These changes are often easy to reverse (by physical methods)

  • In a physical change the substances remain with the same chemical properties. For example, snow, water and ice are all made of molecules of H2O.

Chemical change - is a change where a new substance is produced

  • During chemical changes (usually referred as chemical reactions), new chemical substances are formed have very different properties from the reactants;

  • There are signs that show that a new substance was produced (we can identify is a chemical reaction has occured or not):

→ A color change;

→ A flame being formed;

→ A precipitate being formed;

→ Bubbles of gas being produced;

→ A smell is detected;

→ Change of temperature occurs

  • Most chemical reactions are very difficult to reverse;

  • During a chemical reaction there are energy changes. Energy can be given out (exothermic) or taken in (endothermic). The majority of chemical reactions are exothermic with only a small number being endothermic.

  • A chemical reaction can happen faster or slower.

  • Rate of reaction - measures how fast a reaction happens.

  • The rate of reaction can be affected by several factors.

Factors that affect the rate of reaction

How

The surface area of a solid reactant

The rate (speed) of a reaction increases when the surface area of a solid reactant is increased

The concentration of reactants in solutions

The rate of reaction increases when the concentration of a reactant in solution is increased

The pressure of reactants in reactions in reactions with gases

The rate of reaction increases when the pressure of the gases reacting also increases

The temperature

The rate of reaction increases when the temperature increases

The presence of a catalyst

Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction. So, when we have a catalyst present in a reaction, the rate will increase.

Surface area of solid reactants:

  • Reactions involving solids take place on the surface, so when they have a higher surface area, the rate of the reaction will be higher.

  • Thin powders have a larger surface area than big lumps, which is why it is easier to mix powders than big pieces (thin powders have a more available spaces, so they have greater contact with each other and it´ll be easier to react.

The concentration of reactants in solutions:

  • Comparing the 2 reactions we can observe that the graph line for a higher concentration has a steeper gradient at the start and becomes horizontal sooner. This means, we can get a greater amount of products faster.

  • Increasing the concentration of the reactants in a sollution will increase the frequency of collisions between the reactants, so there will be more opportunities for reaction.

  • This shows that increases the concentration of the reactants in a solution, the rate of reaction will increase.

The pressure of reacting gases:

  • Comparing the reactions with gases we can observe the graph line for a higher pressure has a steeper gradient at the start and becomes horizontal sooner. This means, we can get a greater amount of products faster.

  • Increasing the pressure will increase the frequency of collisions between the reactant gases, creating more opportunities for reaction.

  • This shows that increasing the pressure of gases in a reaction, makes the rate of reactin increase.

Temperature:

  • Increasing the temperature, the particles will be more energized and start to move faster, so the collision frequency will rise and there will be more opportunities for reaction.

  • That is why we compare the same reaction at a low temperature and at a higher temperature, the last one has a steeper gradient at the start and becomes horizontal sooner. That means we cam get a larger amount of products faster.

Catalyst:

  • Catalysts are substances which speed up the rate of a reaction without themselves being changes or consumed in the reaction.

  • The mass of a catalyst at the beggining and at the end of a reaction is the same and they do not form part of the equation.

  • The graph line for the same reaction but with catalyst has a steeper gradient at the start and becomes horizontal sooner compared to a reaction without a catalyst.

  • This shows that with a catalyst, the rate of reaction will increase

Collision Theory of reaction rate:

Collision Theory - is a theory that says that a chemical reaction happens only when the particles of the reactants collide with enough energy to start the reaction. - Ea, activation energy-

  • For a reaction to start, collisions must happen between the involved particles of the reactants.

  • Each reaction has its own activation energy (Ea)

  • The only way for a reaction to happen is when the particles collide with eachother and their combined energy is higher that their activation energy.

The Collision Theory is used to explain why several factors affect the rate of reaction.

  • If the collision frequency increases, there is higher change for a reaction to take place.

  • Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. Higher concentration means more collisions and more opportunities for reactions.

  • Increasing the temperature, the particles get energized and move faster. By moving faster the collision frequency increases and the chance for a reaction takes place also increases.

The Collision Theory also explains why the use of a catalyst increases the rate of reactions.

  • If we use a catalyst, it will reduce the activation energy (Ea) of a reaction (the particles need less energy to collide and start reacting).

  • By using a catalyst, the frequency of collisions increases. Therefore, there are more chances for reactions to take place.

  • The mountain analogy helps us understand. We need more enery to push the rock if we have to climb a higher mountain and we will arrive to the other side later. So, if we reduce the activation energy (Ea), by using a catalyst, we will get the products faster and with less energy.

