Chemistry - C8 / Topic 4(?) - Rates and equilibrium
Read through front and back of 'Analysis of Indigestion Tablets and Hydrochloric Acid'
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Read through method of experiment on 'Investigating chemical cells'.
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Read through pink booklet on 'Required practical 4 - temperature changes'- Investigation of the temperature change which takes place when an acid is neutralised by an alkali.
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Read through pink sheet on 'Required practical 5b - measuring the volume of gas produced' - Indigestion tablets and hydrochloric acid.
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Read through practical method and look at / learn safety symbols.
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Read through second half of inside page and that whole booklet.
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Read through, understand and remember method for Required practical 5a - observing colour change - Investigate how changes in concentration affect the rates of reactions - graph on next page
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Remind yourself / learn definitions e.g. accuracy, calibration etc.
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Using Bond Energies to Estimate Energy Changes Occurring During Chemical Reactions.
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YOU CAN DO THIS GCSE :)))))
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What are 2 important points about the reaction?
1) It is reversible 1) The forward reaction is exothermic. Thus we would not expect total conversion of reactants to products and that a position of equilibrium would be reached.
Read over rest of those questions - 'Fuel cell questions'.
:)
In the experiment you did, what type of reaction was heat of solution?
Endothermic. In our experiment, -11°C.
How do you calculate rate of reaction in this case?
= quantity of product formed / time take = volume of carbon dioxide formed / time taken
How are reversible reactions represented and what does this symbol mean?
A + B ⇌ C + D Where the ⇌ sign means the react can proceed in either direction.
Go over questions on both graphs in 'Reaction rate graphs' booklet.
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Go over sheet and graph before it on 'Calculating rates of reactions'. Answers are on next sheet.
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Go over that booklet - 'Hydrogen as a fuel'
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Go over white 'Rates of Reaction' question pack.
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Go over white sheets on 'Rates of reaction' - AQA Science - Exam-style questions.
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Look at extension questions in that booklet.
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Look at graph showing 'amount of time it takes for paper cases to fall to the floor'. Were there any more sheets for that experiment?
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Look at practical diagram and table we collected data in.
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Look at sheet on 'Using bond energy data' and learn how to draw diagram on the back.
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Look at whole of that pink booklet. You can do it! :)
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Make sure / do your best to know method for that experiment too.
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Make sure you know different between ammonia, ammonium and ammonium hydroxide - symbols etc. Is ammonia (solution?) same as ammonium hydroxide?
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N.B. All reversible reactions are e̲x̲o̲t̲h̲e̲r̲m̲i̲c̲ in one direction and e̲n̲d̲o̲t̲h̲e̲r̲m̲i̲c̲ in the other.
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N.B. If a change is imposed on a system in dynamic equilibrium, the position of equilibrium will shift to oppose that change.
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N.B. Therefore we must NOT forget the signs when considering bond making and bond breaking. bond breaking's sign of ΔH = + bond breaking's sign of ΔH = -
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Pressure
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Read 'How to draw graphs on Excel'.
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Read 1st half of inside page of that booklet and understand too. Check over something to do with more reactive metals displacing others? Displacement reaction?
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Read over how we did each experiment and try to find out if those types of reactions are always exo/endothermic. = Read over that sheet. Be able to recognise those types of reactions.
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Read through back of that sheet on 'Investigation of a reversible reaction'.
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Read through booklet on 'Using the glossary of scientific terms'.
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How do you know that as the time increases for the reaction to complete, the concentration of the line must be higher?
(As concentration increases, rate of reaction decreases). The steeper the gradient, the higher the rate of reaction. More concentrated reactions, take more time to complete and level out.
.Go over back of that sheet on 'Carrying out Calculations'
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Do questions on next white sheet on 'Indigestion Tablets and Hydrochloric Acid'.
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Go over 'Bond energies' question sheet, particularly go over mistakes as only got 16/25. YOU CAN DO IT! :)
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Go over 'Exam Question' on white sheet.
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Go over 'Some Calculations Using Bond Energies'. Make sure you can do them and recognise which is exo/endo. Get + or - sign right.
