chemistry remove revision

HideShow resource information

bonding: Ionic bonding

ionic bonding -  is the transfer of electrons.

  • ionic bonds form only between metal and non metal atoms.
  • in ionic bonding atoms either lose or gain electrons through the transfer of electrons, to form charged particles (called ions) which are then strongly attracted to each other. becuase of the attraction of opposite charge (+/-).
  • this strong attraction is known as electrostatic attraction - this gives ionic compounds a high melting and boiling point.

sodium chloride : a sodium atom transfers its outer electron to a chlorine atom , giving it a positive sodium ion and a negative chlorine ion.

  • this results in the ions being stable because they have 8 electrons in their outer shells.

STRUCTURE: compounds with ionic bonding always have giant ionic structures. the ions are in a closely packed lattice arrangement by he attraction of the oppisitely charged ions.

 a lattice with a charge of 2+ and 2- ions will be held together with a stronger force of of attraction than a lattice of 1+ and 1- ions.                                                                                                properties include: high melting/boiling points, conduct electriciy when dissolved in water.      

1 of 15

Bonding: covalent bonding

Covalent bonding:

  • they are formed between atoms of a non metal and a non metal. (different or the same)
  • they make covalent bonds by sharing their electrons with other atoms.
  • they form to give a stable arrangement as hey will have a full outer shell.
  • the bonded atoms form a unit called a molecule.
  • in a covalent bond there is a strong attraction between the shared electrons and the nucleus.  e.g. H2 , O2 , Cl2

the number of paired electrons determines the how many bonds it has. e.g. a nitrogen molecule has three shared pairs of electrons = a triple bond

STRUCTURE: Simple molecular - the atoms within the molecule are held together by very strong covalent bonds, but the force of attraction between the intermolecular forces are very weak. properties -:low melting/boiling point, usually a gas or a liquid at room temperature. 

Giant covalent structure - all atoms are bonded to each other by strong covalent bond (no charged ions). there are lots of these bonds which means alot of energy is needed to break them.                properties-: high melting/boiling point, don't conduct electricity (except for graphite), insoluble in water.      Examples: graphite and diamond

2 of 15

Bonding: Metallic bonding

Metallic bonding: metals are held together by metallic bonding. the attraction between the positive ions and the negatie electrons keeps the structure together.

STRUCTURE: metals have giant structures of positive ions surounded by a sea of delocalised (free) electrons.

Properties: conduct heat and electricity, malleable.

3 of 15

Lattice Structure

Rates of reaction

Definition:

  • Rate is a mesure of how fast a reaction goes.
  • to measure the rate of reaction, you need to do an experiment to measure a change in mass or volume over a period of time.

The Collision Theory in rates of reaction:

For a chemical reaction to occur, the reactant particles must collide. But collisions with too little energy do not produce a reaction.

  • The particles must have enough energy for the collision to be successful in producing a reaction.
  • The rate of reaction depends on the rate of successful collisions between reactant particles
5 of 15

Measuring Rates of reaction: 1

There are two ways to find the rate of a reaction: 

  • measure the rate at which a reactant is used up
  • measure the rate at which a product is formed

factors effecting the rate of reaction:

1. temperature

2. concentration

3. Catalyst

4.Size of particles or surface area

6 of 15

Measuring Rates of reaction: temperature, concentr

(http://www.bbc.co.uk/staticarchive/e5e463d56f8fe9d5a819836d0f5866469458c6e5.gif)

Temperature:

  • If the temperature is increased:

    • the reactant particles move more quickly
    • they have more energy
    • the particles collide more often, and more of the collisions result in a reaction
    • the rate of reaction increases
7 of 15

Measuring Rates of reaction: surface area

Effect of surface area:

The rate of a chemical reaction can be raised by increasing the surface area of a solid reactant. This is done by cutting the substance into small pieces, or grinding it into a powder.

