ENERGY AND CHEMICAL TESTS

Chemistry

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energy

Reactions are Exothermic or Endothermic

An EXOTHERMIC reaction is one which GIVES OUT ENERGY to the surroundings usually in the form of heat and usually shown by a rise in temperature

An ENDOTHERMIC reaction is one which TAKES IN ENERGY from the surroundings usually in the from of heat and usually shown by a fall in temperature

Energy must always be supplied to break bonds and energyis always releases when bonds form:

  • during a chemical reaction, old bonds are broken and new bonds are formed
  • energy must be supllied to break existing bonds - so bond breaking is an endothermic process and energy releases when new bonds are formed - so bond formation is an exothermic process
  • in an endothermic reaction, the energy required to break old bonds is greater than the energy releases when new bonds are formed
  • in an exothermic reaction, the energy releases in bond formation is greater than the eneregy used in breaking old bonds
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energy and fuels

Fuel energy is calculation using calorimetry:

You can simply burn the fuel and use the flame to heat up some water. It uses a metal contained made of copper because copper conducts heat so well.

  • Put 50g of water in the copper can and record its temperature, weigh the spirit burner and lit, put the spirit burner underneath the can, and light the wick and heat the water (stirring constantly) until temperature is about 50°C, put out the flame using the burner lid and measure final temperature of water and weigh the spirit burner and lid again

Fuels release energy which we use in loads of ways e.g. to generate electricity, to heat our house, to power cars, lorries, trains, planes etc.

It can have various effects on the environment. Burning fossil fuels releases CO2which is a greenhouse gas which causes global warming and other types of climate change. The price of crude ol has a big economic effort - it is linked to the supply (the less there is, the more expensive it is). Same for natural gas and coal.

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bond energies

Energy level diagrams show if its Exo- or Endo- thermic:

  • an exothermic reaction - the products would be at a lower energy than the reactants.
  • the inital; rise in the line representing the energy needed to break the old bonds is calls the activation energy
  • an endothermic reaction - the products are at a higher energy than the reactants
  • the different in heigh represents the energy taken in during the reaction

The activation energy is lowered by catalysts - it represents the minimum energy needed by reacting particles for the reaction to occur. A catalyst makes it happen easier/quicker by reducing initial energy needed.

Bond energy calculations - every chemical bond has a particular bond energy associated with it. It varies slightly depending what compound the bond occurs in.

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energy and food

Food energy is often measured in calories and kilocalories and you get your energy from food:

As with all fuels different foods produce different amouts of energy. The composition of the food determines how much energy it produces. Foods with high proportions of fats and oils produce relatively large amounts of energy. Carbohydrates produce some energy, bt less than fats and oils. Proteins contain about as much energy as carbohydrates.

Taking in more fuel than you use means the excess is stored.

Your body needs energy to perform all your daily activities e.g. breathing, heart beating etc. Chemical reactions in your cells that go on all the time need energy. The energy in food is releases by the process of respiration, where glucose reacts with oxygen to produce carbon dioxide, water, and energy.

Glucose + Oxygen -> Carbon Dioxide + Water + Energy

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tests for cations

Flame tests indentify metal ions!

  • Lithium, Li+ burns with a crimson-red flame
  • Sodium, Na+ burns with a yellow-orange flame
  • Potassium, K+ burns with a lilac flame
  • Calcium, Ca2+ burns with a brick-red flame
  • Barium, Ba2+ bruns with a green flame

Some metals form a coloured precipitate with NaOH:

  • Calciums, Ca2+ colour of precipitate is WHITE
  • Copper (II), Cu2+ colour of precipitate is BLUE
  • Iron (II), Fe2+ colour of precipitate is SLUDGY GREEN
  • Iron (III), Fe3+ olour of precipitate is REDDISH BROWN
  • Aluminium, Al3+ colour of precipitate is WHITE at first, then redissolves to form a COLOURLESS solution
  • Magnesium, Mg2+ colour of precipitate is WHITE
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test for anions

Testing for carbonates - check for CO2- you can test to see if a gas is carbon dioxide by bubbling it through limewater, if so the limewater turns milky/cloudy. Can use this to test for carbonatres since carbonates react with dilute acids to form cardon dioxide.

Some carbonates change colour when they decompose - a colour change during a reaction can give you clues to identify a substance. EXAMPLE: copper carbonate turns from green to black and stays black when cool and zinc carbonate turns from white to yellow but when it cools down it turns back to white.

Tests for sulfates and halides - Sulfate ions = add a dilute HCl followed by barium chloride solution. A white precipitate of barium sulfate means the original compound was a sulfate. Chloride, Bromide or Iodide ions = add dilute nitric acid followed by silver nitrate solution. Chloride gives a white precipitate of silver chloride, Bromide gives a cream precipitate of silver bromide and Iodide gives a yellow precipitate of silver iodide.

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tests for organic compounds

Organic compounds burn when heated - they burn in air wit a yellowy-orange and/or blue flame. The more cartbon in the compound the more yellow and smoky the flame is. When theres plenly of air available, burning a hydrocarbon produced carbon dioxide and water. If amount of air is reduced then carbon monoxide (poisonous gas) and carbon (soot) can also be produced. Solid organic compounds will char (surface will get scorched with black marks of carbon)

Compounds with C=C bonds decolourise bromine water - if organic compound is unsaturated it'll decolourise bromine warter. If its saturated the bromine will stay brown.

Empirical Formula - find the mass of each element in the compound (multiply the mass of co2 produced by the proportion of C in co2, to find the mass of hydrogen multiply the mass of h20 produced by the proportion of H in h20), then divide these masses of C and H by the atomic masses of C and H (to find the number of moles) then divide both answered by the smallest one, this gives the simplest rario of atoms of each element.

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instrumental methods

Machines can also alalyse unknown substances - they are useful for  medical purposes, police forensic work, environmental analysis, checking whether and athlete has taken a banned substance, analysis of products in industry, etc.

Advantages = Can be operated by technicians, lab methods need trained chemists to do everything. More accurate than lab methods, and can detect the tiniest amounts of substances and much faster than lab methods and  tests can be automated.

Disadvantages = Its very expensive to buy, run and maintain the machines.

Atomic absorption spectoscopy indentifies metals - Bit like a flame test machine, used for identifying metals. The pattern of light absorbed by the metals in the sample are analysed. Each metal present in the sample produces a different pattern. Its much faster and more reliable than can be done with human eye. The steel industry uses atomic absorption spectroscopy to check the composition of the steels it produces. Takes minutes compared to days with lab method.

Other techniques identify elements or compounds - 1) Infrared (IR) Spectrometry. 2) Ultraviolet (UV) Spectroscopy. 3) Nuclear Magnetic Resonance (NMR) Spectroscopy. 4) Gas-Liquid Chromatopgraphy. 5) Mass Spectrometry.

 

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