The City SchoolPrep Girls North Nazimabad

Science NotesClass 8

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Energy transfer diagrams

Energy transfer diagrams show the locations of energy stores and energy transfers. For example, consider the energy transfers in the simple electrical circuit below.

We can show the transfers like this:

The battery is a store of chemical energy. The energy is transferred by electricity to the lamp, which transfers the energy to the surroundings by light. These are the useful energy transfers - we use electric lamps to light up our rooms.

But there are also energy transfers that are not useful to us. In the example above, the lamp also transfers energy to the surroundings by heating. If we include this energy transfer, the diagram looks like this:

Sankey diagrams

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. This Sankey diagram for the lamp shows that it transfers most of the energy by heating, rather than by light:

Notice that the total amount of energy transferred to the surroundings is the same as the amount of electrical energy. We say that the energy has been conserved. Energy is always conserved, it is never "lost" or "wasted", although some energy transfers are useful and some are not.

Energy Transfer Diagrams and Efficiency

Energy Transfer Diagrams

Most of the machines or devices we use transfer energy from one form to another. Input energy is taken in by the device in one form and transformed to output energy in another form.

An energy transfer diagram or a Sankey diagram is used to show the transfer of energy across a process or a device. It is a flow diagram in which the widths of the arrows show the relative amounts of each type of energy.

An energy transfer diagram for a power station along with a Sankey diagram is shown below:

 

Efficiency

The efficiency of a device is calculated using the following formula:

Device Light Bulb Energy Saving Light Bulb

 

Energy Transfer Diagram

Electrical Energy Heat and Light Energy Electrical Energy Heat and Light Energy

Sankey Diagram

Efficiency Efficiency = 5/100 x 100Efficiency = 5%

Efficiency = 15/60 x 100Efficiency = 25%

Explanation Energy cannot be created nor destroyed. It can only be transformed from one form to another. Thus the 100J of electrical energy is transformed to 5 J of light energy and 95J of heat energy. In the case of the light bulb the 95J of energy transferred as heat is wasted energy as it is not useful because the purpose of the device is to produce light.

An ordinary light bulb works on the principle of a thin wire (filament) being heated by the resistance to the electrical current. At a temperature of about 1100°C it glows with a bright white light. As the electrical energy is required in heating the wire hence this is why

Energy saving light bulbs work on the principle of fluorescence. Here the electrical energy is supplied to electrodes which generate fast moving electrons that pass through a tube containing mercury gas. On collision with the mercury atoms ultraviolet light is produced which then collides with the phosphor atoms coated around the tube converting the ultraviolet to visible light. Here a greater proportion of the electrical energy is converted to useful light, thus the energy saving light bulb is a more efficient device.

most of the energy is given off as heat. Lamps which give of light when hot are called incandescent.

 

Tip

Energy cannot be created nor destroyed. It can only be transformed from one form to another (the law of conservation of energy).

When energy is transformed or transferred only part of it can be usefully transformed or transferred. The energy which is not usefully transformed or transferred is referred to as wasted energy.

Both the useful energy and the wasted energy which is transformed or transferred are eventually transferred to their surroundings which become warmer. As the energy spreads out it becomes more difficult to use for further energy

transformations.

The greater the percentage of the energy that can be usefully transformed by a device the higher its efficiency.

A Sankey diagram gives a visual illustration of an input/output situation. It is drawn to scale - there are lots of variations as to how they are drawn - only thing they have in common is that the width of the 'arms' represents the energy transferred but the length of the 'arms' does not!

Sankey diagrams allow us to visualize flow through a process or system more easily than a table of numerical data can.

They show not only the energy transfers involved but also the quantitative distribution of values in the transfers.

 

Sankey diagrams do add an 'indisputable expressive power to mathematical rendering of a system'. When constructed properly, Sankey diagrams represent flow in a manner that can be perceived by anyone, instantly.

However, Sankey diagrams can be difficult, time-consuming, and uninteresting to produce by hand - very tedious to draw! The benefits of being able to generate these diagrams automatically, anytime, are obvious to anyone who has tried to draw one and commercial computer packages for their production are available.

They are used not only in physics and engineering to demonstate how energy is distributed but also for cash flow in businesses.

there is a specific way in which the Sankey diagrams is to be drawn.

