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Chemistry Revision C3
Transcript of Chemistry Revision C3
The reaction stops when this is used up
The amount of product formed is directly proportional to the amount of limiting reactant
All other reactants are described as being 'in excess' (i.e. there is lots available) Limiting Reactant Measuring Rate of Reaction Example reaction:
Magnesium + Hydrochloric Acid --> Magnesium Chloride + Hydrogen
To measure the gas released, use either a gas syringe or an upturned measuring cylinder filled with water.
The gas syringe was more accurate.
The amount of gas produced in a certain time is the rate of reaction. Rate of Reaction The rate of reaction is the gradient of the line, The change in y
The change in x Effect of Concentration on Reaction Rate Example reaction: The disappearing cross
Sodium thiosulfate +hydrochloric acid --> Sodium chloride + sulfur dioxide + water
Na S 0 + 2HCl --> 2NaCl + S + SO + H O 2 5 3 2 2 This is a precipitate - a solid that appears when two solutions are mixed.
It will obscure a cross drawn on the paper below the flask. Concentration Concentration of y = Volume of y x 0.25
50 e.g. Concentration of sodium thiosulfate =
Volume of thiosulfate x 0.25
50 Reaction Rate and Temperature As the temperature increases, the time taken for the reaction decreases.
When particles are heated, they gain energy. This means that they more faster. The collision rate increases.
A successful collision means the reaction takes place.
Some collisions are not successful because nothing happens.
There are more successful collisions at higher temperatures because the particles have more energy. Collisions Increasing reaction rate causes rate of reaction to increase because particles must collide before they react. The more frequently they collide, the faster the (rate of) reaction will be.
Particles can only react if they collide with enough energy (activation energy). The more energy the particles have, the more energy is transferred during the collision and the more likely it is that the collisions will be successful.
This is called the collision theory. How does changing the concentration cause rate of reaction to change? The concentration of a solution is a measure of how much solute is dissolved in the solvent.
When solutions are more concentrated their particles become more crowded. They do not gain energy but they do have more collisions. How does changing pressure cause rate of reaction to change? When gases are put under pressure, their particles become more crowded. They do not gain energy but they do have more collisions.
The collision frequency between reactant particles increases and so the rate of reaction increases.
For example, high pressure is used to speed up the rate of reaction of explosions. How does changing temperature cause rate of reaction to change? When particles react at higher temperature, they have more kinetic energy. This means they move around more quickly and therefore are more likely to collide.
The collision frequency between reactant particles increases and so the rate of reaction increases.
Also, as the particles have more energy, each collision is likely to be successful. This increases the rate of reaction as well. Surface Area Increasing the surface area of a reactant (e.g. from lump to powder form) increases the rate of reaction.
This is because a greater surface area increases the frequency of collisions between particles.
Combustible substances burn easily (e.g. flour). When released as a fine powdery dust, they can explode if they come into contact with a spark. Catalysts A catalyst is a substance that changes the rate of a reaction and is unchanged at the end of the reaction.
Only a small amount of catalyst is needed to catalyse large amounts of reactants (because it isn't used up in the reaction).
Catalysts are often specific to one particular reaction (especially enzymes). Reacting Masses Mass is conserved during all chemical reactions.
Therefore, mass of reactants = mass of products.
e.g. Magnesium + Oxygen --> Magnesium Oxide
2Mg + 0 --> 2MgO (s) 2(g) (s) This is because atoms are never lost or gained during a chemical reaction. Relative Atomic Mass The relative atomic mass is the mass of an atom compared to the 1/12th the mass of a carbon atom.
The relative mass of...
Carbon is 12
Hydrogen is 1
Oxygen is 16
Nitrogen is 14 Li Lithium 3 7 Relative Formula Mass The relative formula mass is the mass of the atoms in a compound added together.
For example, the relative formula mass of...
