rates

Slide 1: 

RATES OF REACTION

Rates of reaction : 

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.

Rates of reaction : 

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

Speeding up reactions : 

Speeding up reactions Anything that increases the chance of effective collision increases the rate (speed) of reaction. Factors include: Increased surface Area Increased concentration Increased temperature Use of a catalyst

Surface area : 

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 Very fast Slow Surface area

Slide 7: 

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

Concentration : 

Concentration Reactions in solution involve dissolved paticles that must collide before reaction is possible. The more crowded (concentrated) the solution, the faster the reaction. Collisions infrequent Collisions frequent

Pressure : 

Pressure Reactions involving gases are affected by the pressure of the gases present. If we cover one end of a bicycle pump and push in the plunger we increase the pressure. What we are doing is squeezing the gas molecules closer together or making them more concentrated. And so - pressure speeds up gas reactions Low pressure High pressure

Slide 11: 

The Haber Process, in which nitrogen reacts with hydrogen to form ammonia, is carried out at 200 atmospheres pressure. How and why will this affect the rate of reaction? The particles will be 200 times closer together and so will collide much more often. The reaction will be much faster. compress

Temperature : 

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. Fewer collisions per second More collisions per second

Slide 13: 

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.

Slide 14: 

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.

Activation energy : 

Activation energy Chemical reactions involve the formation of bonds between atoms but often before new bonds can be formed old ones have to be broken. This means that there has to be enough energy (activation energy)to start breaking the old bonds before a reaction can occur. Activation energy needed to break existing bonds Energy is given out as new bonds form

Activation energy : 

Most reactions are exothermic (give out heat) overall but there is still a need for energy to get the reaction started. Activation energy Activation energy

Slide 17: 

Why doesn’t petrol catch fire when it is poured through air? Why is just one spark enough to create a major explosion? Energy is needed to break the bonds in petrol before new bonds can be formed by a reaction with oxygen. Once some of the bonds in one petrol molecule have been broken the subsequent reaction with oxygen gives out enough energy to break the bonds in several other petrol molecules - and so on.

Catalysts : 

Catalysts For chemical reactions to occur: Existing bonds have to begin breaking so that new ones can be formed. The molecules have to collide in such a way that the reacting parts of the molecules are brought together. Catalysts can help with either or both of these processes. A catalyst is a substance that speeds up a reaction without getting used up in the process.

Slide 20: 

In the presence of a nickel catalyst vegetable oil and hydrogen react to form margarine. Nickel adsorbs hydrogen gas onto its surface in such a way that the bond holding the hydrogen molecule together becomes stretched. This partial breaking of the bond lowers the activation energy making hydrogen more reactive. Catalysts

Slide 21: 

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

Slide 22: 

Catalysts are used in the manufacture or application of a huge number of products. Catalysts

Slide 23: 

Inside car engines some of the nitrogen and oxygen from the air combine to form poisonous nitrogen oxide. Inside the exhaust system a catalyst encourages decomposition back into nitrogen and oxygen. Copy the energy profile for the uncatalysed reaction and draw in new lines showing how the presence of a catalyst will alter the profile.

Measuring reaction rates : 

Measuring reaction rates Rate implies we are measuring how things change over a period of time. To measure the rate of a reaction we have to track the manner in which the amount of product (or reactant) changes over time.

Slower and slower : 

Slower and slower Reactions do not proceed at a steady rate. They 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. Percentage completion of reaction

Rates and Graphs : 

Rates and Graphs These show the increasing amount of product or the decreasing amount of reactant. Steep gradient Fast reaction Shallow gradient Slow reaction Steep gradient Fast reaction Shallow gradient Slow reaction

Slide 27: 

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

Some Reaction Rates Experiments : 

Some Reaction Rates Experiments The following slides describe the four chemical reactions that are commonly used as examples.

Acid and marble : 

Marble chips are calcium carbonate. They react with acid to evolve a gas. CaCO3(s) + 2HCl(aq)  CaCl2(aq) + H2O(l) + CO2(g) The gas given off can be collected in a syringe and readings taken every 30 seconds or so. Acid and marble

Acid and marble : 

Measure the agreed mass of marble chips Set up the syringe, flask and connector Measure the acid / water. Add the marble chips and quickly insert the bung and start stop clock. Take syringe readings at 30 second intervals. Acid and marble

Acid and marble : 

Acid and marble

Acid and metal : 

Acid and metal Reactive metals (eg. Magnesium) react with acid to evolve hydrogen gas. As the gas given off leaves the flask the total mass of the flask and its contents decreases slightly. Readings of the mass(g) can be taken. Typically at 1 minute intervals. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g) 11.80 11.77 11.74 11.73 11.72 11.71 11.71