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Go over questions on back of last page of that booklet.
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Consider the system: A + B ⇌ C + D 1. What could you say about the four chemicals if a state of dynamic equilibrium is reached? 2. What could you say if there is more C + D (products) and less A + B (reactants)? 3. Conversely, what could you say if at equilibrium there is more A + B (reactants) and less C + D (products)?
1. If a state of dynamic equilibrium is reached, chemical A, B, C and D will be present in the closed system with constant, but not necessarily equal, concentrations. 2. The position of equilibrium lies to the right. 3. The position of equilibrium lies to the left.
What is the equation for hydrogen being oxidized by reaction with oxygen.
2H₂(g) + O₂(g) → 2H₂O(I)
What is needed to break such bonds, and what is this?
Energy is needed to break such bonds. This energy is known as the bond energy. The bond energy is the energy needed to break one mole of the specified bonds.
How and why did sub-division (change in state? or change in surface area?) affect the rate of reaction?
As surface area increases, rate of reaction increases. More collisions per second, in this case between Magnesium and H⁺ ions. This means there are more successful collision, and thus an increased rate of reaction.
How does an electrochemical cell generate electricity?
An electrochemical cell generates electricity by using chemical reactions.
What are the advantages and disadvantages of using hydrogen fuel cells?
Advantages: Does not produce problem pollutants e.g. no NOx, no CO, no CO₂, no SO₂, no soot = environmentall friendly. Disadvantages: 1. 2 methods - one consumes methane, produces CO₂, needs higher temp. and more energy. More energy goes into producing than released by fuel cell!!! Other method involves electrolytic breakdown of water but involves burning of fossil fuels. Alternative is use wave/wind power but there are problems. 2. Transport and storage of hydrogen - systems have to be built (pipeline. hydrogen stations etc.), large vols of H₂ under pressure, burns easily. Additionally, car can only carry small amount, limiting distance. Liquefying gas has safety implications. 3. Problems in fuel cell design - individual fuel cells dont give high enough current, limited by how quickly gases can move through porous electrodes and react. Also cells produce low P.D (1.2V). To overcome, cells combined in series and parallel = greater p.d. and current respectively. This causes battery to be very large, needing her temps and pressures. Catalysts may be used. Fuel cell can get very hot. 4. Cost of Fuel Cell - first fuel cells cost a lot of money to produce and not very efficient. Now, cost-effective hydrogen fuel cells being made, already prototype cars powered by them. However before such cars exist, all the above problems have to be addressed.
What evidence is there that a fair test was conducted by ensuring the same mass of calcium carbonate was used in each experiment?
All four of the experiments produce the same amount of carbon dioxide when complete (36cm³). NOT that they all finish at the same time because they don't, due to different concentrations therefore different rates of reactions.
What are the uses of ammonia?
Ammonia is a very important chemical. It is used to make: 1. Artifical Fertilizers - e.g. ammonium nitrate, sulphate and urea. This is most important use of ammonia. 2. Nitric Acid - oxidized to give the nitric acid. (some of which is reacted with ammonia to give ammonium nitrate) 3. Man-made Fibres - e.g. Nylon 4. Other uses - includes making dyes detergents etc.
What can allow chemists to calculate a rate of reaction?
Any measurable physical change per unit time.
How and why did temperature affect the rate of reaction?
As temperature increased, the rate of reaction increased. The H⁺ ions would have more kinetic energy as temperature increases. There would be more collisions per second with the particles having activation energy. There would be more successful collisions (per second?) and hence the rate of reaction is increased.
Look at graph, next in folder on graph paper. Use the graph to estimate how much sodium hydroxide solution was needed to neutralise 30cm³ (in method I think) of dilute hydrochloric acid. Show your working on your graph.
As the concentration of the acid & alkali were the same, and they react in 1:1 ratio from the equation. Since we used 30cm³ of acid in the equation, we expected to need to use 30cm³ alkali (for neutralisation). From my graph, I have the value as 26cm³ (extrapolated from where the two lines of best fit cross, I think, should it cross at 30 on y axis?) NaOH (alkali) needed for neutralisation.
