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Unit 1 - Chemical Changes and Structure

This unit provides an overview of chemical changes, reaction rates, collision theory, and atomic structure. Learn about measuring reaction rates, the factors that affect reaction rates, and the arrangement of electrons in atoms.

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Unit 1 - Chemical Changes and Structure

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  1. Unit 1 - Chemical Changes and Structure

  2. Reaction Rates

  3. Reaction Rates • During the course of a chemical reaction, reactants are being converted into products. • Measurement of the rate of reaction involves measuring the ‘change in the amount’ of a reactant or product in a certain time. • The rate of reaction changes as it progresses, being relatively fast at the start and slowing towards the end. • What is being measured is the averagerate over the time interval chosen. • Reactions can be followed by measuring changes in concentration, mass and volume.

  4. Nat 5 Where property = mass/volume/concentration The above is used when there is no change in mass/volume/concentration measured, for example during a colour change reaction. Higher

  5. Calculate the reaction rate; • During the first 5 seconds • Between 10-15 seconds • Between 20-30 seconds • Of the whole reaction (7-0/5-0 = 7/5 = 1.4cm3/s) (12-10.5/15-10 = 1.5/5 = 0.3cm3/s) (13-12.75/30-20 = 0.25/10 = 0.025cm3/s) (13-0/30-0 = 13/30 = 0.43cm3/s)

  6. Collision Theory A chemical reaction can only occur if there is a successful collision between reactant molecules. From S3 we know that we can speed up a chemical reaction by; • Decreasing particle size (increasing surface area) • Increasing concentration (of reactant) • Increasing temperature • Adding a catalyst

  7. Collision Theory – Particle Size • The smaller the particle size, the higher the surface area. • The higher the surface area, the greater the number of collisions that can occur at any one time. • The greater the number of collisions, the faster the reaction. • Therefore the smaller the particle size, the faster the reaction rate.

  8. Collision Theory – Concentration • The higher the concentration, the higher the number of particles. • The higher the number of particles, the greater the chance of collisions that can occur. • The greater the number of collisions, the faster the reaction. • Therefore the higher the concentration, the faster the reaction rate.

  9. Collision Theory – Temperature • The higher the temperature, the higher the energy the particles have. • The higher the energy, the faster the particles move. • The faster the particles move, the greater the chance that they can collide • The greater the number of collisions, the faster the reaction. • Therefore the higher the temperature, the faster the reaction rate.

  10. Catalysts • A catalyst speeds up a chemical reaction without getting used up or changed itself. • Catalytic converters are used in exhaust systems to turn harmful gases into less harmful gases. Platinum, rhodium (expensive transition metals) are used as the catalyst used in catalytic converters. https://www.youtube.com/watch?v=rmtFp-SV0tY

  11. Atomic Structure and Bonding

  12. The atom is made up of a dense centre called the nucleus, which contains protons and neutrons Modern Day Model of the atom The electrons are very light and are found in a space around the nucleus called the electron shell.

  13. Mass Number 16 O Symbol 8 Atomic Number

  14. Important Atomic Number = No. of ____________ No. of Protons = No. of ____________ (when the atom is neutral!) Mass Number = No. of __________ + No. of _________ No. of Neutrons = Mass Number - ____________ __________ number atomic protons electrons neutrons protons

  15. What if the element isn’t neutral? when there is a charge the no of protons and electrons are no longer equal. (i.e. the no of electrons change)

  16. Isotopes

  17. Isotopes are atoms of the same element (same number of protons) but have different number of neutrons. This means for isotopes,the atomic number stays the same but the mass number changes. Many elements exist as 2 or more isotopes. isotopes

  18. The relative atomic mass (R.A.M.) of an element is the average mass number for a sample of that element. R.A.M is related to isotopes. relative atomic mass

  19. The relative atomic mass of Copper (Cu) is 63.5 63 65 Cu Cu 29 29 What does this tell you about the proportion of the two types of isotopes in a sample of copper? example from whiteboard (think number line)

  20. Electron Arrangements Electrons are arranged in shells (or energy levels.) Lithium has the electron arrangement 2,1 so there are two electrons in the electron shellclosest to the nucleus, and one in the next shell: X X X Li

  21. The 2,8,8,2 Rule For the first 20 elements (hydrogen to calcium) we follow the 2, 8, 8, 2 rule. A maximum of 2 electrons are allowed in the 1st electron shell, 8 electrons in the second, 8 electrons in the 3rd and 2 electrons in the 4th. Each electron shell must be full before the next one is started.

