9.1 - Enthalpy changes Flashcards
What is enthalpy?
Enthalpy (H) is a measure of the heat energy in a chemical system.
(System = the atoms, molecules or ions making up the chemicals)
What is the law of conservation of energy?
One of the fundamental rules of science that states energy cannot be created or destroyed.
When a chemical reaction involving an enthalpy change takes place, heat energy is transferred between the system and the surroundings.
The system is the chemicals e.g. reactants and products.
The surroundings are the apparatus e.g. the thermometer and apparatus, the lab and anything that isn’t the chemical system.
What is the enthalpy change?
In a chemical reaction, the reactants and products are likely to have different enthalpies.
△H = H(products) - H(reactants)
△H can be positive or negative, depending on if the products contain more or less energy than the reactants.
How can enthalpy change be determined?
△H can be determined experimentally by measuring the energy transfer between the system and the surroundings.
If energy is transferred from the system to surroundings = exothermic
If energy is transferred from the surroundings to the system = endothermic
When is a reaction exothermic?
When energy is transferred from the system to the surroundings.
Due to the conservation of energy, any energy loss by the system is balanced by the same energy gain by the surroundings.
△H is negative.
Temperature of surroundings increases as they gain energy.
When is a reaction endothermic?
When energy is transferred from the surroundings to the system.
Due to the conservation of energy, any energy gain by the system is balanced by the same energy loss by the surroundings.
△H is positive.
Temperature of surroundings decreases as they lose energy.
What is activation energy?
During chemical reactions, the bonds in the reactants need to be broken by an input of energy.
New bonds in the products can then form to complete the reaction.
The energy input required to break bonds acts as an energy barrier to the reaction, known as the Activation energy.
Activation energy is the minimum energy required for a reaction to take place.
Why do reactions with small activation energies take place rapidly?
Because the energy needed to break bonds is readily available from the surroundings.
What are standard conditions?
Standard pressure = 100 kPa
Standard temperature = 298K (25 degrees C)
Standard concentration = 1 mol dm-3
Define the standard enthalpy change of reaction
The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation, under standard conditions, with all reactants and products in their standard states.
Define the standard enthalpy change of formation
The standard enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions
What is the standard enthalpy change of formation for an element?
From its definition, enthalpy change of formation for an element refers to the formation of one mole of an element from its element.
So there is no change, and all elements have an enthalpy change of formation of 0kJ mol-1.
Define the standard enthalpy change of combustion?
The standard enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.
Define the standard enthalpy change of neutralisation
The standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O,
under standard conditions, with all reactants and products in their standard states.
0K in °C
0K = -273°C
