C7 Energy Changes

• The amount of energy in the universe before a reaction is the same as after • If energy is transfered to surrounds, products have less energy than reactants, by the amount transferred

• Energy required to break bonds (endothermic) • Energy released when bonds are formed (exothermic) • If more energy is required to break bonds (so more endothermic) than to make then (exothermic) = endothermic

Explanation: - A reaction where more energy is released than taken in - Reactants have more energy than products - (+) temperature change - (-) energy change Uses: - Hand warmers - Combustion - Respiration

Explanation: • More energy is taken in than released • Products have more energy than reactants • more energy is used to break bonds than make • (-) temperature change • (+) energy change Uses: • Photosynthesis • Thermal Decomposition • Ice packs

Particles must collide with more energy than the activation energy

Minimum amount of energy with which particles must collide with for a reaction to happen

Energy needed to break bonds - Energy released when bonds formed = energy change • If negative change its exothermic (NEGATIVE energy EXits the room) • If positive change it endothermic

A cell which contains chemicals that react together to produce electricity

• Difference in reactivity between metals used • SA of the metals • Concentration of the electrolyte

Several cells connected in series to increase voltage

• Non rechargeable batteries • Chemicals inside the cell run out and the reaction which produces electricity stops

• Can be connected to a power supply forcing the reaction to reverse • Lithium ion batteries

• Place two different electrodes in an electrolyte (solution with sodium ions) • Connect them with a wire to eachother and a volt metre

• Difference of reactivity of a metal • Metals want to give up e- to form positive ions • The greater the tendency to form positive ions the more reactive the metal • The difference in reactivity between metals equates to the voltage

• A cell which uses the combustion of hydrogen • Its only product is water • Reaction is very exothermic and release lots of energy • Energy is turned into electricity instead of heat

• H2 enters the cell • H2 is oxidised, H+ ions formed • H+ move to the cathode through electrolyte • O2 eneters from the air • O2 combines with electrons an H+ ions at the cathode • Water produced

• H2 —-> 2H+ + 2e- • O2 + 4H+ +4e- ——> 2H20

2H2 + O2 —->2H20

Ad: - Cheap Dis: - They run out - Must be disposed of properly due to toxic chemicals inside

Ad: -Reversible reaction Dis: - More expensive than alkali

Ad: - Water only product - Can (often dont) get H2 energy from renewable resource - Continuous supply of fuel = continuous energy Dis: - Usually H2 comes from finite resource - Very explosive - Difficult to store (its a gas) - Expensive - No infrastructure (like petrol stations

• Add 30cm3 of HCl and 5cm3 of NaOH • MEasure the temperature • Keep adding 5cm3 of NaOH until 40cm3 has been added • Measure the temperature after every 5cm3 added

• Temp vs Volume added • Intial increase (straight line) then decrease (straight line) • reaction is exothermic so increases temp • when reaction is finished the excess NaOH cools the liquid down • Nuetralisation is value of volume where the two lines cross over