chemical lesson 6 - students website · chemistry module 1- lesson 6 5 redox reactions -...

Chemistry Module 1- Lesson 6

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Chemical lesson 6

Study guide - Notes – Questions

This week we look at two main types of chemical reactions and also the factors that influence them.

Your weekly program. Develop your own method by all means but this will get you going.

Read the notes attached here. All questions and the final assessment are based on what is in the notes.

Read the chapter sections in the reading section ( the text has much greater detail than what you are expected to know) especially look at diagrams and figures

At this stage not much will make sense but that is OK

With your text as reference and this study sheet go through the Power point presentation. Make notes where needed.

MANDATORY watch all of the embedded video links (you will need internet access to do this)

Optional - Listen to the audio file of a live lesson; be aware that there will be long pauses with not much going on at certain points.

Refine your notes / mind maps on the key concepts outlined in these weekly study sheets. Check your understanding

Now go through your homework questions and answer those

Your study resources

Textbook

Audio files of the actual lessons

Weekly study sheet

Power Point slides

YouTube links and YouTube as a general resource

College forum site

Molecule kit


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Textbook reference Bettleheim Edition 9 Please read these sections Chapter 4 sections 4.6 to 4.8

Key Concepts to understand

Precipitation

Redox - Oxidation and reduction

Heat of reaction

Exothermic and endothermic

Entropy

Activation energy

Transition state

Intermediate steps

Factor that affect reaction rates o Catalysts o Concentration of reactants o Temperature o Medium of the reaction

Use the links in the power point presentation. Good sources for chemistry videos are Bozeman science http://www.bozemanscience.com/ Crash course chemistry – YouTube Socratic.org - https://socratic.org/

http://www.bozemanscience.com/


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Study Notes

Classification of reactions

In these notes we cover redox reactions and precipitation reactions. The focus will be on the redox reactions as they are of great importance in biological systems. Firstly we will briefly cover precipitation reactions.

Precipitation reactions

Precipitation Reactions occur when cations and anions of aqueous solutions combine to form an insoluble ionic solid, called a precipitate.

Aqueous Definition: Aqueous is a term used to describe a system which involves water. The word aqueous is also applied to a solution or mixture in which water is the solvent. When a chemical species has been dissolved in water, this is denoted by writing(aq) after the chemical name. Precipitate Definition: An insoluble solid that emerges from a liquid solution. The emergence of the insoluble solid from solution is called precipitation. Often the precipitate emerges as a suspension. Whether or not such a reaction occurs can be determined by using the solubility rules for common ionic solids. You will see these rules in the text book. You do not need to know these rules for this course. Since not all aqueous reactions form precipitates, one must consult the solubility rules before determining the state of the products.

Being able to predict these reactions allows scientists to calculate what ions are present in a solution, and allows industries to form chemicals by extracting certain elements from these reactions.

The determining factors of the formation of a precipitate can vary. Some reactions depend on temperature, such as solutions used for buffers, while others are dependent only on solution concentration. The solids produced in precipitate reactions are crystalline solids. This solid can be suspended throughout the liquid or fall to the bottom of the solution. The fluid that remains is called the supernatant liquid. The two parts (precipitate and supernate) can be separated by various methods, such as filtration, centrifuging, or decanting.

http://www.chemicool.com/definition/insoluble.html

http://www.chemicool.com/definition/solution.html

http://www.chemicool.com/definition/suspension.html


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Redox Reactions - Oxidation-Reduction

Redox is a shorthand way of saying oxidation and reduction reactions. Redox reactions, or oxidation-reduction reactions, primarily involve the transfer of electrons between two chemical species. This is a very large group of reactions. The compound that loses an electron is said to be oxidized, the one that gains an electron is said to be reduced. A good memory aid for this is the acronym

OIL RIG

. Oxidation involves loss of electrons Reduction involves gain of electrons There are also specific terms that describe the specific chemical species. A compound that is oxidized is referred to as a reducing agent, while a compound that is reduced is referred to as the oxidizing agent. In the following equation you will see the movement of electrons from Iron Fe to Copper Cu. Iron is oxidised as it loses electrons (reducing agent) and Copper is reduced as it accepts electrons (oxidised).

This is potentially very confusing if you try to learn both what oxidation and reduction mean in terms of electron transfer, and also learn definitions of oxidising and reducing agents in the same terms. To help make sense of this use the following reasoning. If you wanted to know, for example, what an oxidising agent did in terms of electrons: An oxidising agent oxidises something else.


