4.1 Energetics Flashcards
What is thermochemistry
Study of heat changes during chemical reactions
What happens to bonds during a chemical reaction
Eg what happens to energy when you break bonds and make bonds
Chemical bonds break and new ones are formed
Energy must be put in to break bonds, and energy is given out when bonds are broken.
As a result most chemical reactions have an energy change
The overall energy change may result in energy being taken in or given out
What is an exothermic reaction
Give an example
A reaction that gives off energy so increases the temperature of the surroundings
Neutralising an acid with an alkali is exothermic
Also all combustion reactions
What is an endothermic reaction
Give an example
A reaction that takes in heat so decreases the temperature of the surroundings
the breakdown of calcium carbonate (limestone) to calcium oxide and C02
Also heating copper sulphate:
Heating copper sulphate reaction equation
It is endothermic
CuSO4.5H20 –> CuSO4 + 5H20
Here, it is endothermic because energy is being put in to break the bonds between the water molecules and copper sulphate.
Heat is given in to break bonds so temp decreases
It goes from blue copper sulphate to white anhydrous copper sulphate and water.
What happens when you react white anhydrous copper sulphate and water together
It is an exothermic reaction, and heat is given out
CuSO4 + 5H2O –> CuSO4.5H20
So from white to form blue
What does the amount of heat taken in or given out in a reaction depend on
What is energy change measured in
It depends on the quantity of the reactants
Measured in KJ/mol-1
Combustion of methane equation
CH4 + 2O2 –> CO2 + 2H20
It is exothermic
The moles are very important because 1 mol of methane burns in 2 mols of oxygen, and 890KJ of are given out only when this happens.
Give an example of a useful enthalpy change
When fuels are burnt (exothermic reactions) lots of energy is released from them
Eg coal is mostly carbon
Carbon dioxide is the only product in its combustion
So C(s) + O2(g) –> CO2(g)
This only happens when 1 mol of carbon is burned completely so the most oxidised product is formed
