Rates of reactions A car rusting – is a slow reaction – it takes a long time to happen A tanker explosion – happens very quickly !

Rates of reaction Reactions can be very fast, like fireworks or explosives, but they can also be very slow – such as an apple turning brown.

Time taken for the change
Rate of reaction Rate of reaction is defined as the change in concentration per unit time of any one reactant or product Do a few examples. Then plan the experiment. Change in concentration of the chosen reactant/product Time taken for the change Rate =

6.1 Monitoring the rate of production of oxygen from hydrogen peroxide, using manganese dioxide as a catalyst. NB: Wear your safety glasses. 1. Measure out 5 cm3 of hydrogen peroxide and dilute to 50 cm3 with water. Place it in the conical flask. 2. Weigh about 0.5 g manganese(IV) oxide into the small test tube, and use the thread and stopper to suspend the test tube in the conical flask. Avoid contact between the manganese(IV) oxide and the hydrogen peroxide. 3. Place sufficient water in the trough to allow the graduated cylinder to be filled with water and inverted over the beehive shelf. Using a teat pipette, inject air into the graduated cylinder until the water level is at the 10 cm3 mark. 4. Arrange the delivery tube for the oxygen produced to be collected in the graduated cylinder by displacement of water. 5. Loosen the stopper momentarily to allow the thread to fall into the flask and shake vigorously, thus bringing the manganese(IV) oxide into contact with the hydrogen peroxide. The stop-clock should be started as this contact is made. Record the total volume of gas in the graduated cylinder every 30 seconds. Readings should be taken at eye level. 1. Why is the slope of the graph steepest in the early stages of the reaction? Since rate is proportional to concentration, the greatest rate, indicated by the steepest slope, is evident in the early stages when the concentration of hydrogen peroxide is at a maximum. 2. At what stage is the reaction complete? When the graph becomes horizontal. 3. What would be the effect on the graph of doubling the amount of manganese(IV) oxide? The increased surface area of catalyst would speed up the reaction, giving a steeper slope and an earlier completion. The volume of oxygen produced would be unchanged. 4. Would doubling the manganese(IV) oxide create a practical difficulty? Explain your answer. Yes. The production of oxygen could become too quick for accurate monitoring. 5. What would be the effect on the graph of doubling the concentration of hydrogen peroxide? Increasing the concentration of a reactant would speed up the rate, as indicated by a steeper slope. Doubling the concentration would produce double the final volume of oxygen. 6. Would doubling the concentration of hydrogen peroxide create a practical difficulty? Explain your answer. Yes. The capacity of the collection vessel could be exceeded.

Interpretation of reaction rate graphs
Ask what the volume is after 1 minute 2 minutes, approx 1.5 minutes extra – why is this not accurate?

Total time taken for the change
Higher level only The average rate You can work out the average rate of reaction.. Average rate of reaction = GET THEM TO WORK OUT SME EXAMPLES ON PAPER AND FROM A GRAPH Total change in the concentration of a product /reactant Total time taken for the change

Average rate from the graph

Percentage completion of reaction
Slower and slower Reactions start fast and get slower and slower. This is not surprising because the reactant concentration (and the chance of collision) gets lower and lower as time progresses. fast 25 slower 75 very slow 100 stopped product reactants Percentage completion of reaction

Rate graphs and reactant concentrations
Amount of product Time All product Mix of reactant And product Reactant Concentration falls Rate of Reaction falls reactants product Gradient of graph decreases All reactant

The instantaneous rate
Higher level only The instantaneous rate The instantaneous rate is the rate at a particular moment in time during the reaction. You can find the instantaneous rate by looking at the graph…

Finding the instantaneous rate
Higher level only Finding the instantaneous rate Pick the moment in time that you want to measure the instantaneous rate for. Draw a tangent line to the curve at this point Find the slope of this line Slope = y2- y1 x2 – x1 4. Make sure units are correct ! Pick

Point 2: (1.5, 46) x2 y2 Point 1: (0.5, 28) x1 y1 :Slope = y2- y1
x2 – x1 = 46 – 28 1.5 – 0.5 = 18 1 Units 18cm3 min-1

Point 2: (1.5, 46) x2 y2 Point 1: (0.5, 28) x1 y1 Slope = 18
Units 18cm3 min-1 Slope = 18 Instantaneous rate = 18 cm3 min-1