Why is the hydrogen fuel cell attractive and what can it be used to do?
Attractive because: 1. Overall conversion of chemical to electrical energy is quite efficient (and electrical engines are much more efficient than combustion engines.) 2. The product of the oxidation process is water. Can be used to drive an electrical motor, as well as other electrical devices.
How is the process/reaction made more efficient?
Cooling rapidly the mixture of gases that exit the catalyst chamber, thus condensing ammonia to give liquid ammonia - which is removed. Less of it can breakdown in reverse reaction. Unreacted nitrogen and hydrogen remain gaseous and are recycled.
What does adding a catalyst do then?
Catalysts speed up forward and reverse reactions b̲y̲ ̲t̲h̲e̲ ̲s̲a̲m̲e̲ ̲a̲m̲o̲u̲n̲t̲. Adding a catalyst allows equilibrium to be attained f̲a̲s̲t̲e̲r̲ but will n̲e̲i̲t̲h̲e̲r̲ ̲i̲n̲c̲r̲e̲a̲s̲e̲ ̲n̲o̲r̲ ̲d̲e̲c̲r̲e̲a̲s̲e̲ a percentage yield of a product.
How can chemist often move the position of equilibrium and what is this important to?
Chemists can often move the position of equilibrium by imposing different conditions on the system, i.e. changing: Temperature Pressure This is often important in industry, for instance to maximise product yields (i.e. shifting the position of equilibrium to the right.
What must there be for a chemical reaction to occur?
Collisions between reactant particles.
Why is there a decrease in the mass during the reaction?
Because a gas is produced, CO₂.
Is bond breaking exothermic or endothermic? Therefore, what is bond making?
Bond breaking = endothermic. e.g. H-H → 2H ΔH = +436 kJ/mol The reverse of this change is exothermic because we are referring to bond making. e.g. 2H → H-H ΔH = -436kJ/mol
How are electrochemical cells created?
By harnessing the different in reactivity between two metals.
Therefore, how must you increase the rate of reaction?
By increasing the number of successful collisions per second.
How is the ammonia made?
By reacting the purified gases in the ratio of 1 volume of nitrogen to 3 volumes of hydrogen (the ratio obtained from the equation).
How is ammonia manufactured?
By the Haber Process.
What governs the conditions used?
Compromise aimed at: i) Giving reasonable equilibrium yield of ammonia. ii) Giving reasonable rate. iii) Minimizing costs. iv) Minimizing energy requirements and energy waste (less effects on environment).
How was the catalyst (iron) chosen?
Catalyst speeds up rate by giving new reaction pathway with lower activation energy. U̲s̲i̲n̲g̲ ̲a̲ ̲c̲a̲t̲a̲l̲y̲s̲t̲ ̲g̲i̲v̲e̲s̲ ̲a̲ ̲f̲a̲s̲t̲ ̲r̲a̲t̲e̲ ̲a̲t̲ ̲a̲ ̲l̲o̲w̲e̲r̲ ̲t̲e̲m̲p̲e̲r̲a̲t̲u̲r̲e̲ ̲t̲h̲a̲n̲ ̲w̲o̲u̲l̲d̲ ̲b̲e̲ ̲n̲e̲e̲d̲e̲d̲ ̲i̲n̲ ̲a̲b̲s̲e̲n̲c̲e̲ ̲o̲f̲ ̲t̲h̲e̲ ̲c̲a̲t̲a̲l̲y̲s̲t̲.̲ ̲A̲l̲s̲o̲, more favourable in terms of equilibrium yield. Added benefit = less energy to attain equilibrium at lower operating temp required. Ultimately, less fossil fuel burnt, less adverse effect on environment. (Additionally, careful design of plant means energy release via chemical reaction can also be used productively in other parts of factory - rather than being lost to environment). BBC Bitesize: The use of an iron catalyst increases the rate of the reaction, but it does not alter the percentage yield of ammonia.
Explain how the presence of a catalyst may change the rate of a reaction.
Catalysts change the rate of chemical reactions but are not used up during the reaction. Different reactions need different catalysts. See later notes on catalysts. ADD MORE LATER? IMAGE NEEDED (not really)
What is the symbol for activation energy?