The input is from the left of the diagram. The wanted (useful) output is to the right. All unwanted (wasted) output is made to go vertically down.

Remember the total input always equals the total output - but an efficient system will have a high percentage of useful output.

States of Matter gas-liquid-solid revision notes

EXAMPLES OF THE THREE PHYSICAL STATES OF MATTER

GASES eg the air mixture around us (including the oxygen needed for combustion) and the high pressure steam in the boiler and cylinders of the steam locomotive. All of these gases are 'invisible', being colourless and transparent, so note that the 'steam' you see outside of the locomotive is actually fine liquid droplets of water, formed from the expelled steam gas condensing when it meets the cold air - the 'state change' of gas to liquid (same effect in mist and fog formation).

LIQUIDS eg water is the most common example, but so are, milk, hot butter, petrol, oil, mercury or alcohol in a thermometer.

SOLIDS eg stone, all metals at room temperature (except mercury), rubber of walking boots and the majority of physical objects around you. In fact most objects are useless unless they have a solid structure!

On this page the basic physical properties of gases, liquids and solids are described in terms of structure, particle movement (kinetic particle theory), effects of temperature and pressure changes, and particle models used to explain

these properties and characteristics. Hopefully, theory and fact will match up to give students a clear understanding of the material world around them in terms of gases, liquids and solids - referred to as the three physical states of matter.

The changes of state known as melting, fusing, boiling, evaporating, condensing, liquefying, freezing, solidifying, crystallising are described and explained with particle model pictures to help understanding. There is also a mention of

miscible and immiscible liquids and explaining the terms volatile and volatility when applied to a liquid.

1.1. The Three States of Matter, gas-liquid-solid particle theory models

WHAT ARE THE THREE STATES OF MATTER? WHY ARE THEY LIKE WHAT THEY ARE? HOW CAN WE EXPLAIN HOW THEY BEHAVE? CAN PARTICLE MODELS HELP US UNDERSTAND THEIR PROPERTIES and

CHARACTERISTICS? WHY IS IT IMPORTANT TO KNOW THE PROPERTIES OF GASES, LIQUIDS AND SOLIDS?

What is the KINETIC PARTICLE THEORY of gases, liquids and solids? CAN WE MAKE PREDICTIONS BASED ON THEIR CHARACTERISTIC PROPERTIES?

 1.1a. The particle model of a Gas

WHAT IS THE GASEOUS STATE OF MATTER? WHAT ARE THE PROPERTIES OF A GAS? HOW DO GASEOUS PARTICLES BEHAVE? How does the kinetic particle theory of gases explain the properties of gases? A gas has no fixed shape or volume, but always spreads out to fill any container. There are almost no forces of attraction between the particles so they are completely free of each other. The particles are widely spaced and scattered at random throughout the container so there is no order in the

system. The particles move linearly and rapidly in all directions, until frequently colliding with each other and the

side of the container. The collision of gas particles with the surface of a container causes gas pressure, on bouncing off a surface

they exert a force in doing so. With increase in temperature, the particles move faster as they gain kinetic energy, this increases gas

pressure and/or the volume of the container.

Using the particle model to explain the properties of a Gas

Gases have a very low density (‘light’) because the particles are so spaced out in the container (density = mass / volume).

o Density order: solid > liquid >>> gases Gases flow freely because there are no effective forces of attraction between the gaseous particles - molecules.

o Ease of flow order: gases > liquids >>> solids (no real flow in solid unless you powder it!)o Because of this gases and liquids are described as fluids.

Gases have no surface, and no fixed shape or volume, and because of lack of particle attraction, they always spread out and fill any container (so gas volume = container volume).

Gases are readily compressed because of the ‘empty’ space between the particles. o Ease of compression order: gases >>> liquids > solids (almost impossible to compress a solid)

Gas pressureo When a gas is confined in a container the particles will cause and exert a gas pressure which is

measured in atmospheres (atm) or Pascals (1.0 Pa = 1.0 N/m2) - pressure is force/area on which force is exerted.

The gas pressure is caused by the force created by millions of impacts of the tiny individual gas particles on the sides of a container.

