Fuels/food + oxygen: exothermic - amount of energy released depends what's used - energy release measured in J (or calories: 1cal = 4.2J)
Use calorimeter to measure energy released when substances burn - simple calorimeter: water in glass beaker/metal can, substance underneath burnt and heats water, temperature rise of water depends on energy released - amount of energy transferred calculated by: Q = mcΔT (Q:amount of energy transferred - J, m:mass - g, c:specific heat capacity - J/g°C, ΔT: temperature change - °C)
Simple calorimeters don't give accurate results: heat lost to surroundings - but can be used to compare energy released by different fuels
To compare energy released by burning substances, energy change in kJ/g or /mole can be used - calculate /mole by multiplying /g by relative fomula mass
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C3.3.2 - Energy Transfers in Solutions
Energy transferred to or from solution when reaction takes place in it - reduce energy transfers to surroundings by doing reaction in container
In energy calculations, assume solutions behave like water - 1cm³ = 1g and c = 4.2J/g°C
When solid is added to water/aqueos solution, assume volume doesn't change
If we know no of moles involved in calculated reaction we can calculate energy change in kJ/mol
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C3.3.3 - Energy Level Diagrams
Show energy changes for reaction on energy level diagram - difference between energy levels of reactants and products is energy change of reation - exothermic reaction: goes down
During chemical reaction, reactants bonds must be broken for reaction to happen - breaking bonds is endothermic: energy taken in - activation energy: minimum energy needed to break bonds
New bonds are formed - energy released: exothermic
Catalysts increase rate of reaction by providing different pathway with activation energy that's lower
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C3.3.4 - Calculations Using Bond Energies
In chemical reaction, energy needed to break bonds and released when bonds are formed - difference in these changes makes reaction exo or endothermic
Energy needed to break and make bonds between atoms is bond energy - use it to calculate overall energy change of reaction - bond energy measured in kJ/mol
Balanced equation for reaction is needed to calculate energy change for reaction
To calculate energy change, calculate: energy needed to break bonds, energy needed to make bonds and difference between the two
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C3.3.5 - Fuel Issues
Fossil fuels - non-renewable, cause pollution - urgent need to develop alternatives
Hydrogen as a fuel - burns easily, releases large amount of energy/g, no CO2 produced - just H2O, can be burnt in combustion engines or used in fuel cells to power vehicles, can be produced from renewable resources, difficulties include supply, storage and safety problems
Vehicles using fuel cells need to match performance, convenience and costs of petrol/diesel vehicles
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