Acid and metal : 

Measure the agreed volume of acid / water into the conical flask. Have a loose plug of cotton wool to prevent “spitting” of droplets of liquid. Have a piece of magnesium of known mass ready. Add the magnesium, place the cotton wool in the neck and start taking mass readings immediately. 11.80 11.77 11.74 11.73 11.72 11.71 11.71 Acid and metal

Acid and metal : 

Acid and metal

Decomposition of Hydrogen Peroxide : 

Decomposition of Hydrogen Peroxide Hydrogen peroxide decomposes into water and oxygen. Oxygen gas is given off and can be measured using a gas syringe or a balance. The reaction is catalysed by a wide range of solids. 2H2O2(aq)  2H2O(l) + O2(g) Remember the catalyst NEVER produces more product - just quicker

Acid and Sodium Thiosulphate : 

Acid and Sodium Thiosulphate In this reaction sulphur is precipitated which makes the solution turn cloudy. The effect of changing conditions such as temperature or concentrations can be studied by measuring how long it takes to produce enough sulphur to make the solution opaque (non see-through). Na2S2O3(aq) + 2HCl(aq)  2NaCl(aq) + H2O(l) + SO2(g) + S(s)

Slide 40: 

Measure the agreed volume of thiosulphate / water into the conical flask. Prepare a piece of paper with a cross drawn on it. Measure the required volume of acid in a measuring cylinder. Add the acid to the flask, start the clock, swirl the flask. Look down through the flask until the cross disappears. Note the time. Look down here

Slide 41: 

Imran studied the effect of temperature upon the time it took for the flask to go cloudy. Sketch a graph of the results. Using the reactions at 25oC and 40oC, explain how the time taken lets you work out the relative rate of reaction.

Some Rates Questions : 

Some Rates Questions

Slide 44: 

A pupil performed an investigation into the rate of reaction between a metal and an acid. The results below where obtained. i) Plot a graph of gas volume (y-axis) against time (x-axis) ii) When was the rate of reaction fastest? iii) Use the graph to find the volume of gas produced after 35 seconds. iv) Use the graph to tell after how long the reaction stopped. v) On the graph sketch a line showing the experiment repeated at a higher temperature.

Slide 45: 

Experimental Results 0 50 100 150 200 250 300 0 10 20 30 40 50 60 70 80 90 Time / seconds Volume / cm3 ii) The reaction was fastest at about 25 seconds as the gradient of the line is highest at this point. iii) About 175 cm3 iv) About 55 seconds. v) Higher temperature reaction is in red. Answer

Slide 46: 

A flask was connected to a gas syringe by a glass delivery tube. 30cm3 of water and 0.5g of manganese dioxide were added to the flask. Then 5cm3 of hydrogen peroxide was added and the stopper quickly fitted. Readings of the volume of gas produced were taken every 10 seconds. i) Plot a graph of volume of gas (y-axis) against time (x-axis). Label this curve A. ii) Without emptying the flask another 10cm3 of water and a further 5cm3 of hydrogen peroxide were added. Sketch the shape of the second experiment and label it B.

Slide 47: 

Notes: Curve B is an experiment with half the concentration of hydrogen peroxide. This should produce about half the rate as shown by a line with half the gradient of A. However, the same amount was added so 58cm3 of gas will still be produced. A B Time (s) Answer

Slide 48: 

i) The results above were obtained from an experiment where the loss in mass was recorded as lumps of zinc reacted with hydrochloric acid. Plot a graph of mass loss (y-axis) against time (x-axis). On the graph sketch the lines you would expect if a) the concentration of acid was reduced, b) the temperature was increased.

Slide 49: 

(b) (a) Answer

Slide 50: 

Which of these would speed up the rate at which magnesium dissolves in acid? Cool the acid. Cut up the magnesium. Add water. Coat the magnesium in oil.

Slide 51: 

Why does breaking up solids increase the rate of reaction? Makes more solid. Creates more energy. Increases surface area. Increases the concentration.

Slide 52: 

Why does temperature increase the rate of reaction? Acts as a catalyst. Increases the concentration. Increases number of molecules. Makes collisions more frequent and harder.

Slide 53: 

Why does a catalyst increase the rate of reaction? Provides a route with a lower activation energy. Helps provide energy for the reaction. Increases the speed of reactant molecules. Reduces the number of molecular collisions.

Slide 54: 

Why do most reactions start fast and get slower and slower? They run out of energy. They run out of catalyst. The concentration of reactant molecules gets less and less. The surface area increases.