10/11/2018 10/11/2018 Energy Changes AQA 2016 Chemistry topic 5

5.1 Exothermic and Endothermic Reactions 10/11/2018 5.1 Exothermic and Endothermic Reactions

Grouping reactions Can you put these reactions into groups? Burning 10/11/2018 Photosynthesis Can you put these reactions into groups? Burning Self-heating cans Hand warmer packs Cooling packs

Exothermic and Endothermic Reactions 10/11/2018 Exothermic reactions Ones that GIVE OUT energy to the surroundings Endothermic reactions Ones that TAKE IN energy from the surroundings

Energy level diagrams Energy level 10/11/2018 Energy level Activation energy – the energy needed to start the reaction Energy given out by reaction Reaction progress

Exothermic vs endothermic: 10/11/2018 EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration) ENDOTHERMIC – less energy is taken in than given out (e.g. photosynthesis)

Chemical Reactions recap 10/11/2018 A chemical reaction is when atoms are basically rearranged into something different. For example, consider burning methane: Methane + oxygen Carbon dioxide + water Energy is NEVER created or destroyed. Therefore the total amount of energy after the reaction must have been the same as before.

Endo and exo reactions in more detail (HT only) 10/11/2018 For a reaction to happen you have to break bonds and then make new ones. How is energy involved? Step 1: Energy must be SUPPLIED to break bonds (the “activation energy”): Energy Energy Step 2: Energy is RELEASED when new bonds are made: A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC.

Measuring energy in methane 10/11/2018 CH4 + 2O2 2H2O + CO2 To burn methane you have to break all of these bonds: And then you have to make these ones:

Bond energies (HT only) 10/11/2018 C-H = 435 Kj O=O = 497 Kj Total for breaking bonds = 4x435 + 2x497 = 2734 KJ/mol H-O = 464 Kj C=O = 803 Kj Total for making bonds = 2x803 + 4x464 = 3462 KJ/mol Total energy change = 2734-3462 = -728 KJ/mol

Drawing this on an energy diagram: 10/11/2018 3462 Kj 2734 Kj -728 Kj More energy is given out (3462) than is given in (2734) – the reaction is EXOTHERMIC. The total (“nett”) energy change is –728 Kj/mol. An endothermic reaction would have a positive energy change.

Bond energy values C-H = 435 KJ/mol O-H = 464 KJ/mol O=O = 497 KJ/mol 10/11/2018 C-H = 435 KJ/mol O-H = 464 KJ/mol O=O = 497 KJ/mol C=O = 803 KJ/mol C-O = 360 KJ/mol C-C = 346 KJ/mol

Another example: ethanol 10/11/2018 C2H5OH + 3O2 2CO2 + 3H2O Total for breaking bonds = 5x435 (C-H) + 360 (C-O) + 464 (O-H) + 3x497 (O=O) +346 (C-C) = 4836 KJ/mol Total for making bonds = 4x803 (C=O) + 6x464 (O-H) = 5996 KJ/mol Energy change = 4836-5996 =-1160 KJ/mol

Another example: methanol 10/11/2018 2CH3OH + 3O2 2CO2 + 4H2O Total for breaking bonds = 6x435 (C-H) + 2x360 (C-O) + 2x464 (O-H) + 3x497 (O=O) = 5749 KJ/mol Total for making bonds = 4x803 (C=O) + 8x464 (O-H) = 6924 KJ/mol Energy change = 5749-6924 (divide this by two as we are dealing with two molecules of methanol) = -587.5 KJ/mol

Example reactions (HT only) 10/11/2018 Reaction Temp. after mixing/OC Exothermic or endothermic? Sodium hydroxide + dilute hydrochloric acid Sodium hydrogencarbonate + citric acid Copper sulphate + magnesium powder Sulphuric acid + magnesium ribbon

5.2 Chemical Cells and Fuel Cells (Chem only) 10/11/2018 5.2 Chemical Cells and Fuel Cells (Chem only)

Electric Current 10/11/2018 Here’s a battery (a “cell” really) powering a lightbulb: - + e- How does the cell work? e- A “battery” is technically a combination of cells.

How cells are made Basically, cells are made out of chemicals: 10/11/2018 Basically, cells are made out of chemicals: Carbon rod with metal cap cathode Manganese oxide Ammonium chloride (electrolyte) Zinc casing anode In other words, a battery is basically two DIFFERENT metals in contact with an electrolyte. The voltage produced by the cell depends on the type of metals and electrolyte.

Increasing reactivity Metals in Batteries 10/11/2018 Here are some common metals and their potentials. What do you notice about the potential compared to their reactivity? Potassium Sodium Calcium Magnesium Aluminium Carbon Zinc Iron Lead Hydrogen Copper Silver Gold Increasing reactivity Metal Potential / V Lithium -3.04 Potassium -2.92 Calcium -2.76 Sodium -2.71 Magnesium -2.37 Zinc -0.76 Copper +0.34 Silver +0.80 Gold +1.50

Alkaline vs Rechargeable Batteries 10/11/2018 In non-rechargeable batteries the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable. In rechargeable batteries the chemical reactions are reversed using an external electric current.

The whole process is called a “redox” reaction. Fuel Cells 10/11/2018 Fuel cells work by using the reaction between hydrogen and oxygen to produce an electric current: The hydrogen atom loses an electron (this is called “oxidation”) They then pass through the electrolyte The oxygen atoms gain two electrons each (“reduction”). The whole process is called a “redox” reaction.

Fuel Cell equations (HT only) 10/11/2018 Here are the equations for what happens in a fuel cell: At the cathode, oxygen gains electrons and reacts with water to make hydroxide ions: O2 + 4e- + 2H2O 4OH- Then, at the anode, hydrogen combines with the hydroxide ions to make water and electrons: 2H2 + 4OH- 4H2O + 4e- Overall – 2H2 + O2 2H2O

Words – construct, pollute, energy, efficient Fuel Cells 10/11/2018 Fuel cells have many advantages over traditional fuels: They do not _______ They are very _______ They transfer ______ directly They are simple to ______ Words – construct, pollute, energy, efficient

Fuel Cells 10/11/2018 Fuel cells are also useful as a potential energy source for __________ as: They provide _______ They are very _______ They have no ____ ____ They are _________ They are also being considered for cars but have two main disadvantages: fuel cells often have poisonous ________ and producing the hydrogen and oxygen will use energy elsewhere. Words – lightweight, spacecraft, water, moving parts, efficient, catalysts