Ea
What 4 things did you test for as factors affecting rates of reaction?
Effect of light Effect of concentration Effect of temperature Effect of sub-division
If a reversible reaction takes place in a closed system then a state of what will be attained?
Equilibrium (or 'balance').
Summarise this reaction.
Exothermic and releases energy. Product of the oxidation is water, is not a pollutant. Obvious candidate for using hydrogen as a fuel would be motor vehicles. But it is inefficient. much energy released by burning gas is lost as heat to surroundings, instead of driving the vehicle.
Is the combustion of methane exothermic or endothermic?
Exothermic.
In the experiment you did, what type of reaction was heat of crystallization ?
Exothermic. In our experiment, +24°C
In the experiment you did, what type of reaction was heat of dilution?
Exothermic. In our experiment, +48°C.
In the experiment you did, what type of reaction was heat of neutralisation?
Exothermic. In our experiment, +5°C
In the experiment you did, what type of reaction was heat of displacement (heat of redox reaction)?
Exothermic. In our experiment, +7°C
Explain how changing the surface area of any solid reactants may change the rate of a reaction.
For the same mass of solid reactant: Decreasing particle size increases total surface area. Increasing total surface area allows more collisions per second between reactants. This increases the number of successful collisions per second, hence increasing the rate of reaction. Powders have larger surface area! See text below image.
In the modern world we require huge amounts of energy and there are various ways by which this energy is generated. One of the main ways involves burning fossil fuels. What are the two very obvious problems with burning fossil fuels?
Fossil fuels are running out. Burning them causes a great deal of pollution.
Complete the sentences. From this example we can see that: In __________ reactions the energy released when new bonds are ____ in the products is more than that needed to break existing bonds in the _________. In ___________ reactions the ______ needed to break ________ bonds in the reactants is more than the energy released when new bonds are made in the products.
From this example we can see that: In exothermic reactions the energy released when new bonds are made in the products is more than that needed to break existing bonds in the reactants. In endothermic reactions the energy needed to break existing bonds in the reactants is more than the energy released when new bonds are made in the products.
How do fuel cells work (I think this is what the answer is saying)?
Fuel cells are fed with an external source of fuel (e.g. , hydrogen), plus oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a voltage. The electricity provided by the fuel cell can be used to run an electric vehicle or to power other equipment.
What is meant by the term exothermic reaction?
Heat is produced by the reactants and given off to the surroundings.
What is meant by the term endothermic reaction?
Heat is taken in from the surroundings.
What are the typical conditions used?
