woodchurch science

here’s a link to the guardian’s tech page which in this episode discusses smart materials and nanotechnology applications. good for a listen while you’re doing something else. you can even download it for your mp3 player.

you will notice that at the end of each post (including this one) there is a clickable link (listen to this podcast) which enables you to hear the post as well as reading it. you could even download each post to your ipod or mp3 player to playback at your leisure. (note that mp3 downloads are blocked in school – sorry).

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this podcast gives the user an introduction to energy level diagrams for year 11 GCSE chemistry coursework.
the presentation lasts approximately 4 minutes and you will need to refer to the diagrams in the text.

most collisions do not result in chemical change. before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the activation energy. this is shown on the energy level diagrams below (sometimes called reaction profiles).

going up to the top hump represents bond breaking on reacting particle collision. the purple arrow up represents this minimum energy needed to break bonds to start the reaction, that is the activation energy. going down the other side represents new bonds forming in the reaction products. the red arrow down represents the energy released in an exothermic reaction. however, it does not matter whether the reaction is an exothermic or an endothermic in terms of energy change, it is the activation energy which is important in terms of temperature and reaction speed.

now heated molecules have a greater average kinetic energy, and so at higher temperatures, a greater proportion of them have the required activation energy to react. this means that the increased chance of successful higher energy collision greatly increases the speed of the reaction, depending on the fraction of molecules with enough energy to react.

for this reason, generally speaking, a low activation energy reaction is likely to be fast, and a high activation energy reaction much slower, reflecting the trend that the lower the energy barrier to a reaction, the more molecules are likely to have sufficient energy to react on collision.

now i will try to resolve an apparent confusion for some gcse students!

with increase in temperature, there is an increased chance of collisions, due to the more energetic situation – but this is a minor factor when considering why rate of a reaction increases with temperature. the minimum energy needed for reaction, the activation energy, stays the same when temperature increases.

however, the average increase in particle kinetic energy caused by the absorbed heat means that a much greater proportion of the molecules now have the minimum or activation energy to react. shown as the blue shaded segment for the low temperature and the blue+red segments for the higher temperature.

it is this increased chance of a successful higher energy collision leading to product formation, that is the major factor, and this effect increases more than the increased frequency of particle collisions, for a similar rise in temperature.

this is usually only fully discussed at a-level, but it may impress your teacher for gcse coursework if you look up the maxwell-boltzmann distribution of kinetic energies.

it is often quite difficult to get over some of these ideas without considering the diagrams and graphs of probability versus particle kinetic energy, but that is up to you!

thanks to doc brown for the diagrams and to you for listening – look out for more podcasts from woodchurch science.

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this podcast gives the user an introduction to collision theory for year 11 GCSE chemistry coursework.
the presentation lasts approximately 5 minutes.

2. the theory of how reactions happen is called collision theory.

reactions can only happen when the particles collide, but most collisions are not successful in forming the product despite the high rate of collisions.

the reason is that particles have a wide range of kinetic energy, but only a small fraction of particles have enough energy to break bonds and bring about chemical change. The minimum kinetic energy required for any reaction is known as the activation energy.

the minority high kinetic energy collisions between particles which do produce a chemical change are called successful or fruitful collisions. a successful collision results in new products forming.

the rate-controlling factors described below are to do with the chance of collisions. the particle theory of gases and liquids, and the particle diagrams shown in your notes and revision guides together with the explanations below, will all help you understand or describe in your coursework what is going on.

3. factors affecting the rate of chemical reactions.

3a, effect of concentration.

if the concentration of any reactant in a solution is increased, the rate of reaction is increased.
increasing the concentration, increases the chance of a successful collision between reactant particles because there are more of them in the same volume and so increases the chance of product formation.

e.g. increasing the concentration of acid increases the frequency or chance at which they hit the surface of marble chips to dissolve them.
or, e.g. increasing the concentration of acid increases the frequency or chance at which they will collide with sodium thio sulphate particles causing sulphur cloudiness to form faster.

3b, effect of stirring.

it is sometimes forgotten that stirring the mixture is an important rate factor.
if the reacting mixture is not stirred evenly, the reactant concentration in solution becomes much less near the solid, which tends to settle out at the bottom of the flask.

therefore, at the bottom of the flask the reaction prematurely slows down, distorting the overall rate measurement and making the results uneven and therefore inaccurate. the unevenness of the results is even more evident by giving the reaction mixture the odd stir! you may get jumps in the graph!

3c, effect of surface area of solid.

if a solid reactant or a solid catalyst is broken down into smaller pieces, the rate of reaction increases.
the speed increase happens because smaller pieces of the same mass of solid have a greater surface area compared to larger pieces of the solid. therefore, there is more chance that a reactant particle will hit the solid surface and react.