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Oxidation is loss of electrons (OIL RIG). That means that an oxidising agent takes electrons from that other substance. So an oxidising agent must gain electrons. Or you could work it out like this: An oxidising agent oxidises something else. That means that the oxidising agent must be being reduced. Reduction is gain of electrons (OIL RIG). So an oxidising agent must gain electrons. To broaden the definition a bit further the following diagram also defines redox in terms of the movement of Hydrogen and oxygen in a reaction.

Assigning Oxidation States

The Oxidation State (OS) corresponds to the number of electrons, e-, that an atom loses, gains, or appears to use when joining with other atoms in compounds. When determining the OS of an atom there are seven guidelines to follow:

The OS of an individual atom is 0.

The total OS of all atoms in: a neutral species is 0 and in an ion is equal to the ion charge.

Group 1 metals have an OS of +1 and group 2 an OS of +2

The OS of fluorine is -1, when in compounds

Hydrogen generally has an OS of +1 in compounds

Oxygen generally has an OS of -2 in compounds

In binary metal compounds, group 17 elements have an OS of -1, group 16 of -2, and group 15 of -3.

(Note: The sum of the OS’s is equal to zero for neutral compounds and equal to the charge for polyatomic ion species.)


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These rules allow the movement of electrons and reactions to be predicted. For this course you need to be aware of this concept of oxidation states only.

Example of an Oxidation Reduction Reaction

The reaction between hydrogen and fluorine is an example of an oxidation-reduction reaction:

H2 + F2 → 2 HF

The overall reaction may be written as two half-reactions:

H2 → 2 H+ + 2 e− (the oxidation reaction)

F2 + 2 e− → 2 F− (the reduction reaction)


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There is no net change in charge in a redox reaction so the excess electrons in the oxidation reaction must equal the number of electrons consumed by the reduction reaction. The ions combine to form hydrogen fluoride:

H2 + F2 → 2 H+ + 2 F− → 2 HF

Importance of Redox Reactions

Oxidation-reduction reactions are vital for biochemical reactions and industrial processes. The electron transfer system in cells and oxidation of glucose (respiration) in the human body are two examples of redox reactions. Photosynthesis is another redox reaction. There are five main groups of redox reactions namely: Respiration, corrosion, combustion, electrochemical (batteries) and bleaching. Redox reactions are often written in pairs separating out the oxidizing and reducing changes. Respiration & Photosynthesis

Cellular respiration, for instance, is the oxidation of glucose (C6H12O6) to CO2 and the reduction of oxygen to water. You can see the summary reaction above. Photosynthesis is essentially the reverse of the redox reaction in cell respiration. Photosynthesis involves the reduction of carbon dioxide into sugars and the oxidation of oxygen in water into O2. Biological energy is frequently stored and released by means of redox reactions.


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Combustion

In many redox reactions, such as combustion, it can be much more difficult to see how the electrons are moving around.

In the combustion of methane we can see that the oxidation state of carbon has changed from -4 to +4. Carbon is able to exist in several oxidation states.


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This picture shows all of the oxidation states for this reaction. You can see that oxygen has gained electrons and carbon lost them. Rusting –Corrosion In this example of corrosion Iron gives up electrons becoming a Cation with the electrons reacting with water and oxygen to form hydroxide ions.


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Electrochemical reactions These redox reactions are the basis for batteries and galvanizing and electro- plating of metals. The electron flow between the metals is carried by the wire and registers on the voltmeter. Magnesium gives up electrons as the anode and Copper accepts the electrons.

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Bleaching

Chlorine is a good example of an oxidising agent that gains electrons (RIG) as it is reduced in the reaction. Chlorine is widely used in our society for this property.


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Heat of reactions – endothermic and exothermic

Many chemical reactions release energy in the form of heat, light, or sound. These are exothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness or entropy.

Entropy is a term used to describe the degree of disorder in any system, Zero entropy is achieved at extremely low temperatures. In the lab, exothermic reactions produce heat or may even be explosive.

There are other chemical reactions that must absorb energy in order to proceed. These are endothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur.

When endothermic reactions absorb energy, a temperature drop is measured during the reaction. Endothermic reactions are characterized by heat flow into the reaction.


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Examples of Endothermic and Exothermic Processes

Photosynthesis is an example of an endothermic chemical reaction. In this process, plants use the energy from the sun to convert carbon dioxide and water into glucose and oxygen. This reaction requires 15MJ of energy (sunlight) for every kilogram of glucose that is produced:

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

A simpler example is melting ice – The ice absorbs energy and changes phase to liquid from that has a higher level of embodies energy.