CaO = 40 + 16 = 56
KOH = 39 + 16 + 1 = 56
Na O = 23 +23 +16 =62
Mg(OH) = 24 + (2 x 16) + (2 x 1) = 58 2 2 We can use the relative formula to check that mass is conserved in any reaction. Tests For Elements Carbon dioxide - Turns cloudy in lime water - white precipitate
Hydrogen - Squeaky pop when put near glowing splint
Oxygen - Relights a glowing splint Reacting Masses CuO + H SO --> CuSO + H O
80 98 160 18
CuCO --> CO + CuO
124 44 80 2 4 4 2 3 2 How much copper sulphate would you expect to make from 80g of copper oxide? 160g
How much copper sulphate would you expect to make from 4g of copper oxide? 8g
How much copper oxide would you expect to make from 12.4g of copper carbonate? 8g This is only 'expected' because some might be lost (e.g. stuck to the flask or escape as gas) or not all the reactants react completely, or 'side' reactions happen that you may/may not be expecting (e.g. a substance reacts with an element in the air). Thermal decomposition Percentage Yield In real life we don't always get what we expect!
Some materials get stuck to flasks and test tubes, some escape into the air and some simply don't react completely.
So although, in theory, you could make the amount you've worked out, you probably won't.
Chemists call how much they actually made compared to how much they expected to make the 'yield'. Predicted yield = maximum mass of product that can be made
Actual yield = actual mass of product collected Percentage Yield Percentage Yield = Actual Yield x 100
Predicted Yield First, write out a balanced symbol equation.
Then work out the RFM for each substance.
Then work out the amount of sodium chloride you should get (predicted yield).
Finally, work out the percentage yield using the equation. Working Out Percentage Yield In an experiment, 5.60g of sodium reacted with an excess of chlorine. The mass of sodium chloride collected is 9.72g What is the percentage yield?
Equation: 2Na + Cl --> 2NaCl
RFM: 2 x 23 2 x 35.5 2 x (23 + 35.5)
Predicted yield: 5.6/46 x 117 = 14.2g
Actual yield: 9.78g
Percentage yield = actual/predicted x 100
= 9.78/14.2 x 100
= 68.9% Example Percentage Yield Question 2 C3 Atom Economy Atom economy = formula masses of desired products x 100 formula masses of all the products MgCO + H SO --> MgSO + H O + CO 3 2 4 4 2 2 120 18 44 E.g. Atom economy = 120/182 x 100 = 66% Atom Economy Atom economy is a way to measure the amount of atoms that are wasted when a certain chemical is made.
If something has a higher atom economy, it is 'greener'.
If something has 100% atom economy if there are no waste products. The Importance Of Atom Economy Atom economy is a comparison of how much waste product is produced compared to how much useful product. When a reaction occurs some of the products will be useful and some waste. Atom economy is calculated by comparing the relative atomic mass of the useful products with the waste products as a percentage. Those that are waste products will require disposing of and are usually bad for the environment, making this an expensive process. This means the reaction is expensive to perform and can cut profit a company can make from it, or mean it cannot afford to do the reaction at all. A reaction can have 100% atom economy if all of the products are useful. Companies will try and get the highest atom economy so they can make the most profit. Hot Or Cold? H + Cl --> 2HCl 2 2 H-H + Cl-Cl --> H-Cl + H-Cl These bonds get broken when a reaction occurs. This takes in energy, therefore, this step is endothermic. These bonds get formed when a reaction occurs. This gives out energy, therefore, this step is exothermic. If the bond braking stage involves more energy, the reaction is endothermic.
If the bond making stage involves more energy, the reaction is exothermic. Measuring The Energy Released Calorimetry is a method used to measure the amount of energy given out when fuels combust.
The fuel is used to heat some water and the temperature of the water is measured.
Energy released = mass of water x 4.2 x temperature change
E=mc T Change Mass of water = Volume of water
c = specific heat capacity Energy In Chemicals When methane undergoes combustion is excess oxygen, carbon dioxide, water and energy are formed.
