Chemistry Unit 2

AQA CHEMISTRY UNIT 2 (EXAM IN JAN 2012)

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Rates of Reaction

The rate of reaction tells us how quickly reactants become products.

We can measure how quickly reactants are used up or how quickly products are formed.

Measuring a rate involves measuring an amount and the time it takes.

Rate= amount of reactant used/time OR amount of product formed/time.

We can measure the rate of a reaction that gives off a gas by measuring the volume or mass of gas produced over time.

Ractions happen when particles collide with enough energy to bring about a change. The particles must collide with enough energy to change into new substances.

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Rates of Reaction

Factors that affect the frequency o energy of collisions will change the rate of a reaction.

The minimum energy they need is called the 'activation energy'.

Factors that increase that chance of collisions or the energy of the particles will increase the rate of the reaction. Increasing the: Temperature, concentration of solutions, pressure of gases, surface area of solids & using a catalyst will increase the rate of a reaction.

Breaking large pieces of a solid into smaller pieces exposes new surfaces and so increases the surface area. This means there are more collisions in the same time and so a powder reacts faster than large lumps of a substance.

Increasing the temperature increases the speed of the particles in a reaction mixture. This means they collide more often, which increases the rate of reaction. Aswell as colliding more frequently they collide with more energy, which also increases the rate of reaction.

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Rates of Reaction

A small change in temperature has a large affect on reaction rates because  both the number of collisions and energy of collisions are increased/decreased.

At ordinary temperatures a rise of 10 degrees will roughly double the rate of many reactions.

A decrease in temperature will slow reactions down.

Cold: Slow movement, less frequent collisions, little energy.

Hot: Fast movement, more frequent collisions, more energy.

Low Concentration/Low pressure

High concentration/High pressure

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Rate of Reactions

If the concentration of a solution increases there are more particles dissolved in the same volume. This means the dissolved particles are closer together and so they collide more often. Increasing the concentration therefor increases the rate of reaction.

Concentration of solutions are easures in moles per cubic decimetre (mol/dm^3)

Increasing the pressure of a gas puts more molecules into the same volume and so they collide more frequently. This increases the rate of reactions that have gases as reactants.

Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. So 10cm^3 of hydrogen contains the same number of molecules as 10cm^3 of oxygen at the same temperature and pressure.

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Rates of Reactions

Measuring concentrations of solutions in moles per cubic decimeter (mil/dm^3) means we can measure out equal numbers of particles of the solutes by taking equal volumes of the same concentration.

Catalysts change the rates of chemical reactions.

Most catalysts are used to speed up reactions.

The catalysyt is left at the end of the reaction and so it can be used over and over again. Catalysts work by lowering the activation energy of a reaction so that more collisions result in a reaction.

Catalysts can reduce the energy cost and time needed for a reaction. They often only work with one type of reaction.

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Energy & Reactions

Exothermic reactions transfer energy to the surroundings

Endothermic reactions transfer energy from the surroundings

When chemical reactions take place energy is transferred as bonds are broken and made.

Exothermic= temperature increase

Endothermic=temperature decrease

Reversible reactions are exothermic in one direction and endothermic in another.The energy changes of both are always equal.

increase temp=increase amount of products from endothermic reactions. decrease temp= increase amount of products from exothermic reactions

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Energy & Reactions

If a reaction is exothermic- An increase in temperature decreases the yield of the reaction , so the amount of products formed is lower.

-A decrease in temperature increases the yield of a reaction, so the amount of products formed is larger.

If a reaction is endothermic- An increase in temperature increases the yield of a reaction, so the amount of products formed is larger.

-A decrease in temperature decreases the yield of a reaction, so the amount of products formed is lower.

Changes in pressure affect the yield of reversible reactions that have different numbers of molecules of gases in the reactants and products. Increase in pressure will increase the yield of a reaction that has fewer molecules of gases in the products than in the reactants.

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Energy & Reactions

If a reaction produces a larger volume of gases: -An increase in pressure decreases the yield of the reaction, so the amount of products formed is lower.

-A decrease in pressure increases the yield of the reaction, so the amount of products formed is larger.

If a reaction produces a smaller volume of gases: -An increase in pressure increases the yield of the reaction, so the amount of products formed is larger.

-A decrease in pressure decreases the yield of the reaction, so the amount of products formed is lower.

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Energy & Reactions

In the haber process- N2+3H2<-->2NH3

Four molecules of reactant gases produce two molecules of ammonia gas.

Increasing the pressure will produce more ammonia however it also increases the cost of the process so a compromise of a reasonably high pressure is used.The reaction to produce ammonia is exothermic, So lower temperatures give higher yields.The reaction is slower at lower temperatures because the rate decreases and the catalyst does not work aswell, so a compramise temperature is used. The conditions are chosen to produce ammonia as economically as possible.

You can tell is a reversible reaction will be affected by changes in pressure as the reaction will have a different number of molecules of gases in the reactants compared with the products.

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Acids, Alkalis & Salts

When acids are added to water they form hydrogen ions,H+(aq). This makes the solution acidic.

When alkalis are added to water they form Hydroxide ions, OH-(aq)

Acids have pH value below 7 and alkalis have pH value above 7

Bases react with acids and neutralise them. Alkalis are bases that dissolve in water.

ACID+METAL---> SALT+HYDROGEN

Bases are metal oxides or metal hydroxides. They react with acids to form a salt and water-

ACID+BASE---> SALT+WATER

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Acids, Alkalis & Salts

1)Metal or base that is insoluble in water, is added a little at a time to the acid until all of the acid has reacted.2) Mixture is then filtered to remove excess solid, leaving a solution of the salt.3) Solid salt is made when water is evapourated from the solution so that it crystalises.

