Energy
The unit of energy is the Joule (J)
Forms of energy include kinetic, gravitational potential, elastic potential, chemical potential, electric potential, nuclear potential, thermal (internal energy), and radiant energy.
Law of conservation of energy
The total energy of an isolated system remains constant- it is said to be conserved over time.
Energy can be neither created nor be destroyed, but it can change form.
Exothermic and endothermic reactions
In an exothermic reaction there is less energy in the chemical bonds after the reaction and energy released as heat.
In an endothermic reaction energy is taken in (producing cooling) as there is more energy in the chemical bonds after the reaction.
Exothermic reactions include combustion and acid-base neutralization.
Endothermic reactions include electrolysis and addition polymerization.
Energy level diagrams show the energy change in the chemical bonds as reactants change to products in a chemical reaction.
Temperature
Particles of a substance at a particular temperature do not all have the same speed.
Temperature is a measure of the average kinetic energy of the particles.
The Kelvin scale of temperature is an absolute scale of temperature with zero Kelvin at zero kinetic energy.
Temperature in K = Temperature in °C + 273
Measuring energy changes in reactions
One gram of water needs 4.18J (1 calorie) to be heated by 1°C.
The heat released in a reaction can be calculated from the temperature change of heated or cooled water and the mass of water.
Solutions have a specific heat capacity close to water and the value 4.18J/g/°C can often be used.
Enthalpy, H
Enthalpy has the unit of Joules and is similar to energy, but takes account of the space taken up by the material.
For exothermic reactions the change in enthalpy, ΔH < O and for endothermic reactions ΔH > O
Standard enthalpy, ∆H⊖ is used to compare energy changes in reactions and is measured in kJ/mol of reactant.
Standard enthalpy for a reaction can be calculated from enthalpy change ÷ number of moles of reactant.
Bond enthalpies
Bond enthalpy (bond energy) is the average energy in a particular chemical bond between two atoms.
Bond enthalpies can be used to compare the strength of bonds and to calculate the overall enthalpy change in a reaction using ∆H = (Total bond enthalpy of products) – (Total bond enthalpy of reactants)
Combustion of fuels
Some common chemical fuels include wood, coal, methane (natural gas), ethanol, petrol (gasoline), kerosene (paraffin) and hydrogen. These combine with oxygen in the air during combustion.
Many fuels are hydrocarbons or carbohydrates, and complete combustion gives carbon dioxide and water.
The energy density of a fuel can be calculated from the heat released by the fuel ÷ the mass of the fuel and has the units J/g or J/kg.
Incomplete combustion may produce particulate pollution or toxic carbon monoxide.
Impurities in fuels include sulphur, which is oxidized to sulphur dioxide and nitrogen which is oxidized to nitrogen oxides. These gases cause acid rain.
Carbon dioxide is a greenhouse gas as it absorbs infrared radiation. The concentration of carbon dioxide in the atmosphere has increased by almost one half since the industrial revolution, and this has contributed to a global average temperature change of around 0.8°C.
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