High pressure - about 200 atmospheres (atm) Temperature - about 450°C Catalyst - iron
How was the condition of pressure (200 atm) chosen?
Higher pressure = more molecules in given volume. Greater likelihood of successful collisions, increase rate of reaction. However main reason: A̲ ̲h̲i̲g̲h̲e̲r̲ ̲p̲r̲e̲s̲s̲u̲r̲e̲ ̲f̲a̲v̲o̲u̲r̲s̲ ̲p̲r̲o̲d̲u̲c̲t̲i̲o̲n̲ ̲o̲f̲ ̲a̲ ̲g̲r̲e̲a̲t̲e̲r̲ ̲p̲r̲o̲p̲o̲r̲t̲i̲o̲n̲ ̲o̲f̲ ̲a̲m̲m̲o̲n̲i̲a̲ ̲i̲n̲ ̲t̲h̲e̲ ̲e̲q̲u̲i̲l̲i̲b̲r̲i̲u̲m̲ ̲m̲i̲x̲ ̲o̲f̲ ̲g̲a̲s̲e̲s̲.̲ Equation shows decrease in number of gas molecules present during reaction. 4mol → 2mol (CHECK) Another main reason: H̲i̲g̲h̲ ̲p̲r̲e̲s̲s̲u̲r̲e̲ ̲f̲a̲v̲o̲u̲r̲s̲ ̲t̲h̲e̲ ̲r̲e̲a̲c̲t̲i̲o̲n̲ ̲t̲h̲a̲t̲s̲ ̲r̲e̲s̲u̲l̲t̲s̲ ̲i̲n̲ ̲a̲ ̲r̲e̲d̲u̲c̲t̲i̲o̲n̲ ̲i̲n̲ ̲t̲h̲e̲ ̲n̲u̲m̲b̲e̲r̲ ̲o̲f̲ ̲g̲a̲s̲ ̲m̲o̲l̲e̲c̲u̲l̲e̲s̲ ̲p̲r̲e̲s̲e̲n̲t̲ ̲(̲t̲h̲u̲s̲ ̲a̲ ̲g̲r̲e̲a̲t̲e̲r̲ ̲y̲i̲e̲l̲d̲ ̲o̲f̲ ̲a̲m̲m̲o̲n̲i̲a̲,̲ ̲s̲e̲e̲ ̲e̲q̲u̲a̲t̲i̲o̲n̲ ̲f̲o̲r̲ ̲e̲q̲u̲i̲l̲i̲b̲r̲i̲u̲m̲.̲)̲ But high pressure is v̲e̲r̲y̲ expensive. Extremely high pressure = safety problems. BBC Bitesize: The pressure chosen for the Haber process is a compromise. A high pressure increases the percentage yield of ammonia but very high pressures are expensive.
What does the rate of a chemical reaction tell us?
How fast reactants are turned into products. Being able to speed up and slow down chemical reactions is important in everyday life and industry. You will meet several methods by which the rate of reaction can be f̲o̲l̲l̲o̲w̲e̲d̲in the class practical.
What was the word and balanced symbol equation for the reaction we tested?
Hydrochloric acid + sodium hydroxide → sodium chloride + water HCl + NaOH → NaCl + H₂O
If reactants are being converted to ________ at the same rate that products are being converted to __________ in a reversible system, there will then be what?
If reactants are being converted to products at the same rate that products are being converted to reactants in a reversible system, there will then be what? No overall change in the concentration or quantity of reactants or products.
Explain the effect of changing pressure in a dynamic equilibrium.
If you r̲a̲i̲s̲e̲ ̲t̲h̲e̲ ̲p̲r̲e̲s̲s̲u̲r̲e̲ on the system the position of equilibrium will oppose that change and m̲o̲v̲e̲ ̲t̲o̲ ̲t̲h̲e̲ ̲s̲i̲d̲e̲ ̲w̲i̲t̲h̲ ̲a̲ ̲l̲o̲w̲e̲r̲ ̲v̲o̲l̲u̲m̲e̲ (i.e. the side having fewer gas molecules). if you l̲o̲w̲e̲r̲ ̲t̲h̲e̲ ̲p̲r̲e̲s̲s̲u̲r̲e̲on the system the position of equilibrium will shift to oppose that change and m̲o̲v̲e̲ ̲t̲o̲ ̲t̲h̲e̲ ̲s̲i̲d̲e̲ ̲w̲i̲t̲h̲ ̲a̲ ̲h̲i̲g̲h̲e̲r̲ ̲v̲o̲l̲u̲m̲e̲ (i.e. the side having more gas molecules). If there are e̲q̲u̲a̲l̲ ̲g̲a̲s̲ ̲v̲o̲l̲u̲m̲e̲s̲ on each side, altering pressure will have no effect on the position of equilibrium.
Explain the effect of changing temperature in a dynamic equilibrium.
If you r̲a̲i̲s̲e̲ ̲t̲h̲e̲ ̲t̲e̲m̲p̲e̲r̲a̲t̲u̲r̲e̲ (supplying heat energy), the equilibrium will shift in the e̲n̲d̲o̲t̲h̲e̲r̲m̲i̲c̲ ̲d̲i̲r̲e̲c̲t̲i̲o̲n̲ to oppose this change and use up the extra heat that has been supplied. If you r̲e̲d̲u̲c̲e̲ ̲t̲h̲e̲ ̲t̲e̲m̲p̲e̲r̲a̲t̲u̲r̲e̲ (removing heat energy), the equilibrium will shift in the e̲x̲o̲t̲h̲e̲r̲m̲i̲c̲ ̲d̲i̲r̲e̲c̲t̲i̲o̲n̲ to give out more heat.