(http://www.bbc.co.uk/staticarchive/f553e97c9d3c44d5a7550de602adbdc8a0a7d074.gif)

If a solid reactant is broken into small pieces or ground into a powder:

  • its surface area increases
  • more particles are exposed to the other reactant
  • there are more collisions
  • the rate of reaction increases
8 of 15

measuring the rates of reaction: Catalyst

Effect of catalysts:

A catalyst is a substance that can increase the rate of a reaction. The catalyst itself remains unchanged at the end of the reaction it catalyses. Only a very small amount of catalyst is needed to increase the rate of reaction between large amounts of reactants.

catalystreaction catalysed iron     making ammonia from nitrogen and hydrogen platinum      making nitric acid from ammonia vanadium(V) oxide     making sulphuric acid

9 of 15

Calculating rates of reaction

The faster the rate, the more reactant is used, or product is made, in a given time. The faster the reaction, the steeper the line on a graph showing total product against time will be.

rate of reaction = total amount of reactant used or product made ÷ time taken

  • A reaction finishes when one of the reactants is all used up. No more product is made, so the line on a graph of total product against time will become horizontal.
10 of 15

Measuring Rates of reaction: Methods

1. Precipitation

  • This is when the product of the reaction is a precipitate which forms a cloudy solution.
  • Observe a marker through the solution and measure how long it takes for it to disappear.
  • The quicker the marker disappears, the quicker the reaction.

2. Change in mass

  • measuring the speed of a reaction that produces a gas can be carried out on a mass balance.
  • As the gas is released the mass disappearing is easily measured on the balance.
  • the quicker the reading on the balance drops, the quicker the reaction.

3. The volume of gas given off

  • this involves the use of a gas syringe to measure the volume of gas given off.
  • The more gas given off during a given time, the faster the reaction.
  • a graph of gas volume against time could be plotted to give a rate of reation graph.
11 of 15

Rate of reaction: experiments

1. Reaction of hydrochloric acid and marble chips:

  • measure the volume of gas released with a gas syringe, take readings at regular intevals.
  • Make a table of the readings and plot them as a graph, the time is the independant variable(x) and volume is the dependant variable(y).
  • repeat experiment with the same volume of acid and the same mass of marble chips, but with smaller bits of marble, then to follow do the experiment with powdered marble chips.
  • This will show that the increase in surface area causes more frequent collisions, so he reaction rate is faster in the sammer and powdered marble chips.

2. Reaction of magnesium metal with dilute HCL:

  • This reaction is good for measuring the effects of increased concentration.
  • The reaction gives off hydogen gas, which we can measure he loss in mass with a balance as the gas is given off.
  • In this experiment time is the independant variable(x) and the mass loss is the dependant variable(y).
  • The experiment will show, that with a higher concentration of acid the reaction is faster.
12 of 15

Energy tranfer in reactions

1. Energy must always be suplied to break bonds:

Energy must be suplied to break existing bonds ( give out energy)  - bond breaking is an endothermic process.The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to get colder. e.g:

  • Electrolysis
  • The reaction between ethanoic acid and sodium carbonate
  • The thermal decomposition of calcium carbonate in a blast furnace

Energy is released when new bonds are formed ( take in energy ) - bond formation is an exothermic process.The energy is usually transferred as heat energy, causing the reaction mixture and its surroundings to become hotter. e.g:

  • Combustion (burning)
  • Many oxidation reactions, for example rusting
  • Neutralisation reactions between acids and alkalis
13 of 15

Energy change diagrams

Exothermic reeaction:                                                  Endothermic reaction:

Energy diagram for an exothermic reaction (http://www.bbc.co.uk/staticarchive/2f70326aedb5723204b4b9dd9c9f4c37f9f39689.jpg)Energy diagram for an endothermic reaction (http://www.bbc.co.uk/staticarchive/ef51e506a620e9d39642c54e1d5f5cbfdc1770b2.jpg)

14 of 15

moles

15 of 15

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all chemistry remove revision resources »