2008 QUESTION 3 (a) DRAW: circle (ball, sphere, ring) with flames rising (5) (b) WRITE: 2H2O2 → 2H2O + O2 (c) DRAW: apparatus with hydrogen peroxide and catalyst separate (3) arrangement for mixing at a precise time (6)

4.0 – 6.0 cm3 min–1 (from correctly drawn tangent) (6)
[Allow 3 marks for tangent drawn at 5 minutes]

EXPLAIN: greatest rate // highest concentration of H2O2 // most collisions (3)
(ii) g 78 ÷ 1000 ÷ 24 = mol × 32 = 0.104

Objectives for today To learn the effects of concentration and temperature on reaction rate

6.2 Factors affecting the rate of a reaction

Rates of reaction Chemical reactions occur when particles of reactant collide with enough energy to react.

Factors affecting the rate of a reaction:
Concentration of reactants Particle size Temperature Nature of Reactants Catalysts

Collisions infrequent
Concentration Reactions in solution involve dissolved particles that must collide before reaction is possible. The more crowded (concentrated) the solution, the faster the reaction. The rate of a reaction increases with concentration of the reactants due to more effective collisions of reactants Collisions infrequent Collisions frequent

How would increased concentration change this graph?
Steeper shape and also higher colume of oxygen made

Extra surface for molecules to collide with.
Surface area The reactions of solids can clearly only take place at the surface of the solid. If we break a solid into smaller pieces we get more area and a faster reaction. Molecules collide with the surface of the solid Extra surface for molecules to collide with.

Surface area If we grind up a solid to a powder we massively increase the surface area. We therefore massively increase the rate of any reaction Slow Very fast

Particle size The reaction rate is faster when the particle size is smaller This is because it increases the surface are of the particles in the reaction and causes more effective collisions

How would smaller particle size change this graph?
Steeper shape and also higher colume of oxygen made

Which glass has the crushed tablet?
An indigestion tablet fizzes in water – but fizzes much faster if it is crushed. Which glass has the crushed tablet? A B Solid Crushed

Dust explosions Any solid material that is a dust will burn in air at an increased rate! If the heat released is great there will be an explosion.. This can happen if any source of electricity is applied to a dust in air… an cause explosions in flour factories, drug powder manufacturing factories, coal mines etc. Static electriccity can cause this

Check your learning… What is the rate of reaction?
How does concentration of reactant effect the rate? Explain How does size of the particle effect the rate? Explain

Studying the effects on reaction rate of (i) concentration and (ii) temperature
2HCl(aq) + Na2S2O3(aq) → 2NaCl(aq) + SO2(aq) + S(s)↓ + H2O(l) The precipitate of sulfur formed gradually obscures a cross marked on paper and placed beneath the reaction flask. The rate of reaction can be determined from the time taken to obscure the cross.

Studying the effects on reaction rate of (i) concentration
Concentration of thiosulfate 0.l M 0.08 M 0.06 M 0.04 M 0.02 M Reaction time (s) 1/time

Draw a graph of 1/time against concentration
Draw a graph of 1/time against concentration. This is effectively a graph of reaction rate against concentration.

Questions on this concentration experiment
What is the effect of increasing the concentration on the reaction time? What is the effect of increasing the concentration on the reaction rate? What is meant by saying that two quantities are directly proportional?

Studying the effects on reaction rate of (ii) temperature
Reaction time (s) 1/time

Draw a graph of 1/time against temperature
Draw a graph of 1/time against temperature. This is effectively a graph of reaction rate against concentration.

Questions on this temperature experiment
What is the effect of raising the temperature on the reaction time? What is the effect of raising the temperature on the reaction rate? Suggest two factors responsible for the result observed. Suggest a reason why it is not recommended to carry out the experiment at temperatures higher than about 60 0C. Which is the limiting reactant in the temperature experiment? The reaction time is decreased. The rate is increased. The higher temperature results in greater kinetic energy of the particles present. This causes: (i) more collisions per unit time, and (ii) a greater proportion of the collisions to have the activation energy needed for products to form. Both (i) and (ii) result in a rate increase. The reaction occurs so quickly that it is not possible to measure the time100 cm3 of 0.05 M Na2S2O3 contains: 100/1000 x 0.05 = moles Na2S2O3 5 cm3 of 3 M HCl contains: 5/1000 x 3 = moles HCl According to the balanced equation, the reacting ratio is Na2S2O3 : HCl = 1:2 The amounts used are in the ratio Na2S2O3 : HCl = 0.005: = 1 : 3 Clearly Na2S2O3 is the limiting reactant. accurately.