Catalysts are very important in the industry because they make the processes faster and less expensive, and since the catalysts do not take part in the reactions and they can be re-used.

Each reaction needs a different catalyst:

→ We use iron as a catalyst in the Haber Process (industrial process to produce ammonia) to speed up the process and make it less expensive.

→ We use anadium oxide as a catalyst in the Contact Process (industrial process to produce sulfuric acid)

Some catalysts act by adsorption - Adsorption is the attachement of molecules to a solid surface. Catalysts bring the molecules close together, increasing the chance of a reaction. And catalysts also turn the bonds of the reactants weaker, making the breaking them and easy and less energetic process.

Combustion Reactions:

A combustion reaction occurs when a substance reacts with oxygen, releasing energy in the form of heat (so, it is an exothermic reaction)

  • Its very common to see a flame during these reactions (it occurs a burning)

  • During combustion, the substance involved is oxidised (gains oxygen)

→ An example is when wood reacts with air, it can combust.

Examples:

An example of a Combustion Reaction is the combustion of natural gas (methane). This is a reaction that produces a large amount of energy that we use in our homes and in industry. Thats why we call this kind of substance fuel.

Another example of a Combustion Reaction is the Respiration that occurs in our cells. Cells do respiration to produce energy to live and evolve. During this reaction, glucose reacts with oxygen to produce energy and 2 sub-products (carbon dioxide and water).

—>Another example of a Combustion Reaction is and Oxidation Reaction. In this case the substance gets oxidised, but not a great amount of energy is released.

Oxidation Reactions:

An oxidation Reaction occurs when a substance reacts with oxygen, and gets oxidised (loss of electrons), and also energy is released. Therefore, an oxidation reaction is an exothermic reaction.

Example:

An example of an Oxidation Reaction is passing air over heated copper. The surface of copper will be oxidised (a dark coat covers the copper surface). During this reaction it was released a little amount of energy but it was released a little amount of energy but it was not very exothermic and no flame is produced.

2Cu + O2 → 2CuO

Copper + Oxygen → Copper oxide

Reduction Reactions:

A reduction reaction occurs when a substance reacts and loses oxygen.

Example:

An example of a Reduction Reaction is passing hydrogen over heated copper oxide. The dark coat (oxide) covering surface will disapear and the surface turms pink/orange again because the obtained product is copper. But observing this reaction, we can see that copper oxide loses oxygen (reduction reaction), but in other hand hydrogen gains an oxygen (oxidation reaction).

Summary: Copper was reduced (lost oxygen) and Hydrogen was oxidised.

Since reduction never takes place without oxidation, we call these reactions Redox reactions or Oxidation-reduction reactions.

Oxidation-Reduction Reactions or REDOX reactions - in terms of OXYGEN:

  • Oxidation and reduction reactions take place together at the same time in the same reaction;

  • 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;

Example: Identifying the loss and gain of oxygen in an equation

zinc oxide + carbon → zinc + carbon monoxide

ZnO + C → Zn + CO

→ In this reaction, the zinc oxide (ZnO) has been reduced since lost oxygen;

→ The carbon atom (C) has been oxidised since gained oxygen.

Reducing Agent - is an element or compound that will remove oxygen from other substance. Usually, they are Hydrogen, Carbon and Carbon monoxide. the reducing agent is the one that will receive the oxygen (will be oxidised). “Its the one that steals the oxygen“.

Oxidising Agent - is an element or compound that will add oxygen to another substance. Usually, they are Oxygen, Hydrogen peroxide, Potassium manganate (VII) and Potassium dichromate (VI). The oxidising agent is the one that will lose oxygen (will be reduced). “Its the nice one, that offers oxygen“.

Redox - in terms of ELECTRONS:

  • Redox reactions can also be defined in terms of electron transfer.

  • Oxidation is a reaction in which an element, ion or compoung loses electrons:

→ The oxidation number of the element is increased.

→ This can be shown in a half equation, ex: when silver reacts with chlorine, silver is oxidised and forms silver ions (a neutral element was transformed to a positive ion because lost an electrons):

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, ex: when oxygen reacts with magnesium, oxygen is reduced to oxide ions (a neutral element was transformed to a negative ion):

O2 + 4e- → 2O2-

Conclusion: Even in reactions that do not involve the transfer of oxygen, we can identify if it is an oxidation or a reduction reaction.

OXIDATION NUMBER - it is a number that shows if an element was oxidised or reduced

TIP TO MEMORISE:

OIL-RIG = Oxidation Is Loss - Reduction Is Gain

MM

Chapter 6: Chemical reactions

Physical change - is a change in the physical state of a substance that not involves its chemical change.