In an endothermic reaction the products are shown at ______ energy than the reactants. ΔH is ________. Energy is _____ __ from the surroundings. What are examples of such changes?
In an endothermic reaction the products are shown at higher energy than the reactants. ΔH is positive. Energy is taken in from the surroundings. Examples: a) Photosynthesis b) Thermal Decomposition c) Dissolving some salts in water (e.g. KCl or NH₄NO₃). Learn diagram. Remember, if you take in more stuff, the amount of stuff (energy) you have increases.
In an exothermic reaction the products are shown at a _____ energy than the reactants. ΔH is ________. Energy is _____ ___ to the surroundings. What are examples of such changes?
In an exothermic reaction the products are shown at a lower energy than the reactants. ΔH is negative. Energy is given out to the surroundings. Examples: a) Combustion of fuels b) Neutralisation c) Redox reactions d) Hydration reactions e) Respiration Learn diagram. Remember, if you lose stuff, the amount of stuff (energy) you have decreases.
What is rate of reaction like in most cases?
In most cases rate of reaction is highest at the start of the reaction and slows as the reaction proceeds, and at the end of a reaction the graphs level off as rate is zero.
Here we will be dealing with reactions involving covalent substances. What do the bonds involve in such substances?
In such substances the bonds involve electron sharing = covalent bonding.
Explain how changing concentrations of solutions or pressure of gases may change the rate of a reaction.
Increasing concentration of reactants in solution and the pressure of reacting gases increases the number of particles per unit volume. This increases likelihood of collisions between reactant particles. This result in increase in number of successful collisions per second and so increases rate of reaction. Like changing from solid to a powder(?) See text below image.
Explain how changing temperature may change the rate of a reaction.
Increasing temperature increases speed and kinetic energy of the reacting particles. This increases: a) number of collisions per second. b) number of colliding particles having the activation energy. Both of these factors result in an increase in number of successful collisions per second, hence increased reaction rate. See text below image.
How and why did light affect the rate of reaction?
It sped up the rate of reaction. The light has given more particles the activation energy so when they collide the collisions are successful. (reaction occurs / increases). should it be: so there are a high number of collisions so higher number of successful collisions? or not?
How was the condition of temperature (450°C) chosen?
Low temp favors reaction which releases heat. Thus left to right reaction = endothermic. T̲h̲e̲r̲e̲f̲o̲r̲e̲ ̲t̲h̲e̲ ̲e̲q̲u̲i̲l̲i̲b̲r̲i̲u̲m̲ ̲y̲i̲e̲l̲d̲ ̲o̲f̲ ̲a̲m̲m̲o̲n̲i̲a̲ ̲i̲s̲ ̲i̲n̲c̲r̲e̲a̲s̲e̲d̲ ̲b̲y̲ ̲u̲s̲i̲n̲g̲ ̲a̲ ̲l̲o̲w̲ ̲t̲e̲m̲p̲e̲r̲a̲t̲u̲r̲e̲.̲ However l̲o̲w̲ ̲t̲e̲m̲p̲e̲r̲a̲t̲u̲r̲e̲s̲ ̲=̲ ̲s̲l̲o̲w̲e̲r̲ ̲r̲a̲t̲e̲ ̲o̲f̲ ̲r̲e̲a̲c̲t̲i̲o̲n̲! Raising temp gives more gas particles energy, more (successful) collisions, higher rate of reaction. T̲h̲u̲s̲ ̲a̲ ̲h̲i̲g̲h̲e̲r̲ ̲t̲e̲m̲p̲e̲r̲a̲t̲u̲r̲e̲ ̲f̲a̲v̲o̲u̲r̲s̲ ̲a̲ ̲g̲r̲e̲a̲t̲e̲r̲ ̲r̲a̲t̲e̲ ̲o̲f̲ ̲r̲e̲a̲c̲t̲i̲o̲n̲.̲ Problem is h̲i̲g̲h̲ ̲t̲e̲m̲p̲e̲r̲a̲t̲u̲r̲e̲ ̲g̲i̲v̲e̲s̲ ̲l̲o̲w̲e̲r̲ ̲e̲q̲u̲i̲l̲i̲b̲r̲i̲u̲m̲ ̲y̲i̲e̲l̲d̲ ̲o̲f̲ ̲a̲m̲m̲o̲n̲i̲a̲.̲ Temperature chosen is c̲o̲m̲p̲r̲o̲m̲i̲s̲e̲ = acceptable equilibrium yield at acceptable rate. BBC Bitesize: The temperature chosen is also a compromise. A high temperature gives a fast reaction but decreases the percentage yield of ammonia. 450°C gives a reasonably fast reaction with a sufficiently high percentage yield of ammonia.