Higher level only Activation energy Molecules must collide with each other to have a chemical reaction Not every collision results in a reaction – the particle must also have enough energy for the reaction to take place Activation energy Ea is the minimum energy that particles need to have when they collide with each other to cause a reaction.

Higher level only Activation energy When a collision between two particles results in a chemical reaction it is called an effective collision The reaction rate is determined by the number of effective collisions If there is an effective collision between two particles bonds will be broken and new ones will be formed The more effective collisons the faster a reaction will go.

Reaction profile diagram
Higher level only Reaction profile diagram A reaction profile diagram is used to show how the energy of the particles in a reaction changes. On average most of the reactant particles have this energy some will have higher and some will have lower temperatures – like ppl on the street The activation energy is high – particles need to have this energy to have an effective collision – only a very small amount will have this energy The products have lower energy than the reactants. Could mention exothermic reaction – heat is given out! Label diagrams is important!

Old bonds start to break
Activation energy Activation energy is the energy needed to start breaking the old bonds before a reaction can occur and new bonds can be formed Reactants New bonds form Old bonds start to break

Activation energy Activation energy Energy in chemicals
Reaction Activation energy Break old bonds Energy taken in Form new bonds Energy given out

Temperature and the rate of reaction
Higher level only Temperature and the rate of reaction An increase in temperature of a reaction causes the kinetic energy of the particles to increase Reactions go faster at higher temperatures because (i) more collisions occur (ii) more of the colliding particles have the minimum activation energy that they need to react. – the particles move around faster and have more energy than before.

Food spoils because of chemical reactions that occur.
Why does food remain usable for so much longer if it is kept in a freezer? The reactions that cause the food to go off will be slower because there will be fewer and “softer” collisions between molecules at a reduced temperature.

Before microwave ovens were common many people used pressure cookers.
It was a pan that stopped the water boiling until it reached about 115oC. How would this help cooking? The molecules move faster and collide more often and with more energy. Cooking times were greatly reduced.

More collisions per second
Temperature At higher temperatures molecules move faster. As a result there are more collisions per second and so a faster reaction occurs. Slow molecules are also less likely to lead to a reaction than fast ones. The rate of a reaction increases with the temperature of the reactants. More collisions per second Fewer collisions per second

Check your learning.. Explain the effect of concentration on reaction rate Explain the effect of particle size on reaction rate Explain the effect of temperature on reaction rate

2010 Q3 Rates (a) IDENTIFY: Sulfur / S (5)
(b) DESCRIBE: 3 points needed Measure 30 cm3 of the 0.20 M thiosulfate solution dilute (make up) to 50 cm3 using deionised water (c) WHY: any of the following So that only one variable is changed / to keep the test valid (fair) / so as to keep the total volume the same / so as not to change the conc. of another species present /

to keep the same depth / should be only two variables (6)

(e) USE: 2.30 ± 0.05 (6) [Use data from graph; mathematical “slip”– loses 1 mark..] (f) WOULD: Decrease (3) JUSTIFY: Reaction rate increases with temperature / more collisions reach activation energy / more effective collisions (3)

By the end of this class you should be able to:
Tell what a catalyst is Tell the difference between a homogenous a heterogeneous catalyst, and autocatalysis Explain why catalysts effect the rate of a reaction Give two examples of enzymes Describe how a catalytic converter works Say why unleaded fuel is so environmentally friendly

Catalysts A Catalyst changes the rate of a reaction but doesn’t get used up itself They are only needed small amounts Their action can be stopped by catalyst poisons Don't need to know about the arsenic Do examples of mno2 and h2o2 Why isnt is t called the symbol ar? Arsenic is a catalyst poison that stops many catalysts in our bodies working