For example - if the number of gaseous particles in a container is doubled, the gas pressure is doubled because doubling the number of molecules doubles the number of impacts on the side of the container so the total impact force per unit area is also doubled.

This doubling of the particle impacts doubling the pressure is pictured in the two diagrams below.

2 x partic

les

===>

P x 2

If the volume of a sealed container is kept constant and the gas inside is heated to a higher temperature, the gas pressure increases.

o The reason for this is that as the particles are heated they gain kinetic energy and on average move faster.

o Therefore they will collide with the sides of the container with a greater force of impact, so increasing the pressure.

There is also a greater frequency of collision with the sides of the container BUT this is a minor factor compared to the effect of increased kinetic energy and the increase in the average force of impact.

o Therefore a fixed amount of gas in a sealed container of constant volume, the higher the temperature the greater the pressure and the lower the temperature the lesser the pressure.

If the ‘container’ volume can change, gases readily expand* on heating because of the lack of particle attraction, and readily contract on cooling.

o On heating, gas particles gain kinetic energy, move faster and hit the sides of the container more frequently, and significantly, they hit with a greater force.

o Depending on the container situation, either or both of the pressure or volume will increase (reverse on cooling).

o Note: * It is the gas volume that expands NOT the molecules, they stay the same size!o If there is no volume restriction the expansion on heating is much greater for gases than liquids or

solids because there is no significant attraction between gaseous particles. The increased average kinetic energy will make the gas pressure rise and so the gas will try to expand in volume if allowed to e.g. balloons in a warm room are significantly bigger than the same balloon in a cold room!

DIFFUSION in Gases:o The natural rapid and random movement of the particles in all directions means that gases readily

‘spread’ or diffuse. The net movement of a particular gas will be in the direction from lower concentration to a

higher concentration, down the so-called diffusion gradient. Diffusion continues until the concentrations are uniform throughout the container of gases, but

ALL the particles keep moving with their ever present kinetic energy! Diffusion is faster in gases than liquids where there is more space for them to move (experiment illustrated

below) and diffusion is negligible in solids due to the close packing of the particles. o Diffusion is responsible for the spread of odours even without any air disturbance e.g. use of perfume,

opening a jar of coffee or the smell of petrol around a garage.o The rate of diffusion increases with increase in temperature as the particles gain kinetic energy

and move faster.o Other evidence for random particle movement including diffusion:

When smoke particles are viewed under a microscope they appear to 'dance around' when illuminated with a light beam at 90o to the viewing direction. This is because the smoke particles show up by reflected light and 'dance' due to the millions of random hits from the fast moving air molecules. This is called 'Brownian motion' (see below in liquids). At any given instant of time, the hits will not be even, so the smoke particle get a greater bashing in a random direction.

A two gaseous molecule diffusion experiment is illustrated above and explained below!

A long glass tube (2-4 cm diameter) is filled at one end with a plug of cotton wool soaked in conc. hydrochloric acid sealed in with a rubber bung (for health and safety!) and the tube is kept perfectly still, clamped in a horizontal position. A similar plug of conc. ammonia solution is placed at the other end. The soaked cotton wool plugs will give off fumes of HCl and NH3 respectively, and if the tube is left undisturbed and horizontal, despite the lack of tube movement, e.g. NO shaking to mix and the absence of convection, a white cloud forms about 1/3rd along from the conc. hydrochloric acid tube end.

Explanation: What happens is the colourless gases, ammonia and hydrogen chloride, diffuse down the tube and react to form fine white crystals of the salt ammonium chloride.

ammonia + hydrogen chloride ==> ammonium chloride NH3(g) + HCl(g) ==> NH4Cl(s)

Note the rule: The smaller the molecular mass, the greater the average speed of the molecules (but all gases have the same average kinetic energy at the same temperature).

Therefore the smaller the molecular mass, the faster the gas diffuses. e.g. Mr(NH3) = 14 + 1x3 = 17, moves faster than Mr(HCl) = 1 + 35.5 = 36.5 AND that's why they meet nearer the HCl end of the tube! So the experiment is not only evidence for molecule movement, it is also

evidence that molecules of different molecular masses move/diffuse at different speeds.

A coloured gas, heavier than air (greater density), is put into the bottom gas jar and a second gas jar of lower density colourless air is placed over it separated with a glass cover.

