Chemistry Unit 2

Chemistry unit 2 exam

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In ionic bonding, atoms lose or gain electrons

-Ionic bonds produce giant ionic structures where the ions are closely packed together in a lattice arrangement.

-There are very strong chemical bonds between all the ions.

-They have high melting and boiling points due to the strong chemical bonds.

-When dissolved, the ions separate and are free to move so the solution can carry electric current.

-They also conduct electricity when molten

POSITIVE IONS = CATIONS

NEGATIVE IONS = ANIONS

Covalent bonding is when atoms share electrons with one another. This way, both atoms feel they have a full outer shell. Examples: H2, Cl2, Hcl, NH3, CH4, H2O, O2

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In ionic bonding, atoms lose or gain electrons

-Ionic bonds produce giant ionic structures where the ions are closely packed together in a lattice arrangement.

-There are very strong chemical bonds between all the ions.

-They have high melting and boiling points due to the strong chemical bonds.

-When dissolved, the ions separate and are free to move so the solution can carry electric current.

-They also conduct electricity when molten

POSITIVE IONS = CATIONS

NEGATIVE IONS = ANIONS

Covalent bonding is when atoms share electrons with one another. This way, both atoms feel they have a full outer shell. Examples: H2, Cl2, Hcl, NH3, CH4, H2O, O2

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Metallic structures, New materials

-Metals consist of a giant covalent structure.

-The metallic bonds involve free electrons which come from the outer shell of every metal atom in the structure.

-Electrons hold the atoms together in a rigid structure.

-Allow atoms to slide over each other causing metals to be malleable.

Smart materials...

-Behave differently depending on conditions (eg temperature)

-Example - Nitinol - Shape memory alloy.

Nanomaterials... -Really really tiny particles (1-100 nanometres across)

-They include fullerenes which are molecules of carbon shaped like hollow balls or closed tubes.

- Nanoparticles are useful because they have a huge surface area.

-They can be made by molecular engineering

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Relative formula mass

Relative atomic mass = A Relative atomic mass is usually same as mass number of element.

Compounds have relative formula mass is all the relative atomic masses added together!

% mass of an element in a compound = A x number of atoms of the element

                                                                ----------------------------------------- x 100

                                                                M (of whole compound)

Empirical formula = The lowest ratio of atoms.....

- List elements

- Write in experimental masses

- Divide by the A (atomic mass) for each element

- Multiply and divide to work out ratio

- Get the ratio in the simplest form.

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Calculating Masses in Reactions, The Mole

-Write out balanced equation (2Mg + O2 --> 2MgO)

-Work out the M (48 --> 80)

-Divide to get one, then multiply to get all (48g of Mg gives 80g of MgO) (Divide by 48, *60)

Mole = 6.023 x 10^23 (602,300,000,000,000,000,000,000)

^^That many atoms of Carbon-12 = 12g

*One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass for that substance*

NUMBER OF MOLES = Mass in g (of element or compound)

                                       -----------------------------------------------------

                                        M (of element or compound)

NUMBER OF MOLES = Volume (in litres) x Moles per litre of solution

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Atom economy, Percentage yield

Atom economy = % of reactants changed to useful products

ATOM ECONOMY = Total M of useful products

                                ------------------------------------- x 100

                                     Total M of reactants

High atom economy is better for profits and environment

Percentage yield compares actual and predicted yield

PERCENTAGE YIELD = Actual yield (g)

                                       ------------------ x 100

                                     Predicted yield (g)

It is impossible to get a 100% yield Reversible reactions

- reactions and products in an equilibrium Filtration

- filtering a liquid to remove solid particles (always results in losing some solid) Transferring Liquids

- Always lose some liquid when transferring because of inside surface Unexpected reactions

- these can cause yield of intended product to go down.

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Rates of reactions

Slow - Rusting of iron, Medium - Metal and acid, Fast - Explosion.

Rate of reaction is dependant on:

Temperature, Concentration, Catalyst and Size of particles.

The speed of a reaction can be measured by either how quickly the reactants are used or by how quickly the product is formed (easier).

RATE OF REACTION = Amount of reactant used or product formed // time

There are 3 ways the rate of a reaction can be measured:

PRECIPITATION - When the product of the reaction is a precipitate which clouds the solution. You can observe how long it takes for a marker to disappear in the solution. Result can be subjective.

CHANGE IN MASS - Gas can be given off so mass lowers. The quicker the mass lowers, the faster the reaction ---- Most accurate method.

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Rates of reaction continued...Collision theory

THE VOLUME OF GAS GIVEN OFF - Involves the use of a gas syringe to measure volume of gas, the more gas, the faster the interval. Gas syringes are accurate to the nearest millilitre

Hydrochloric acid and marble chips - Marble chips in acid, repeat experiment with same mass but chips more crushed up.

Magnesium metal with dilute HCl - Reaction is good for measuring the effects of increased concentration. 

COLLISION THEORY 

Temperature - Increase of temp makes particles move quicker and collide more.

Concentration - Increase means there are more particles of reactants to collide.

Surface area - Larger means more area to work on so more useful collisions.

Catalysts - Solid catalyst gives reacting particles a surface to stick to and increase number of successful collisions by lowering activation energy.

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Catalysts

A catalyst is a substance which changes the speed of a reaction without being changed or used up itself.

Catalysts lower the activation energy, making it easier for them to happen so a lower temperature can be used.

Solid catalysts are best when as a fine powder giving them a large surface area.

Catalysts help reduce costs in industrial reactions.....