Enzymes Enzymes are biological catalysts
Catalase is a biological catalyst that speeds up the rate of the following reaction: H ½ O2 + H20 Need to know two examples of enzymes Catalase is a common enzyme found in nearly all living organisms. Its functions include catalyzing the decomposition of hydrogen peroxide to water and oxygen.[1] Catalase has one of the highest turnover rates of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.[2] Hydrogen peroxide is a harmful by-product of many normal metabolic processes: To prevent damage, it must be quickly converted into other, less dangerous substances. To this end, catalase is frequently used by cells to rapidly catalyze the decomposition of hydrogen peroxide into less reactive gaseous oxygen and water molecules.[13] The true biological significance of catalase is not always straightforward to assess: Mice genetically engineered to lack catalase are phenotypically normal, indicating that this enzyme is dispensable in animals under some conditions.[14] Some human beings have very low levels of catalase (), yet show few ill effects. It is likely that the predominant scavengers of H2O2 in normal mammalian cells are peroxiredoxins rather than catalase. Catalase works at an optimum temperature of 37 °C, which is approximately the temperature of the human body. Catalase is usually located in a cellular organelle called the peroxisome.[15] Peroxisomes in plant cells are involved in photorespiration (the use of oxygen and production of carbon dioxide) and symbiotic nitrogen fixation (the breaking apart of diatomic nitrogen (N2) to reactive nitrogen atoms). Fresh pig's liver and celery are rich sources of catalase. The experiment: grind a pigs liver with a mortar and pestle and put it trough filter paper – keep the filtrate as this is the enzyme extract Have some h2o2 in a boiling tube and have the stopper loosely on top Add in some and watch fizzing – you can test with a glowing splint Demo – with a conical flask on a heat proof mat Need a wet cloth! You can also do this with some cut up celery To measure the rate you can put some celeryin eith some h202 (3%) in a syringe and measure the rate that the syringe is pushed back

Enzymes Amylase is a biological catalyst that speeds up the rate of the following reaction: Starch maltose Need to know two examples of enzymes Catalase is a common enzyme found in nearly all living organisms. Its functions include catalyzing the decomposition of hydrogen peroxide to water and oxygen.[1] Catalase has one of the highest turnover rates of all enzymes; one molecule of catalase can convert millions of molecules of hydrogen peroxide to water and oxygen per second.[2] Hydrogen peroxide is a harmful by-product of many normal metabolic processes: To prevent damage, it must be quickly converted into other, less dangerous substances. To this end, catalase is frequently used by cells to rapidly catalyze the decomposition of hydrogen peroxide into less reactive gaseous oxygen and water molecules.[13] The true biological significance of catalase is not always straightforward to assess: Mice genetically engineered to lack catalase are phenotypically normal, indicating that this enzyme is dispensable in animals under some conditions.[14] Some human beings have very low levels of catalase (), yet show few ill effects. It is likely that the predominant scavengers of H2O2 in normal mammalian cells are peroxiredoxins rather than catalase. Catalase works at an optimum temperature of 37 °C, which is approximately the temperature of the human body. Catalase is usually located in a cellular organelle called the peroxisome.[15] Peroxisomes in plant cells are involved in photorespiration (the use of oxygen and production of carbon dioxide) and symbiotic nitrogen fixation (the breaking apart of diatomic nitrogen (N2) to reactive nitrogen atoms). Fresh pig's liver and celery are rich sources of catalase. The experiment: grind a pigs liver with a mortar and pestle and put it trough filter paper – keep the filtrate as this is the enzyme extract Have some h2o2 in a boiling tube and have the stopper loosely on top Add in some and watch fizzing – you can test with a glowing splint Demo – with a conical flask on a heat proof mat Need a wet cloth! You can also do this with some cut up celery To measure the rate you can put some celeryin eith some h202 (3%) in a syringe and measure the rate that the syringe is pushed back

Catalysts and activation energy
Higher level only Catalysts and activation energy The catalyst provides an alternative reaction pathway with a lower activation energy Therefore more reaction molecules have the energy required for effective collisions to occur… the rate of reaction increases

Nature of Catalysts Homogeneous catalysts are in the same physical state as the reactants H ½ O2 + H20 KI (aq) Action• catalyst and reactants are in the same phase • reaction proceeds through an intermediate species of lower energy • there is usually more than one reaction step • transition metal ions are often involved - oxidation state changes