Alcohol + Oxygen --> Carbon Dioxide + Water Exothermic Or Endothermic To work out if a reaction is exothermic or endothermic:
Make sure you have a balanced symbol equation.
Draw out all the molecules, showing all the bonds.
Add up all the bond energies for the bonds that are broken.
Add up all the bond energies for the bonds that are formed.
Do this sum: energy of bonds broken - energy of bonds formed
If the number is negative, the reaction is exothermic.
If the number is positive, the reaction is endothermic. Exothermic Or Endothermic Example C + O --> CO 2 2 C O=O O=C=O Bonds Broken Bonds Formed 0 + 498 805 x 2 Bond energies (kJ):
C=O 805 Given in exam 498 1610 498 - 1610 = -1112
So it is exothermic Allotropes Of Carbon Carbon can exist is 3 forms known as allotropes:
These have different properties because their carbon atoms are joined in different ways.
In each case however, the atoms are joined by strong covalent bonds to form a giant covalent structure. Diamond Uses: Jewellery, ends of drills/machinery, etc.
Properties: Lustrous (shiny), colourless, very hard, high melting point (because of strong covalent bonds), etc.
Diamonds are very hard because they have many strong covalent bonds, creating a strong, rigid, 3D structure.
Diamond has a high electrical resistance because it has no delocalised electrons. delocalised - free/moving electrons Graphite Uses: Pencil 'lead', electrolysis, lubricant (replaces oil in machinery.
Properties: Soft/breaks easily, high melting point (used for electrolysis), slippery (because of its layers), black, etc.
Graphite can conduct electricity because of the delocalised electrons. They also have a high melting point.
Graphite is used in pencil 'lead' because it flakes off easily and sticks to the paper. It is soft and slippery. It is used in some engine oils because it is slippery and a good lubricant. Both of these are due to the weak intermolecular forces between layers, allowing them to slide over each other. Glass has the same structure as diamonds Structures Of Carbon Allotropes Diamond Graphite Fullerenes Each C (carbon) is strongly bonded to 4 others. Each C atom is strongly bonded to 3 others. Weak intermolecular forces between layers caused by delocalised electrons. Fullerenes Fullerenes are hollow and the space inside is large enough to contain atoms or other molecules. Scientists have discovered how to 'cage' drug molecules inside fullerenes. They can be coated with chemicals that cause them to target cancer cells - in this way the drug can be delivered to its target without damaging normal cells. Nanotubes can be used as catalysts when they are stacked side by side. They have a huge surface are compared to volume allowing a high collision frequency. Their properties allow them to attach other substances (e.g. nitrogen or iron) to its surface. Nanotubes fullerenes joined together strongest and stiffest material Batch Or Continuous? Continuous Processes:
The haber process
Making ethanol (reacting ethene with steam)
Purifying water - making it safe to drink Batch processes:
Making fertilisers from ammonia and acids (nitric acid)
Making ethanol (by fermentation) Continuous Process The product is continuously produced 24/7.
Used to make chemicals that are needed in very large quantities.
e.g. ammonia via the haber process.
Advantage: Most machines run automatically so labour costs are low.
Disadvantage: Machines are dedicated to making one product only (can't be changed). Batch Process Makes small(er) batches on demand.
Used to make pharmaceutical drugs.
Advantage: Products can be easily changed, e.g. one batch of paracetamol followed by a batch of aspirin.
Disadvantage: Large labour costs. Extracting Chemicals The plant is crushed.
And mixed with a solvent.
The mixture is filtered.
And purified (by mixing with another solvent which will only allow the drug to dissolve and no other chemicals).
The solvent is evaporated.
It is checked for purity using chromatography. This depends on the property of solubility.
The melting and boiling point can also be used to test for purity. The melting/boiling point can be measured; impurities alter the temperature at which a drug melts.