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Chemical reactions


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Chemical reactions

  1. 1. Chemical Reactions and Energetics Grade 10 IGCSE
  2. 2. What you need to be able to do and understandEnergetics in Chemical Reactions  Interpret data obtained from Relate the terms exothermic and experiments concerned with speed of endothermic to the temperature reaction. changes observed during  Describe the application of the above chemical reactions. factors to the danger of explosive Demonstrate understanding that combustion with fine powders (e.g. exothermic and endothermic flour mills) and gases (e.g. mines). changes relate to the  Describe and explain the effects of transformation of chemical temperature and concentration in energy to heat (thermal terms of collisions between reacting energy), and vice versa. particles (concept of activation energy will not be examined).Speed of Reactions  Define catalyst as an agent which Describe the effect of increases rate but which remains concentration, particle unchanged. size, catalysis and temperature on the speeds of reactions. Redox Describe a practical method for  Define oxidation and reduction in investigating the speed of a terms of oxygen loss/gain, and reaction involving gas evolution. identify such reactions from given Devise a suitable method for information. investigating the effect of a given  Define redox in terms of electron variable on the speed of a transfer, and identify such reactions reaction. from given information.
  3. 3. Endothermic and Exothermic Reactions During a chemical reaction there is always an energy changeExothermic: give out energy (heat, light or sound) reactants → products + energyThe total energy is the same but the products havelower energy than the reactants.Endothermic: take in energy reactants + energy → productsThe total energy is the same but the products havehigher energy than the reactants since energy wastaken in. A song to help you remember the difference: =XgiCn1IpvzM
  4. 4. Which is which- Endothermic or Exothermic? • A cold pack (for sport injuries) • Endothermic • The combustion of fuels • Exothermic • Firing a cannon • Exothermic • Cooking a steak • Endothermic • Cellular respiration • Exothermic • Photosynthesis • Endothermic
  5. 5. Is this an exothermic or endothermic reaction? Remember energy is measured in kilojoules (kJ)CaCO3(s) → CaO(s) + CO2(g) the energy change is +178kJ The + sign shows that energy is taken in This is an endothermic reaction Fe(s) + S(s) → FeS(s) the energy change is -100kJ The - sign shows that energy is given out This is an exothermic reaction A little fun
  6. 6. Rates of ReactionsA rate is a measure of how fast or slow something is. Reactions can be slow orfast. Some fast reactions include: How can you measure the rate of • Fireworks reaction? • Wood burning • Florescent light bulbs In general, to find the rate of a reaction, you should measure: Some slow reactions include: • Rust forming on a car • the amount of a reactant used up/unit • Food rotting time • Cellular respiration OR • the amount of a product produced/unit time A song to introduce rates of reactions X9Xo6zm_kM
  7. 7. Factors that change reaction rates The rate of a reaction depends on how many successful collisions there are in a given unit of timeIf we want to increase/decrease the number ofcollisions what would we change? • Temperature • Concentration • Surface area (particle size) More animations on The Collision Theory: We will use this animation to change some variables to see the fecting.htm effect on the product ons/collision.swf
  8. 8. Effect of concentration or temperature on the rate of a reaction If the concentration or temperature of a reactant is increased the reaction goes faster.When the reactants have a higher temperature, they move faster, thereby increasingthe chance of a collision.When the reactants have a lower temperature, they move slower, thereby decreasingthe chance of a collision.When the reactants are less concentrated there is less chance of a collision.When the reactants are more concentrated there is more chance of a collision. An animation on temperature: Why does this graph have a plateau? ersciences/chemistry/animation s/collision_theory.asp
  9. 9. Effect of surface area (size of particle) on the rate of a reaction If the surface area of a reactant is increased the reaction goes faster.Remember a collision involves surfaces.Particles that are smaller have more surfaces to react with.When the reactants are smaller there is more chance of a collision.When the reactants are larger there is less chance of a collision. Why does this graph have a plateau?
  10. 10. Effect of a catalyst on the rate of a reaction. If a catalyst is added the reaction goes faster.A catalyst is a substance that speeds up a chemical reaction but remains chemicallyunchanged itself. Catalysts lower the activation energy needed to run the chemical reaction – so it goes faster. The reactants are able to react in a way that requires less energy. This means that more collisions now have enough energy to be successful Enzymes are biological catalysts This animation give a good basic overview of enzymes A cool video showing catalysts! mations/enzyme.swf
  11. 11. labs:The effect of temperature on rate of reactionThe effect of particle size of rate of reactionHow would you design a lab to investigateeither one of these using steradent tablets? You have 10 min to discuss this with your neighbour then you have 15 min to put something in writing. What do you think happens when you dissolve an alka setlzer tablet in space? After which time we will do one of them.
  12. 12. What happens when the reaction goes too fast? BOOM!!!There have been many examples when DUST (i.e. high surface area) offlour, sugar, wood, or wheat explode. The dust from all of these can catchfire and burn. If the reaction goes too fast you can get an explosion.In mines methane and other flammable gasses can also have explosiveresults if they are in high enough concentrations.
  13. 13. Oxidation and reduction – REDOX - it’s all about oxygenOxidation: oxygen is gained Iron is being oxidised to form iron(III) oxide- this is how rust forms. Fe(s) + O2(s) →Fe2O3(s) Fe gained oxygen Reduction: oxygen is lost If you heat magnesium oxide and carbon you get magnesium metal and carbon monoxide. Magnesium oxide was reduced to magnesium metal. MgO(s) + C(s) →Mg(s) + CO(g) MgO lost oxygen Two examples: which is which? 1- 2PbO2(s) → 2PbO(s) + O2(g) PbO2 lost oxygen 2- N2(g) + O2(g) → 2NO(g) N gained oxygen
  14. 14. But in reality… Oxidation is accompanied by reduction. For example, in the extraction of iron from its ore: reduction So the reaction is Iron(III)oxide lost oxygen called REDOXFe2O3 + 3CO → 2Fe + 3CO2 Carbon monoxide gained oxygen oxidation Because they always occur together we can now define oxidation and reduction a bit differently: Oxidation is a loss of electrons. OIL RIG Reduction is a gain of electrons. OxidationIs Losing Reduction Is Gaining IMPORTANT
  15. 15. REDOX in pictures When magnesium is burned, magnesium oxide is formed. Magnesium has clearly oxidised 2Mg(s) + O2(g) → 2MgO(s) (gained an oxygen) but oxidation and reduction always occur together. So oxygen was reduced. But how does oxygen lose oxygen? oxidation We need to look at the electrons. 2 electrons transfer Mg2+ O2- giving Mg O Mg ODuring the reaction each magnesium atom loses two electrons and each oxygen atomgains two. Therefore magnesium was oxidized (loss of electrons) and oxygen wasreduced (gain of electrons). Writing the half-equations: Oxidation: 2Mg → 2Mg2+ + 4e- Reduction: O2 + 4e- → 2O2-
  16. 16. REDOX without oxygen Remember any reaction in which electrons are transferred is a REDOX 2Na(s) + Cl2(g) → 2NaCl(s) reaction. electron transfer giving Cl- Na+ Na Cl Na ClDuring the reaction each sodium atom loses one electron and each chlorine atomgains one. Therefore sodium was oxidized (loss of electrons) and chlorine wasreduced (gain of electrons). Writing the half-equations: Oxidation: 2Na → 2Na+ + 2e- Reduction: Cl2 + 2e- → 2Cl-
  17. 17. Have you learned anything? Let’s check out this reaction rate online simulation ation/reactions-and-rates