If the glass cover is removed then (i) the colourless air gases diffuses down into the coloured brown gas and (ii) bromine diffuses up into the air. The particle movement leading to mixing cannot be due to convection because the more dense gas starts at the bottom!

No 'shaking' or other means of mixing is required. The random movement of both lots of particles is enough to ensure that both gases eventually become completely mixed by diffusion.

This is clear evidence for diffusion due to the random continuous movement of all the gas particles and, initially, the net movement of one type of particle from a higher to a

lower concentration ('down a diffusion gradient'). When fully mixed, no further colour change distribution is observed BUT the random particle movement continues! See also other evidence in the liquid section below.

A note on 'forces'

Forces between particles are mentioned on this page and some ideas will seem more abstract than others - but think about it ...

o A gas spreads everywhere in a given space, so there can't be much attraction between the molecules/particles.

o Something must hold liquid molecules together or how can a liquid form from a gas?o In fact between liquid molecules there are actually weak forces of attraction called intermolecular forces,

but they can't be strong enough to create a rigid solid structure.o However, in solids, these forces must be stronger to create the rigid structure.

 1.1b. The particle model of a Liquid

WHAT IS THE LIQUID STATE OF MATTER? WHAT ARE THE PROPERTIES OF A LIQUID? HOW DO LIQUID PARTICLES BEHAVE? How does the kinetic particle theory of liquids explain the properties of liquids? A liquid has a fixed volume at a given temperature but its shape is that of the container which holds the

liquid. There are much greater forces of attraction between the particles in a liquid compared to gases, but not

quite as much as in solids.

If there were no intermolecular forces, liquids could not exist.

The particles are quite close together but still arranged at random throughout the container due to their random movement, there is a little close range order as you can get clumps of particles clinging together temporarily (as in the diagram above).

The particles are moving rapidly in all directions but collide more frequently with each other than in gases due to shorter distances between particles.

With increase in temperature, the particles move faster as they gain kinetic energy, so increased collision rates, increased collision energy, increased rates of particle diffusion, expansion leading to decrease in density.

Using the particle model to explain the properties of a Liquid

Liquids have a much greater density than gases (‘heavier’) because the particles are much closer together because of the attractive forces.

Most liquids are just a little less dense than when they are solido Water is a curious exception to this general rule, which is why ice floats on water.

Liquids usually flow freely despite the forces of attraction between the particles but liquids are not as ‘fluid’ as gases.

o Note 'sticky' or viscous liquids have much stronger attractive forces between the molecules BUT not strong enough to form a solid.

Liquids have a surface, and a fixed volume (at a particular temperature) because of the increased particle attraction, but the shape is not fixed and is merely that of the container itself.

o Liquids seem to have a very weak 'skin' surface effect which is caused by the bulk molecules attracting the surface molecules disproportionately.

Liquids are not readily compressed because there is so little ‘empty’ space between the particles, so increase in pressure has only a tiny effect on the volume of a solid, and you need a huge increase in pressure to see any real contraction in the volume of a liquid.

Liquids will expand on heating but nothing like as much as gases because of the greater particle attraction restricting the expansion (will contract on cooling).

o Note: When heated, the liquid particles gain kinetic energy and hit the sides of the container more frequently, and more significantly, they hit with a greater force, so in a sealed container the pressure produced can be considerable!

The natural rapid and random movement of the particles means that liquids ‘spread’ or diffuse. Diffusion is much slower in liquids compared to gases because there is less space for the particles to move in and more ‘blocking’ collisions happen.

o Just dropping lumps/granules/powder of a soluble solid (preferably coloured!) will resulting in a dissolving followed by an observable diffusion effect.

o Again, the net flow of dissolved particles will be from a higher concentration to a lower concentration until the concentration is uniform throughout the container.

Diffusion in liquids - evidence for random particle movement in liquids: o If coloured crystals of e.g. the highly coloured salt crystals of potassium manganate(VII) are dropped

into a beaker of water and covered at room temperature. Despite the lack of mixing due to shaking or convection currents from a heat source etc. the

bright purple colour of the dissolving salt slowly spreads throughout all of the liquid but it is much slower than the gas experiment described above because of the much greater density of particles slowing the spreading due to close proximity collisions.