  • Physical changes (such as melting or evaporating) do not produce any new chemical substances.

  • These changes are often easy to reverse (by physical methods)

  • In a physical change the substances remain with the same chemical properties. For example, snow, water and ice are all made of molecules of H2O.

Chemical change - is a change where a new substance is produced

  • During chemical changes (usually referred as chemical reactions), new chemical substances are formed have very different properties from the reactants;

  • There are signs that show that a new substance was produced (we can identify is a chemical reaction has occured or not):

→ A color change;

→ A flame being formed;

→ A precipitate being formed;

→ Bubbles of gas being produced;

→ A smell is detected;

→ Change of temperature occurs

  • Most chemical reactions are very difficult to reverse;

  • During a chemical reaction there are energy changes. Energy can be given out (exothermic) or taken in (endothermic). The majority of chemical reactions are exothermic with only a small number being endothermic.

  • A chemical reaction can happen faster or slower.

  • Rate of reaction - measures how fast a reaction happens.

  • The rate of reaction can be affected by several factors.

Factors that affect the rate of reaction

How

The surface area of a solid reactant

The rate (speed) of a reaction increases when the surface area of a solid reactant is increased

The concentration of reactants in solutions

The rate of reaction increases when the concentration of a reactant in solution is increased

The pressure of reactants in reactions in reactions with gases

The rate of reaction increases when the pressure of the gases reacting also increases

The temperature

The rate of reaction increases when the temperature increases

The presence of a catalyst

Catalysts are substances which speed up the rate of a reaction without themselves being altered or consumed in the reaction. So, when we have a catalyst present in a reaction, the rate will increase.

Surface area of solid reactants:

  • Reactions involving solids take place on the surface, so when they have a higher surface area, the rate of the reaction will be higher.

  • Thin powders have a larger surface area than big lumps, which is why it is easier to mix powders than big pieces (thin powders have a more available spaces, so they have greater contact with each other and it´ll be easier to react.

The concentration of reactants in solutions:

  • Comparing the 2 reactions we can observe that the graph line for a higher concentration has a steeper gradient at the start and becomes horizontal sooner. This means, we can get a greater amount of products faster.

  • Increasing the concentration of the reactants in a sollution will increase the frequency of collisions between the reactants, so there will be more opportunities for reaction.

  • This shows that increases the concentration of the reactants in a solution, the rate of reaction will increase.

The pressure of reacting gases:

  • Comparing the reactions with gases we can observe the graph line for a higher pressure has a steeper gradient at the start and becomes horizontal sooner. This means, we can get a greater amount of products faster.

  • Increasing the pressure will increase the frequency of collisions between the reactant gases, creating more opportunities for reaction.

  • This shows that increasing the pressure of gases in a reaction, makes the rate of reactin increase.

Temperature:

  • Increasing the temperature, the particles will be more energized and start to move faster, so the collision frequency will rise and there will be more opportunities for reaction.

  • That is why we compare the same reaction at a low temperature and at a higher temperature, the last one has a steeper gradient at the start and becomes horizontal sooner. That means we cam get a larger amount of products faster.

Catalyst:

  • Catalysts are substances which speed up the rate of a reaction without themselves being changes or consumed in the reaction.

  • The mass of a catalyst at the beggining and at the end of a reaction is the same and they do not form part of the equation.

  • The graph line for the same reaction but with catalyst has a steeper gradient at the start and becomes horizontal sooner compared to a reaction without a catalyst.

  • This shows that with a catalyst, the rate of reaction will increase

Collision Theory of reaction rate:

Collision Theory - is a theory that says that a chemical reaction happens only when the particles of the reactants collide with enough energy to start the reaction. - Ea, activation energy-

  • For a reaction to start, collisions must happen between the involved particles of the reactants.

  • Each reaction has its own activation energy (Ea)

  • The only way for a reaction to happen is when the particles collide with eachother and their combined energy is higher that their activation energy.

The Collision Theory is used to explain why several factors affect the rate of reaction.

  • If the collision frequency increases, there is higher change for a reaction to take place.

  • Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. Higher concentration means more collisions and more opportunities for reactions.

  • Increasing the temperature, the particles get energized and move faster. By moving faster the collision frequency increases and the chance for a reaction takes place also increases.

The Collision Theory also explains why the use of a catalyst increases the rate of reactions.

  • If we use a catalyst, it will reduce the activation energy (Ea) of a reaction (the particles need less energy to collide and start reacting).

  • By using a catalyst, the frequency of collisions increases. Therefore, there are more chances for reactions to take place.

  • The mountain analogy helps us understand. We need more enery to push the rock if we have to climb a higher mountain and we will arrive to the other side later. So, if we reduce the activation energy (Ea), by using a catalyst, we will get the products faster and with less energy.