-Increase rate of reaction which saves money as processes are quicker

-Make reaction  happen at lower temp so less energy required

-They can be expensive

- Might need several different catalysts to help ALL reactants

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Energy transfer in reactions, reversible reactions

Exothermic is a reaction which gives out energy to the surroundings in the form of heat.

Examples of exothermic reaction - burning fossil fuels, neutralisation reactions, oxidation reactions.

Endothermic is a reaction which takes in heat energy from the surroundings.

Thermal decompositions are the best example of endothermic reactions...

CaCo3 ----> CaO + CO2

A reversible reaction is one where products of the reaction themselves can react to produce the original reactants.

Reversible reactions reach a DYNAMIC EQUILIBRIUM where reactions are taking place in both directions with the overall effect being nil.

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Reactions, The Haber Process

If temperature or pressure is deliberately altered then the point of equilibrium changes too.

If you raise the temp the endothermic reaction will increase to use up the heat.

If you reduce the temp, the exothermic reaction will increase to give off heat.

If you raise pressure it will encourage reaction which produces less volume.

If you lower pressure it will encourage reaction which produces more volume.

ADDING A CATALYST DOESN'T CHANGE DYNAMIC EQUILIBRIUM!

 (http://www.chemguide.co.uk/physical/equilibria/habereq.gif)

Nitrogen is obtained from air

Hydrogen comes from natural gas

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Covalent substances

Simple molecular substances.....

- Atoms form very strong covalent bonds to form small molecules of several atoms. - Forces of attraction between molecules are very weak.

- Low inter-molecular forces mean low melting and boiling points.

- Most molecular substances are gases or liquids at room temperature.

- They don't conduct electricity because there are no ions.

Giant covalent structures.....

- All atoms bonded by strong covalent bonds. -

Very high melting and boiling points.

- Don't conduct electricity, even when molten. - Usually insoluble in water. - Main examples are diamond and graphite.

DIAMOND - Very rigid covalent structure

GRAPHITE - Layers which are free to slide over one another, free electrons so only non-metal that is a good conductor of electricity.

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Acids and Alkalis, Acid and metals

Acid is a substance with PH less than 7. They form H+ ions in water.

Base is a substance with PH of greater than 7.

Alkali is a base that dissolves in water. Forms OH- ions in water.

Acid + base ----> Salt + Water

H+ + OH- ----> H2O

Acid + Metal ---> Salt + Hydorgen

The more reactive the metal, the faster the reaction will go.

Copper doesn't react with Hcl at all because it is less reactive than hydrogen.

Speed is indicated by how many bubbles of gas are given off.

Name of salt depends on acid and metal used.

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Producing salts....

2HCl+ Mg ---> MgCl2 +H2       Magnesium Chloride

6HCl +2Al ---> 2AlCl3 +3H2     Aluminium Chloride

2HCl + Zn ---> ZnCl2 + H2      Zinc Chloride

H2SO4 + Mg ---> MgSO4 + H2              Magnesium Sulfate

3H2SO4 + 2Al ---> Al2 (SO4)3 + 3H2    Aluminium Sulfate

H2SO4 + Zn ---> ZnSO4 + H2                 Zinc Sulfate

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Oxides, Hydroxides and Ammonia

Some metal oxides and metal hydroxides dissolve in water, these compounds are alkalis. 

ALL metal oxides react with acids to form salt and water.

Acid + Metal oxide --> Salt + Water

Acid + Metal hydroxide ----> Salt + Water

Ammonia can be neutralised with HNO3 to make fertiliser...

NH3        +      HNO3    ---->      NH4NO3

Ammonia +  Nitric Acid ----> Ammonium nitrate

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Making salts and electrolysis

Making soluble salts from insoluble bases - Acid + insoluble metal carbonate/hydroxide. Add to acid till all acid is neutralised and then filter out excess carbonate and evaporate water.

Making insoluble salts - precipitation reactions - If you want to make an insoluble salt then you can use a precipitation reaction.

Making salts by displacement - If a more reactive metal is placed in a salt solution of a less reactive metal then the more reactive one will take the others place. The displaced metal then coats itself onto the more reactive metal.

Electrolysis means splitting up with electricity...

It is the breaking down of a substance using electricity which requires a liquid to conduct the electricity, called the electrolyte.

Electrolytes are usually free ions dissolved in water which make the whole thing work.

For the circuit to be complete, there must be a flow of electrons  which are taken away from ions at the positive anode and given to ions at the negative cathode.

As they gain or lose electrons, they become atoms, or molecules are released.

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Electrolysis continued...

Electrolysis of a salt solution:

At the cathode, 2 hydrogen ions accept 2 electrons and form one H molecule.

At the anode, 2 chloride ions lose their electrons and become one Cl molecule.

NaOH is left in the solution.

Cathode: 2H+   +    2e-  --->  H2

Anode:    2Cl-   ----> Cl2  +  2e-

Concentrated brine is electrolysed industrially...

H gas is given off at cathode, Cl gas given off at anode, Sodium Hydroxide is left.

Useful products = Chlorine ( kill bacteria, plastics, insecticides), Hydrogen(used in haber process, change oils to fats) and Sodium Hydroxide(soap, organic chemicals, oven cleaner)

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Purifying copper by electrolysis

Electrons are pulled off copper atoms at the anode causing them to go into solution as Cu2+ ions.

Offering electrons at the cathode to nearby Cu2+ ions turn them back to copper atoms.

Impurities are dropped at anode as sludge whilst pure copper atoms bond to the cathode.

Reaction at cathode = Cu2+  +  2e' ---> Cu

Reaction at anode = Cu  --->  Cu2+   +    2e-

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