Chlorides=Hydrochloric acid, Nitrates=Nitric acid, Sulfates=Sulfuric acid

An indicator is used to find when an acid has exactly reacted with an alkali to form a salt. Soluble Salt: ACID+ALKALI--->SALT+WATER

There is no visible change when the solutions react so we need an indicator to show when the reaction is complete. The indicator can be removed from the solution after the reaction.

Ammonia solution i an alkali that does not contain a metal. It forms ammonium Salts, such as ammonium nitrate, which are used as fertilisers.

We can make insoluble salts by mixing solutions of soluble salts that contain the ions needed.

Ions can be moved from solutions by precipitation.

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Electrolysis

When electricity is passed through a molten ionic compound or a solution containing ions, electrolysis takes place. The molten ionic solid or solution of ions is called the 'electrolyte'.

The electrical circuit has two conducting rods called 'electrodes' that make contact with the electrolyte. The ions in the electrolyte move to the electrodes where they are dischared to produce elements.

Positivley charged ions are attracted to the negative electrode where they form metals. Hydrogen may be formed at the negative electrode if the ions are dissolved in water.

Negativley charged ins are attracted to the positive electrode where they lose their charge to form non-metallic elements.

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Electrolysis

When positivle charged ions reach the negative electrode they gain electrond to become neutral atoms. Gaining electrons is called' reduction', so the positive ions have been reduced. Ions with a single positive charge gain one electron and those with a 2+ charge gain 2 electrons.

At the positive electrode, negative ions lose electrons to become neutral atoms. This is 'oxidation'. Some non-metal atoms combine to form molecules.

Water contains hydrogen ions and hydroxide ions. When solutions of ions in water are electrolysed, hydrogen may be produced at the positive electrode. This happens if the other positive ions are of metals more reactive then hydrogen.

Anode=Positive, Cathode=Negative

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Electrolysis

Brine is a solution of sodium chloride in water. When it i electrolysed, hydrogen is produced at the negative electrode from hydrogen ions in the water. Chlorine is produced at the positive electrode from the chloride ions. This leaves sodium ions and hydroxide ions(from water) in the solution.

Sodium hydroxide is a strong alkali and has many uses including making soap, paper, bleach, neutralising acids and controling pH.

Chlorine is used to kill bacteria in drinking water and swimming pools, and to make bleach, disinfectants and plastics.

Hydrogen is used to make margirine and hydrochloric acid.

Impurities in copper affect is properties including its conductivity. Copper for use as electrical wires must be very pure. It can be purified by electrolysis, using copper electrodes in a solution of a copper salt. the impure copper is used as the positive electrode and the negative electrode is a thin sheet of pure copper.Copper atoms on the positive electrode are oxidised, losing electrons to form copper ions that go into the solution. At the negative electrode copper ions from the solution are reduced, forming copper metal. The copper is deposited on the negative electrode, which increases in thickness.

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structures & Bonding

The nucleus at the centre of an atom contains two types of particle, called protons and neutrons. Protons have a positive charge and neutrons have no charge. Electrons are negatively charges particles that move around the nucleus. An atom has no overall charge, because the number of electrons is equal to the number of protons and their charges are equal and opposite.

All atoms of an element contain the same number of protons. This number is called the atomic number (or proton number) of the element. Elements are arranged in order of their atomic numbers in the periodic table. the atomic number tells you the number of protons and the number of electrons in atoms of the element.

Electrons in atoms are in energy levels that can be represeted by shells.

Electrons in the lowest energy level are in the shell closest to the nucleus which can hold up to two electrons. The next two shells can both hold up to eight electrons. All the elements in a group of the periodic table have the same number of electrons in their highest energy level (outer shell).

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Structures & Bonding

Compounds are substances in which elements are chemically combined. When elements react their atoms achieve stable arrangements of electrons. Atoms gain or lose electrons  to form ions to share electrons to form covalent bonds.

Ionic bonding holds oppositely charged ions together in giant structures. Strong electrostatic forces of attraction act in all directions. Each ion in the lattice is surrounded by ions with the opposite charge and so is held firmly in place.

A covalent bond is formed when to atoms share a pair of electrons. The number of cavalent bonds an atom forms depends on the number of electrons it needs to achieve a stable electron arrangement.

Metalic bonding occours when the electrons in the highest energy level of an atom delocalise which results in strond electrostatic forcesbetween these electrons and the positively charges metal ions, holding the metal together.

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How much?

Mass numbers

The mass number is the number on top in the periodic table. The mass of a proton is equal to the mass of a neutron. The relative mass of a proton and neutron are both one unit. The total number of protons and neutrons in an atom is called the 'mass number'. The number of protons and electrons are always equal and its the bottom number on the periodic table. The number of neutrons can be different as they are called isotopes. isotopes are atoms of the same element with different numbers of neutrons. The number of neutrons in an atom = mass number - atomic number.

Relative atomic mass

Relative atomic masses compare the masses of atoms. The relative atomic mass of an element is an average value for the iotopes of an element.

the relative atomic mass of an atom is an average value that depends on the isotopes the element contains.

The relative formula mass of a substance is found by adding up the relative atomic masses of the atom in its formula.

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how much?

To work out the mole- the relative formula mass in grams = 1 mole of that substance.

To find the percentage of an element in a compound divide the relative atomic mass of the element by the relative formula mass of the compound and multiply the anwser by 100 to get it as a percentage.

FIND OUT EMPIRICAL FORMULA

Calculate the yield=

Percentage yield= amount of product calculated/maximum amount of product possible     x100!

Atom economy= relative formula mass of useful product/relative formula mass of all products      x100!

 

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Comments

Sophie Hill

THanks these are really good :) 5*! x

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