click the links to activate, they should open up in your browser.
cells
gas exchange in alveoli
mitosis
the ear
the heart

gaddas, pooley are these any good? are they useful?

any access problems then let me know using the comments button at the top of this post.

y7 – 7a cells 7b reproduction 7c environment 7d variation 7e acids and alkalis 7f chemical reactions 7g particles 7h solutions 7i energy 7j electrical circuits 7k forces 7l solar system

y8 – 8a food and digestion 8b respiration 8c microbes and disease 8d ecology 8e atoms and elements 8f compounds and mixtures 8g rocks and weathering 8h rock cycle 8i heating and cooling 8j magnets 8k light 8l sound

y9 – 9a inheritance and selection, 9b fit and healthy, 9c plants and photosynthesis, 9d plants for food, 9e metals and metal compounds, 9f patterns of reactivity, 9g environmental chemistry, 9h using chemistry, 9i energy and electricity, 9j gravity and space, 9k speeding up, 9l pressure and moments

answers are given at the end of each pdf file.

if you want others adding (from other subjects) then please let me know. use comments link.

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.

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