Chemistry - C3.

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Energy transfer in reactions.

  • An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat, which is shown by a rise in temperature.
  • An example of an exothermic reaction is combustion.
  • An endothermic reaction takes in energy from the surroundings, usually in the from of heat, which is shown by a fall in temperature.
  • An example of this is thermal decompostion.
  • You can measure the amount of energy produced by a chemical reaction by taking the temperature of the reactants mixing them in a cup and measuring the temperature.
  • Adding an acid to an alkali is an exothermic reaction.
  • Dissolving ammonium nitrate in water is an endothermic reaction.
  • During a chemical reaction old bonds are broken and new bonds formed.
  • Energy must be supplied to break bonds - bond breaking is endothermic.
  • Energy is always released when bonds form - bond formation is exothermic .
  • In an exothermic reaction, the energy released in bond formation is greater than the energy used in breaking old bonds.
  • In an endothermic reaction, the energy required to break bonds is greater than the energy released when new bonds are formed.
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Measuring the energy content of fuels.

  • This calormetric experiment involves heating water by burning a liquid fuel.
  • If you measure how much fuel you've burned and the temperature of the water, you can work out how much energy is supplied by each gram of fuel.
  • You also need to know the waters specific heat capacity which is 4.2J/g/*C.
  • If you do the same experiment with different fuels, you can compare their energy transferred per gram.
  • If a fuel has a higher energy content per gram you need less fuel to cause the same temperature rise.
  • Calorimetric method - reduce heat loss as much as possible.
  • Three calculations to find energy output per gram...
  • Mass of fuel burned is the final mass subtracted from the intitial mass. 
  • Energy transferred = mass of water x S.H.C. of water x temperature change, or Energy transferred = m x c x /\T ( /\T means change in temp).
  • Energy given out per gram = Energy released / mass of fuel burned.
  • To make it a fair test you keep the conditions the same.
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Chemical reaction rates.

  • The rate of a chemical reaction is how fast the reactants are changed into products.
  • Slow reactions include, rusting of iron and chemical weathering.
  • Reactions with a moderate speed include a metal reacting with dilute acid.
  • Burning is a fast reaction but an explosion is really fast.
  • The rate of a reaction that produces gas can be observed by measuring how quickly the gas is produced, there are two ways of doing this...
  • Measure the change in mass at regular time intervals.
  • Measure the volume of gas given off.
  • The rate of a chemical reaction depends on...
  • The collision frequency of reacting particles (how often they collide). 
  • The energy transferred during a collision. Particles have to collide with enough energy fro the reaction to be successful.
  • The amount of product you get from a reaction depends on the amount of reactant you start with.
  • More reactant means more particles which means more reactions.
  • The amount of product you get is directly proportional to the amount of limiting reactant ( the reactant that's totally used up).
  • Once all the limiting reactant is used up the reaction can't continue. Any other reactant left is in excess.
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Collision theory.

  • More collisions increases the rate of reaction
  • The rate of reaction depends on four things...
  • Temperature - If it is increased the particles move qucker therefore collide more. Higher temperature also increases the energy of collisions.
  • Concentration / pressure - If a solution is more concentrated it means there are more particles of reactant in the same volume which makes collisions more likely. In a gas increasing the pressure means molecules are more crowded, so the frequency of the collisions increases.
  • Large surface area - A large surface area means the particles have more area to work on so the frequency of collisions will increase.
  • A catalyst - a substance which increases the speed of a reaction without being chemically changed or used up. Its works by giving the reacting particles a surfaec to stick to so they can collide.
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Rate of reaction data.

  • On a graph..
  • The steeper the line the faster the reaction.
  • You can calculate the rate of reaction by calculating the slope of the line.
  • To find the average rate during the first 30 seconds draw a line from the volume at the start to the volume produced at 30 seconds the find the slope of the line. 
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Reacting masses.

  • In the periodic table the elements have two numbers the bigger one is the relative atomic mass.
  • If you have a compound like MgCl2 then it has a relative formula mass Mr, which is just all the relative formula masses added together which is 95.
  • During a chemical reaction no atoms are destroyed or created.
  • This means there are the same number and types of atoms on each side of a reaction equation.
  • This means no mass is lost or gain. Mass is conserved.
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Calculating masses in a reaction.