Many reversible reactions have a greater ______ on one side, either of _________ or ________. More volume means that there are more reactants or products and less volume means that there are less gaseous molecules.
Many reversible reactions have a greater volume on one side, either of reactants or reactants. M̲o̲r̲e̲ ̲v̲o̲l̲u̲m̲e̲ means that there are m̲o̲r̲e̲ ̲g̲a̲s̲e̲o̲u̲s̲ ̲m̲o̲l̲e̲c̲u̲l̲e̲s̲ and l̲e̲s̲s̲ ̲v̲o̲l̲u̲m̲e̲ ̲means that there are l̲e̲s̲s̲ ̲g̲a̲s̲e̲o̲u̲s̲ ̲m̲o̲l̲e̲c̲u̲l̲e̲s̲.
To measure rates of reaction experiments must be carried out to follow the progress of a reaction over time. What do common examples of how progress is monitored include?
Mass of reactant lost over time Mass of product gained over time Volume of gas produced over time Transmittance of light through reaction mixture over time.
What is the equation for mean rate of reaction?
Mean rate of reaction = quantity of product formed / time taken
How can you find out the rate of a chemical reaction?
Measuring the amount of reactant used or the amount of a product formed over time. Rate of reaction = Amount of reactant used (or product formed) / time
What are the raw materials involved and where are they obtained from?
Nitrogen - obtained from the air Hydrogen - obtained from natural gas (or from the naptha fraction of crude oil).
Are many mass-produced batteries rechargeable? What does this mean?
No, many mass-produced batteries are not rechargeable. This means that once one pf the reactants has run out the cell stops working and should be disposed of safely.
Will a hydrogen fuel cell run out?`
No, unlike a conventional battery a hydrogen fuel cell does not run out and does not need recharcging. Produces electrical energy as long as hydrogen and oxygen available.
What does adding a catalyst do to the position of equilibrium?
Nothing - adding a catalyst does n̲o̲t̲ affect the position of equilibrium.
Where can the chemical energies of reactants and products and the heat energy exchanged with the surroundings be plotted?
On an energy level diagram.
How can other cells be recharged?
Other cells can be recharged by reversing the reactions at each electrode through the use of an external power supply, such as the batteries in laptops, phones and cars. This process regenerates the original reactants.
What is an alternative use of hydrogen?
Oxidize in a hydrogen fuel cell.
What are the four factors that can change reaction rates?
Presence of a catalyst. Changing the surface area of any solid reactants. Changing concentrations of solutions or pressure of gases. Changing temperature.
Explain what is meant by a fuel cell.
Source of electrical energy from chemical energy.
Look at line graph you made. What does the gradient equal? Gradient = volume of carbon dioxide collected (cm³) / Time (s)
Rate of reaction. e.g. for 0.5mol/dm³: 64/176 = 0.26cm³/s If can't remember above, think about equation of gradient, = cm³ / s
Complete the sentences. Reacting chemicals (_________) have a certain amount of chemical ______ associated with them. Product chemicals (________) have a certain amount of ________ energy associated with them. When the reactants are changed to the products by the chemical ________ there is either ____ or ____ of chemical energy. The change in chemical energy is accompanied by an ________ of ______ (usually heat energy) between the ______ of chemical and its ____________.