Catalysts are very important in the industry because they make the processes faster and less expensive, and since the catalysts do not take part in the reactions and they can be re-used.

Each reaction needs a different catalyst:

→ We use iron as a catalyst in the Haber Process (industrial process to produce ammonia) to speed up the process and make it less expensive.

→ We use anadium oxide as a catalyst in the Contact Process (industrial process to produce sulfuric acid)

Some catalysts act by adsorption - Adsorption is the attachement of molecules to a solid surface. Catalysts bring the molecules close together, increasing the chance of a reaction. And catalysts also turn the bonds of the reactants weaker, making the breaking them and easy and less energetic process.

Combustion Reactions:

A combustion reaction occurs when a substance reacts with oxygen, releasing energy in the form of heat (so, it is an exothermic reaction)

  • Its very common to see a flame during these reactions (it occurs a burning)

  • During combustion, the substance involved is oxidised (gains oxygen)

→ An example is when wood reacts with air, it can combust.

Examples:

An example of a Combustion Reaction is the combustion of natural gas (methane). This is a reaction that produces a large amount of energy that we use in our homes and in industry. Thats why we call this kind of substance fuel.

Another example of a Combustion Reaction is the Respiration that occurs in our cells. Cells do respiration to produce energy to live and evolve. During this reaction, glucose reacts with oxygen to produce energy and 2 sub-products (carbon dioxide and water).

—>Another example of a Combustion Reaction is and Oxidation Reaction. In this case the substance gets oxidised, but not a great amount of energy is released.

Oxidation Reactions:

An oxidation Reaction occurs when a substance reacts with oxygen, and gets oxidised (loss of electrons), and also energy is released. Therefore, an oxidation reaction is an exothermic reaction.

Example:

An example of an Oxidation Reaction is passing air over heated copper. The surface of copper will be oxidised (a dark coat covers the copper surface). During this reaction it was released a little amount of energy but it was released a little amount of energy but it was not very exothermic and no flame is produced.

2Cu + O2 → 2CuO

Copper + Oxygen → Copper oxide

Reduction Reactions:

A reduction reaction occurs when a substance reacts and loses oxygen.

Example:

An example of a Reduction Reaction is passing hydrogen over heated copper oxide. The dark coat (oxide) covering surface will disapear and the surface turms pink/orange again because the obtained product is copper. But observing this reaction, we can see that copper oxide loses oxygen (reduction reaction), but in other hand hydrogen gains an oxygen (oxidation reaction).

Summary: Copper was reduced (lost oxygen) and Hydrogen was oxidised.

Since reduction never takes place without oxidation, we call these reactions Redox reactions or Oxidation-reduction reactions.

Oxidation-Reduction Reactions or REDOX reactions - in terms of OXYGEN:

  • Oxidation and reduction reactions take place together at the same time in the same reaction;

  • 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;

Example: Identifying the loss and gain of oxygen in an equation

zinc oxide + carbon → zinc + carbon monoxide

ZnO + C → Zn + CO

→ In this reaction, the zinc oxide (ZnO) has been reduced since lost oxygen;

→ The carbon atom (C) has been oxidised since gained oxygen.

Reducing Agent - is an element or compound that will remove oxygen from other substance. Usually, they are Hydrogen, Carbon and Carbon monoxide. the reducing agent is the one that will receive the oxygen (will be oxidised). “Its the one that steals the oxygen“.

Oxidising Agent - is an element or compound that will add oxygen to another substance. Usually, they are Oxygen, Hydrogen peroxide, Potassium manganate (VII) and Potassium dichromate (VI). The oxidising agent is the one that will lose oxygen (will be reduced). “Its the nice one, that offers oxygen“.

Redox - in terms of ELECTRONS:

  • Redox reactions can also be defined in terms of electron transfer.

  • Oxidation is a reaction in which an element, ion or compoung loses electrons:

→ The oxidation number of the element is increased.

→ This can be shown in a half equation, ex: when silver reacts with chlorine, silver is oxidised and forms silver ions (a neutral element was transformed to a positive ion because lost an electrons):

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, ex: when oxygen reacts with magnesium, oxygen is reduced to oxide ions (a neutral element was transformed to a negative ion):

O2 + 4e- → 2O2-

Conclusion: Even in reactions that do not involve the transfer of oxygen, we can identify if it is an oxidation or a reduction reaction.

OXIDATION NUMBER - it is a number that shows if an element was oxidised or reduced

TIP TO MEMORISE:

OIL-RIG = Oxidation Is Loss - Reduction Is Gain