  • Example...
  • If 28g of iron reacts with copper sulfate solution what mass of copper will be made?
  • Write down the balanced symbol equation Fe + CuSO4 ---> Cu + FeSO4. 
  • Write down the relative formula masses of reactants and products needed.
  • Fe= 56 and Cu= 64.
  • write down the ratio of reactaning masses
  • 56g : 64g
  • Calculate the scale factor which is the known mass 28g/ relative formula/atomic mass of the reactant 56g
  • 28g / 56g = 0.5
  • Mass of Cu made = scale 0.5 x unknown formula mass of product 64g
  • 0.5 x 64 = 32g
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Atom economy

  • Atom economy of a reaction tells you how much of the mass of the reactants is wasted when manufacturing a chemical.
  • atom economy = total Mr of desired products / total Mr of all products x100.
  • Example...
  • Hydrogen gas is made on a large scale by reacting natural gas (methane) with steam. CH4 + H2O ----> CO + 3H2. Calculate the atom economy...
  • Method...
  • Identify the unseful product - hydrogen gas
  • Work out the Mr of all the product and the useful product.
  • All products = 34. Useful product = 6.
  • 6 / 34 x 100 = 17.6.
  • Reactions with low atom economy use up resources quickly and make lots of waste. This makes reactions unsustainable.
  • Also they aren't profitablle because of this reason.
  • The reactions with the highest atom economy are the ones that only have one product.
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Percentage yield.

  • Percentage yield = actual yield (grams) / predicted yield (grams)  x100.
  • To find this you need to calculate the relitive formula mass of predicted yield (can be done by using the balanced reaction equation) and then divde it by the relitive formula mass of the product you actually got.
  • Yields are always less than 100% due to lots of reason for example...
  • Evaporation.
  • Not all reactants make products - in reversible reactions, the products can turn back into reactants.
  • Filteration - When you filter a liquid to remove solid particles, you nearly always lose a bit of liquid or solid.
  • Transferring liquids.
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Chemical production

  • Pharmaceutical drugs are complicated to make and there's fairly low demand for them. Batch production is often the most cost-effective way to produce small quantities of different drugs to order because...
  • It's flexible - different products can be made using the same equipment and start-up costs are relatively low 
  • But it's labour intensive - equipment nedds to be controlled manually and cleaned and it's hard to keep the same quality.
  • Continuous production runs all the time...
  • Large-scale industrial manufacture of chemicals uses continuous it never stops, runs automatically, consistent quality but start costs are big.
  • Pharmaceutical products cost a lot for lots of reasons...
  • Research + development, trialling, manufacture, cost of energy/material.
  • To extract a substance from a plant, it has to be crushed then boiled and dissolved in a suitable solvent. Then you can extract the substance you want by chromatograpy.
  • Pure substances won't be seperated by chromotography and they have specific melting and boiling points.If a substance is impure, the meting point will be to low and the boiling point to high.
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Allotropes of carbon.

  • Diamonds are sparkly and colourless....
  • Each carbon atom forms four covalent bonds in a rigid giant covalent structure which makes diamond hard, therefore ideal as cutting tools.
  • All the bonds take lots of energy to break this gives diamond a high melting point which is another reason its used as a cutting tool.
  • It doesn't conduct electricity because it has no free electrons or ions.
  • Graphite is black,opaque and has a high melting point....
  • Each carbon atom only forms three covalent bonds, creating sheets of carbon atoms which are free to slide over each other.
  • The layers are held together weakly and can be rubbed on to paper which is how pencil works. This also makes it a good lubricating material.
  • it has three bonds there are delocalised electrons so conducts electricity.
  • Carbon can form lots of covalent bonds with itself so it forms giant molecular structures. They are strong have high melting points and don't dissolve in water and don't conduct electricity, graphite is the exception.
  • Fullerenes are molecules of carbon like closed tubes or hollow balls.
  • They can cage other molecules, form around them and trap them.
  • They can form nanotubes which have large surface areas so can be good industrial catalysts..
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