The same thing happens with dropping copper sulphate crystals (blue, so observable) or coffee granules into water and just leaving the mixture to stand.

o When pollen grains are viewed under a microscope they appear to 'dance around' when illuminated with a light beam at 90o to the viewing direction.

This is because the pollen grains show up by reflected light and 'dance' due to the millions of random hits from the fast moving water molecules.

This phenomenon is called 'Brownian motion' after a botanist called Brown first described the effect (see gases above).

At any given instant of time, the hits will not be even all round the pollen grain, so they get a greater number of hits in a random direction.

 1.1c. The particle model of a Solid

WHAT IS THE SOLID STATE OF MATTER? WHAT ARE THE PROPERTIES OF A SOLID? HOW DO SOLID PARTICLES BEHAVE? How does the kinetic particle theory of solids explain the properties of solids? A solid has a fixed volume and shape at a particular temperature unless physically subjected to some

force. The greatest forces of attraction are between the particles in a solid and they pack together as tightly as

possible in a neat and ordered arrangement called a lattice. The particles are too strongly held together to allow movement from place to place but the particles vibrate

about their position in the structure. With increase in temperature, the particles vibrate faster and more strongly as they gain kinetic energy, so

the vibration increases causing expansion.

Using the particle model to explain the properties of a Solid

Solids have the greatest density (‘heaviest’) because the particles are closest together. Solids cannot flow freely like gases or liquids because the particles are strongly held in fixed positions. Solids have a fixed surface and volume (at a particular temperature) because of the strong particle attraction. Solids are extremely difficult to compress because there is no real ‘empty’ space between the particles, so

increase in pressure has virtually no effect on the volume of a solid. Solids will expand a little on heating but nothing like as much as liquids because of the greater particle

attraction restricting the expansion and contraction occurs on cooling. o The expansion is caused by the increased energy of particle vibration, forcing them further apart causing

an increase in volume and corresponding decrease in density.

Diffusion is almost impossible in solids because the particles are too closely packed and strongly held together with no ‘empty space’ for particles to move through.

2. Changes of State for gas <=> liquid <=> solid

A change of state means an interconversion between two states of matter, namely gas <=> liquid <=> solid

e.g. solid ==> liquid is melting or fusing

liquid ==> gas/vapour (vapor) is boiling, evaporation or vapourisation (vaporisation)

and the reverse processes

gas/vapour (vapor) ==> liquid is condensation, liquefaction/liquefying

liquid ==> solid is freezing, solidifying or crystallising

and there is also

solid ==> gas is sublimation

We can use the state particle models and diagrams to explain changes of state and the energy changes involved.

These are NOT chemical changes BUT PHYSICAL CHANGES, e.g. the water molecules H2O are just the same in ice, liquid water, steam or water vapour. What is different, is how they are arranged, and how strongly they are held

together by intermolecular forces in the solid, liquid and gaseous states.

2a. Evaporation and Boiling (liquid to gas)

explained using the kinetic particle theory of gases and liquids

Because of random collisions, the particles in a liquid have a variety of speeds and kinetic energies. On heating, particles gain kinetic energy and move faster and are more able to overcome the intermolecular forces between the molecules i.e. some particles will have enough kinetic energy to overcome the attractive forces holding the particles together in the bulk liquid.

o Even without further heating, evaporation occurs all the time from volatile liquids, but it is still the higher kinetic energy particles that can overcome the attractive forces between the molecules in the bulk of the liquid and escape from the surface into the surrounding air.

In evaporation* and boiling (both are vaporisation) it is the highest kinetic energy molecules that can ‘escape’ from the attractive forces of the other liquid particles.

The particles lose any order and become completely free to form a gas or vapour.

Energy is needed to overcome the attractive forces between particles in the liquid and is taken in from the surroundings.

This means heat is taken in, so evaporation and boiling are endothermic processes (ΔH +ve). If the temperature is high enough boiling takes place. Boiling is rapid evaporation anywhere in the bulk liquid and at a fixed temperature called the boiling point

and requires continuous addition of heat. The rate of boiling is limited by the rate of heat transfer into the liquid. * Evaporation takes place more slowly than boiling at any temperature between the melting point and

boiling point, and only from the surface, and results in the liquid becoming cooler due to loss of higher kinetic energy particles.