Nature of Catalysts Heterogeneous catalysts are in a different physical state to the reactants H ½ O2 + H20 MnO2 takes place at active sites on the surface of a solid gases are adsorbed onto the surface they form weak bonds with metal atoms In a heterogeneous reaction the reaction occurs on the surface of the catalyst

Nature of Catalysts Autocatalysis is when one of the products of the reaction catalyses it. It causes the reaction to change its rate as it goes along! MnO4 + 8H+ + 5Fe Mn+2 + 5Fe+2 + 4H20

Check your learning.. What is a catalyst?
How do the work to speed up reactions? Name two enzymes How can the nature of a catalyst be different?

Catalytic converters When cars burn fuel they give out gases such as carbon monoxide and nitrogen oxides. Carbon monoxide is a toxic gas and is very harmful Nitrogen oxides are toxic gases. They also contribute to acid rain

Catalytic converters It is made of thin coats of Platinum, Rhodium and Palladium catalysts on a ceramic support material inside a steel case. Catalytic converters are put into cars to make them more environmentally friendly Ceramic make it have a large surface area for the catalyst to work Ask what sort of catayst system do you think this is?

Catalytic converters The following reaction takes place inside the converter: CO + 1/2O CO2 Pt/ Pd/Rh

Catalytic converters 2. The following reaction takes place inside the converter: 2CO + 2NO CO2 + N2 Pt/ Pd/Rh

Catalytic converters The following reaction takes place inside the converter: 2CO + 2NO CO2 + N2 Pt/ Pd/Rh

Catalytic converters 3. The following reaction takes place inside the converter: C8H O CO2 + 9H2O Pt/ Pd/Rh

Catalytic converters Catalytic converters remove the toxic carbon monoxide and nitrogen oxides fumes from the car instead of letting them out into the air. Lead is a catalyst poison of Pt/ Pd/Rh so unleaded petrol must be used for the converter to work properly! It take s a while for them to heat up so cities are still poLlted

Can you.. Tell what a catalyst is
Explain why catalysts effect the rate of a reaction ( activation energy!!) Give two examples of enzymes Describe how a catalytic converter works, giving examples of the reaction Say why unleaded fuel is so environmentally friendly

a. The surface absorption theory
Higher level only a. The surface absorption theory 1. The solid catalyst adsorbs the reactant molecules onto its surface 2. When an effective collision happens the reactant’s bonds break and products are formed. 3. Finally the products are desorbed from the surface of the catalyst. Also could be held in a good position for attack ect – The reactant molecules are held in such a way that the effective collision between the molecules is more likely to happen – the reactants may be held close together for example!

Some catalysts absorb molecules in a way that not only stretches bonds but also brings the reacting parts of reactants right next to each other.

Example – reaction of ethene with hydrogen gas catalyst = Nickel
Example – oxidation of methanol using hot palladium catalyst

Catalysts – mechanism of action
Higher level only Catalysts – mechanism of action b. The intermediate formation theory Catalysts react with the reactants and form an unstable product – called an intermediate The unstable intermediate breakdown very easily to form the products and release the catalyst.

Example The oxidation of potassium sodium tartrate by hydrogen peroxide catalysed by cobalt (II) ions

Try now… HL 2004 q8 OL 2010 Q8

Nature of reactants Reactions for covalent compounds are usually slow: The bonds in the reactants have to be broken and new bonds have to be formed. CH4 + 2O H2O + CO2 + +

Nature of reactants Reactions rates for ionic compounds are usually fast: If the ionic compound is dissolved in water the ions will be pulled apart already and surrounded by water molecules When reactants are mixed the ions can collide and will react very quickly! NaCl (aq) + AgNO3 (aq) AgCl + NaNO3 (aq) What would happen if you evaporated the rest of the water out? How would you separate the agcl etc etc The Ag+ and Cl- ions are more attracted to each other than the water molecules and precipitate out of solution. The Na+ and NO3– ions stay in solution.

Rates of reactions A car rusting – is a slow reaction – it takes a long time to happen A tanker explosion – happens very quickly !

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Rates of reactions A car rusting – is a slow reaction – it takes a long time to happen A tanker explosion – happens very quickly !

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