Reacting chemicals (reactants) have a certain amount of chemical energy associated with them. Product chemicals (products) have a certain amount of chemical energy associated with them. When the reactants are changed to the products by the chemical reaction there is either gain or loss of chemical energy. The change in chemical energy is accompanied by an exchange of energy (usually heat energy) between the system of chemicals and its surroundings.
What is activation energy?
The activation energy is the minimum amount of energy needed to start a chemical reaction. Like an energy barrier that must be overcome if reactions are to occur. Measure of the energy that must be absorbed in order to break existing bonds and separate atoms before new bonds can be made. It can included in the energy level diagram.
What is the minimum amount of energy particles must have to react on collision called?
The activation energy.
What do these reactions rely on and what are chemical cells used to power?
The reactions rely on the differences in reactivity between different metal. Chemical cells are used to power batteries and are essential for the function of many electrical appliances e.g. mobile phones, laptops and torches.
What was the conclusion of your experiment?
The bigger the voltage, the further the metals difference in reactivity.
What can be seen from that graph?
The higher the concentration, the higher the initial rate of reaction.
What happens in a hydrogen fuel cell?
The hydrogen is oxidized by oxygen (from the air) in an electrochemical reaction that generates electricity. In the fuel cell the hydrogen is still converted to water.
How and why did concentration affect the rate of reaction?
The more diluted (= lower the concentration), the slower the reaction. When concentration is decreased: For the same volume of acid, there would be less H⁺ (acid) ions (when (more) diluted). There would likely be fewer collisions per second. Hence, there would be fewer successful collisions so the rate of reaction would go down.
Look carefully at the curve for reaction 2. Describe what happens to the rate of reaction as the reaction proceeds.
The rate is constant until 30cm³. After this the rate of reaction begins to slow down.
Look at this diagram set up and read the text (flip).
The reaction occurring is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
What symbol represents this exchange of heat energy and what are its units?
The symbol ΔH (Upper case delta H). Its units are usually measured in kJ / mol.
What is this kind of system referred to as and why?
The system is referred to as 'dynamic' as both reactions are still occurring and as 'equilibrium' as there is no net change in quantities of either reactants or products.
What is an example of this type of cell?
The zinc-carbon dry cell. In this cell the electrode are mode of zinc (negative terminal) and carbon (positive terminal). This cell produces a voltage of 1.5V. See figure 2: A zinc-carbon cell.
What are the units of rate of reaction?
They are dependant upon the data supplied, but must be 'per unit time'
What is the purpose of the cotton wool?
To stop gas escaping from the flask.
How does a simple electrochemical cell work?
Two metals are joined together with a wire and dipped into an electrolyte then electrons flow through the wire from the more reactive metal to the less reactive metal.
What does covalent bonding happen between?
Two non-metals. http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/atomic/covalentrev1.shtml
How do we usually write out the change and what do + and - mean?
We usually write out the change. followed by the value of ΔH, NOT forgetting to write in the sign. - a negative sign shows heat energy is released to the surroundings. + a positive sign shows heat energy is absorbed from the surroundings.
BBC Bitesize economical considerations for Haber process.
When a chemical is manufactured, the optimum conditions used are the ones that give the lowest cost. These conditions are not necessarily the ones that give the fastest reaction or highest percentage yield. For example: the rate of reaction must be high enough to make enough product each day the percentage yield must be high enough to make enough product each day. Note that a low percentage yield could be accepted if unreacted started materials can be recycled (as they are in the Haber process).
When can chemical reactions only occur?
When particles collide with sufficient energy.
When is a position of equilibrium reached?
When the rate of the forward reaction is equal to the rate of the backward reaction. A + B⇌ C + D
If a graph line has a curve, what part of the reaction's rate can you calculate and how?
You can calculate the initial rate of reaction by meaauring the first part of the line (straight bit).
Aside from any examples your teacher may have provided, these changes are best illustrated by what?
Your study of the manufacture of ammonia - the Haber Process.
What is the word and balanced symbol equation for the Haber process?
nitrogen + hydrogen ⇌ ammonia N₂ + 3H₂(g) ⇌ 2NH₃(g)
What does the term ΔH refer to?
ΔH refers to a change in a given direction.