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An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5Cl2(s) = NaCl(s)

Rate of reactions and activation energy In the table below an exothermic reaction is shown as a graph between energy and reaction progress/time. This shows the different changes a reaction goes through energetically. Reactants need a certain amount of activation energy Ea to commence the process; in strongly exothermic reactions this can be quite small. Once the activation energy is supplied then the chemical bonds break this is called the transition stage, electrons are transferred and the final products formed. Note the symbol ΔH means the change in energy of a reaction. Definition of activation energy - The least amount of energy needed for a chemical reaction to take place. Some elements and compounds react together naturally just by being close to each other, and their activation energy is zero. Others will react together only after a certain amount of energy is added to them. Striking a match on the side of a matchbox, for example, provides the activation energy (in the form of heat produced by friction) necessary for the chemicals in the match to ignite.

Activation energy is usually expressed in terms of joules per mole of reactants. In this process energy is given off and the products are at a lower energy state than the reactants.


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For endothermic reactions the table is reversed. Here a considerable amount of activation energy is needed to commence the reaction. This energy becomes embodied in the new chemical bonds formed.


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Factors that Affect the Chemical Reaction Rate

It's useful to be able to predict whether an action will affect the rate at which a chemical reaction proceeds. There are several factors that can influence the rate of a chemical reaction. In general, a factor that increases the number of collisions between particles will increase the reaction rate and a factor that decreases the number of collisions between particles will decrease the chemical reaction rate. These factors are

o Concentration of reactants o Temperature o Medium o Catalysts

Concentration of Reactants

A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increased reaction rate. Similarly, a higher concentration of products tends to be associated with a lower reaction rate.


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Temperature

Usually, an increase in temperature is accompanied by an increase in the reaction rate. Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. A general rule of thumb for most (not all) chemical reactions is that the rate at which the reaction proceeds will approximately double for each 10°C increase in temperature. Once the temperature reaches a certain point, some of the chemical species may be altered (e.g., denaturing of proteins) and the chemical reaction will slow or stop.

Medium

The rate of a chemical reaction depends on the medium in which the reaction occurs. It may make a difference whether a medium is aqueous or organic; polar or non polar; or liquid, solid, or gaseous.

Presence of Catalysts and Competitors

Catalysts (e.g., enzymes) lower the activation energy of a chemical reaction and increase the rate of a chemical reaction without being consumed in the process. Catalysts work by increasing the frequency of collisions between reactants, altering the orientation of reactants so that more collisions are effective, reducing intramolecular bonding within reactant molecules, or donating electron density to the reactants.

Biological systems make extensive use of catalyst pathways through enzymes. This allows quite detailed reactions to proceed at relatively low temperatures and in the company of many other chemicals to very specific ends.

The presence of a catalyst helps a reaction to proceed more quickly to equilibrium. Aside from catalysts, other chemical species can affect a reaction. The quantity of hydrogen ions (the pH of aqueous solutions) can alter a reaction rate. Other chemical species may compete for a reactant or alter orientation, bonding, electron density, etc., thereby decreasing the rate of a reaction.

http://chemistry.about.com/od/chemicalreactions/a/catalysts-catalysis.htm


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A catalyst permits a different energy pathway for a chemical reaction which has lower activation energy. The catalyst is not consumed in the chemical reaction and it may participate in multiple reactions at a time.

Catalysts often react with reactants to form intermediates that eventually yield the same reaction products and regenerate the catalyst. Note that the catalyst may be consumed during one of the intermediate steps, but it will be created again before the reaction is completed.

The only difference between a catalysed reaction and an uncatalysed reaction is that the activation energy is different. There is no effect on the energy of the reactants or the products. The ΔH for the reactions is the same.

Positive and Negative Catalysts

Usually when someone refers to a catalyst, they mean a positive catalyst, which is a catalyst which speeds up the rate of a chemical reaction by lowering its activation energy. There are also negative catalysts or inhibitors, which slow the rate of a chemical reaction or make it less likely to occur.

Promoters and Catalytic Poisons

A promoter is a substance that increases the activity of catalyst. A catalytic poison is a substance that inactivates a catalyst.


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Lesson 6 Homework Q1 Explain what a redox reaction is and give three examples Q2 The following graph demonstrates what sort of chemical reaction in terms of energy?

Q3 Explain the concept of activation energy Q4 Provide a definition of entropy in your own words Q5 Describe four factors that can influence the rate of chemical reaction These factors are Q6 Describe how catalysts work and what advantage they provide for biological systems Q7 In the following reaction what chemical species are being reduced and which oxidised.


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Q8 A chemical reaction (solution) is proceeding at a set rate at temperature of 10 degrees Celsius. You heat the solution to 20 degrees Celsius. Describe what change you expect


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chemical lesson 6 - students website · chemistry module 1- lesson 6 5 redox reactions -...

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