3d, effect of temperature.

when gases or liquids are heated the particles gain kinetic energy and move faster.
the increased speed increases the chance of collision between reactant molecules and the rate increases.

remember that most molecular collisions do not result in chemical change. before any change takes place on collision, the colliding molecules must have a minimum kinetic energy called the activation energy shown on energy level diagrams (see your notes or revision guides).

there will be more about activation energy in a future podcast.

thanks for listening – look out for more podcasts from woodchurch science.

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this podcast gives the user an introduction to year 11 chemistry coursework for GCSE.
the presentation lasts approximately 5 minutes.

1. what do we mean by rate and how is it measured?

The phrase rate of reaction means how fast is the reaction or the speed of the reaction. It can be measured as the rate of formation of product or the rate of removal of reactant. The speeds of reactions are varied.
for example:
(1) rusting is a slow reaction, you hardly see any change looking at it!
(2) the weathering of rocks is a very slow reaction.
(3) the fermentation of sugar to alcohol is quite slow but you can see the carbon dioxide bubbles forming in the froth!
(4) a fast reaction would be magnesium dissolving in hydrochloric acid or sodium with water.
(5) explosions and burning/combustion reactions would be described as very fast!

2. Why is rates of reaction knowledge important?

time is money in industry, the faster the reaction can be done, the more economic it is.
you need to know how long reactions are likely to take.
hence the great importance of catalysts e.g. transition metals or enzymes which reduce time and save money.

3. how do we measure rate?

a reaction will continue until one of the reactants is used up.
to measure the speed or rate of a reaction depends on what the reaction is, and if what is formed can be measured as the reaction proceeds?

experiment 1.
marble chips (calcium carbonate) reacting with hydrochloric acid.

here a gas is formed from the solid reacting with an acid, which can be collected in a gas syringe.
in this experiment carbon dioxide is formed and collected.
a graph of cm3 of gas versus time is plotted. the slope or gradient of the graph shows that the reaction is fast at the start, but if the reaction is allowed to go on, you can usually measure the final maximum volume of gas and the time at which the reaction stops. however, this is a very poor measure of rate, because the reaction just goes slower and slower as the reactant amounts or concentrations are decreasing.

the shape of the graph is also quite characteristic.
the reaction is fastest at the start when the reactants are at a maximum (i.e. the steepest slope). the gradient becomes progressively less as reactants are used up and the reaction slows down. finally the graph levels out when one of the reactants is used up and the reaction stops.
the amount of product depends on the amount of reactants used.
the initial rate of reaction is obtained by measuring the gradient at the start of the reaction.
a tangent line is drawn through the first part of the graph, which is usually reasonably linear from the origin.
this gives you an initial rate of reaction in cm3 gas per minute.

the rate of a reaction that produces a gas can also be measured by following the mass loss as the gas is formed and escapes from the reaction flask.
the method is ok for reactions producing carbon dioxide or oxygen, but not very accurate for reactions giving hydrogen (here the mass lost is too low for accuracy).

experiment 2.
sodium thio-sulphate and hydrochloric acid.

when sodium thio-sulphate reacts with acid, a yellow precipitate of sulphur is formed and forms the basis of a good project for assessment.
to follow this reaction in your investigation you can measure how long it takes for a certain amount of sulphur to form.
you do this by observing the reaction down through a conical flask, viewing a black cross on white paper.
the x is eventually obscured by the sulphur precipitate and the time is noted.

by using the same flask and paper with x drawn on it you can obtain a relative measure of the speed of the reaction in forming the same amount of sulphur.
the speed or rate of reaction can expressed as x amount of sulphur/time, so the rate is proportional to 1/time for a particular run of the experiment.
you can investigate the effects of
(i) the hydrochloric acid or sodium thiosulphate concentration
(ii) the temperature of the reactants.
to show the effects of changing one of the variables you can plot graphs of:
(i) reaction time versus temperature or concentration, or
(ii) rate of reaction (1/reaction time) versus temperature or concentration.

you could also measure the speed of this reaction by using a light gate to detect the precipitate formation. the system consists of a light beam emitter and sensor connected to a computer and the reaction vessel is placed between the emitter and sensor. the light reading falls as the sulphur precipitate forms.

thanks for listening – look out for more podcasts from woodchurch science

you will very soon start to see this icon at the start of some posts:

when you click on the link provided you can either listen to the post on your computer or download it to your ipod or mp3 player to play back when you want. it will generally only be used for longer posts such as revision notes, so keep your eyes peeled for this exciting innovation.