  22. Coincidence? Maybe. Maybe Not? The number of ______ ________ affect the way that the atom reacts. In other words… Elements with the same number of outer electrons (elements in the same group) have similar chemical properties. outer electrons

  23. Important groups to remember; Group 1 – the alkali metals; all very reactive soft metals. (two examples are ________ and ________) Group 2 – alkaline earth metals; similar to group 1 metals but not as soft or reactive. (two examples are ________ and ________) Group 7 – the halogens; very reactive non-metals. (two examples are ________ and ________) Group 8 – the noble gases; very unreactive non-metals (two examples are ________ and ______) In-between groups 2/3 – transition metals. (two examples are ________ and ________)

  24. Colour in all of the solid elements one colour (easiest thing to do is leave them white!) Colour in all of the liquids one colour (Bromine and Mercury) Colour in all of the gases one colour. (elements 1, 7, 8, 9, 17 and all of group 0) Remember to label what each colour represents. solids liquids gases

  25. Carbon Magnesium Potassium Fluorine Oxygen Nitrogen Beryllium Aluminium Neon Bromine For each of the elements (right) write down the following; Name – Symbol - Atomic number – Mass number - Number of protons – Number of electrons – Number of neutrons – Electron Arrangement – Group number – Metal or non metal – Solid, liquid or gas -

  26. BONDING

  27. Why do atoms bond? Noble gases have a complete outer electron shell. This arrangement of electrons is very stable and therefore other elements want to be like the noble gases. The number of electrons the element needs to lose or gain to be like the noble gases is called the valency. The atoms of other elements can collide together and combine to achieve the full outer electron shell.

  28. The Covalent Bond Two atoms get close enough to each other to collide. X X X X X X X X X X X X X X X X O O step 1 Oxygen with red crosses – 2,6 Oxygen with purple crosses – 2,6

  29. The Covalent Bond The two atoms are attracted to one another through the positive nucleus of one and the negative electrons of the other. X X X X X X X X X X X X X X X X O O step 2

  30. The Covalent Bond The two atoms combine and share enough outer electrons for each of them to become stable (full outer shell) X X X X X X X X O O X X X X X X X X step 3 Oxygen with purple crosses – 2,8 Oxzygen with red crosses – 2,8 O molecule 2

  31. A covalent bond is a shared pair of electrons between non-metal atoms. A covalent bond is held by the attraction of the positive nucleus and negative outer electrons of the different atoms. Important !!

  32. Naming Covalent Compounds If a compound name ends in ‘-ide’ then that compound only contains two elements – e.g. carbon nitride contains carbon and nitrogen only. Sometimes prefixes are used in naming compounds – e.g. silicon dioxide Mono – one Di – two Tri – three Tetra - four

  33. Draw the following covalent elements and compounds using both the lines and the overlapping electron shells (circles.) You only have to draw the outermost shell electrons. Hydrogen Chloride (HCl) Phosphorus Trichloride (PCl3) Water (hydrogen oxide)(H2O) Sulphur Fluoride (SF2) Ammonia (nitrogen trihydride) (NH3) Carbon Dioxide (CO2)*** tricky

  34. symbol valency cross cancel formula Working out formula for covalent compounds

  35. symbol valency cross cancel formula Working out formula Examples • Hydrogen Sulphide • Hydrogen Chloride • Phosphorus Oxide • Carbon Sulphide • Hydrogen Fluoride • Carbon Chloride • Silicon Oxide • Carbon Hydride • Nitrogen Hydride • Carbon Nitride

  36. Shapes of Molecules Formula = CH4 Name =CarbonTetrahydride; Structure is drawn like H C H H H tetrahedral

  37. NH3 H2O HCl pyramidal planar / bent linear

  38. Challenge Your challenge is to; Work out the chemical formula Name the compound (use prefixes if necessary) Draw the line drawing + Draw these molecules using the overlapping circles methods Identify the shape of the molecule use a pencil Complete this task on paper – only use jotter as last resort • Selenium and Iodine • Hydrogen and Fluorine • Carbon and Chlorine • Carbon and Hydrogen • Nitrogen and Hydrogen • Phosphorus and Bromine • Carbon and Sulphur • Silicon and Oxygen

  39. Properties of Covalent Molecules Methane molecule (CH4) This is a covalent molecule. Other examples include water (H2O), oxygen (O2) and candle wax (C8H18). Covalent molecules; have low melting and boiling points can be solid, liquid or gas at room temperature. never conduct electricity (in any state) H C H H H

  40. Covalent Network Substances Some covalent substances do not have individual molecules. Diamond is an example of a covalent network structure. Sand (silicon dioxide) is another example.

  41. This is a diamond structure (carbon atoms only.)

  42. Covalent networks; have extremely high melting and boiling points. are always solid at room temperature. never conduct electricity graphite is the exception to this rule as although it is a covalent network it will conduct electricity. Properties of Covalent Network

  43. Strong Covalent Bond (hard to break) Weak intermolecular Bond (easy to break)

  44. Although the covalent bonds within covalent molecules are strong, the force of attraction between the molecules are weak. These weak forces of attraction don’t require a lot of energy to break and therefore covalent molecules have low melting/boiling pts. Allbonds in a covalent networksare very strong covalent bonds and it takes a lot of energy to break these bonds – i.e. very high melting/boiling points. Covalent Molecular Vs Covalent Network

  45. Low Very high Intermolecular Covalent Very strong Weak

  46. Typical Past Paper Questions - Bonding Which line in the table shows the properties of a covalent molecular compound?

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