2b. Condensing (gas to liquid)

explained using the kinetic particle theory of gases and liquids

On cooling, gas particles lose kinetic energy and eventually become attracted together to form a liquid i.e. they haven't enough kinetic energy to remain free in the gaseous state.

There is an increase in order as the particles are much closer together and can form clumps of molecules.o The process requires heat to be lost to the surroundings i.e. heat given out

2c. Distillation

Simple and fractional distillation involve the processes of boiling and condensation and are described on the Elements, Compounds and Mixtures Part 2 page, where other methods of separation are also described.

The process of distillation involves boiling (liquid ==> gas/vapor) and the reverse process of condensation (gas/vapour ==> liquid)

2d. Melting (solid to liquid)

explained using the kinetic particle theory of liquids and solids

When a solid is heated the particles vibrate more strongly as they gain kinetic energy and the particle attractive forces are weakened.

Eventually, at the melting point, the attractive forces are too weak to hold the particles in the structure together in an ordered way and so the solid melts.

o Note that the intermolecular forces are still there to hold the bulk liquid together - but the effect is not strong enough to form an ordered crystal lattice of a solid.

The particles become free to move around and lose their ordered arrangement. Energy is needed to overcome the attractive forces and give the particles increased kinetic energy of vibration. .

2e. Freezing (liquid to solid)

explained using the kinetic particle theory of liquids and solids

On cooling, liquid particles lose kinetic energy and so can become more strongly attracted to each other. When the temperature is low enough, the kinetic energy of the particles is insufficient to prevent the particle

attractive forces causing a solid to form. Eventually at the freezing point the forces of attraction are sufficient to remove any remaining freedom of

movement (in terms of one place to another) and the particles come together to form the ordered solid arrangement (though the particles still have vibrational kinetic energy.

2f. Cooling and Heating Curves

and the comparative energy changes of state changes gas <=> liquid <=> solid2f(i) Cooling curve: Note the temperature stays constant during the state changes of condensing at temperature Tc, and freezing/solidifying at temperature Tf. This is because all the heat energy removed on cooling at these temperatures (the latent heats or enthalpies of state change), allows the strengthening of the inter-particle forces without temperature fall (the heat loss is compensated by the exothermic increased intermolecular force attraction). In between the 'horizontal' state change sections of the graph, you can see the energy 'removal' reduces the kinetic energy of the particles, lowering the temperature of the substance.

A cooling curve summarises the changes:

gas ==> liquid ==> solid

2f(ii) Heating curve: Note the temperature stays constant during the state changes of melting at temperature Tm and boiling at temperature Tb. This is because all the energy absorbed in heating at these temperatures (the latent heats or enthalpies of state change), goes into weakening the inter-particle forces without temperature rise (the heat gain equals the endothermic/heat absorbed energy required to reduce the intermolecular forces). In between the 'horizontal' state change sections of the graph, you can see the energy input increases the kinetic energy of the particles and raising the temperature of the substance.

A heating curve summarises the changes:

solid ==> liquid ==> gas

.

2g. Sublimation

explained using the kinetic particle theory of gases and solids

Sublimation: o This is when a solid, on heating, directly changes into a gas without melting, AND the gas on cooling

re-forms a solid directly without condensing to a liquid. They usually involve just a physical change BUT its not always that simple!

Theory in terms of particles: o When the solid is heated the particles vibrate with increasing force from the added thermal energy.

If the particles have enough kinetic energy of vibration to partially overcome the particle-particle attractive forces you would expect the solid to melt.

HOWEVER, if the particles at this point have enough energy at this point that would have led to boiling, the liquid will NOT form and the solid turns directly into a gas.

Overall energy absorbed and 'taken in' to the system.o On cooling, the particles move slower and have less kinetic energy.

Eventually, when the particle kinetic energy is low enough, it will allow the particle-particle attractive forces to produce a liquid.

BUT the energy may be low enough to permit direct formation of the solid, i.e. the particles do NOT have enough kinetic energy to maintain a liquid state!

Overall energy released and 'given out' to the surroundings.

 

Appendix 1 some simple particle pictures of ELEMENTS, COMPOUNDS and MIXTURES