3.3 Energetics
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what is the scientific name for energy change ?
Enthalpy change
What is enthalpy change ?
enthalpy change refers to energy changes in reactions that take place at a constant pressure and it is the change in the heat content between products and reactants in an open system
What are exothermic changes?
Exothermic changes give out energy to their surroundings and always have a negative enthalpy value
What are examples of exothermic chemical reactions ?
Freezing, condensing, burning and respiration
What is the enthalpy value for an exothermic change reaction
Negative enthalpy value
What are endothermic changes?
Endothermic changes take in energy from their surroundings and have a positive enthalpy value
What are examples of endothermic changes reactions ?
Melting, evaporation and photosynthesis
In an enthalpy level diagram for an exothermic reaction do the reactants or the products have less energy ?
The products have less energy as the reaction has given out energy into the surroundings
In an enthalpy level diagram for an endothermic reaction do the reactants or the products have less energy ?
The reactants have less energy as they take in energy to form the products which have more energy
What are the standard conditions for an enthalpy change to take place give the value for pressure, temperature and solutions.
Temperature= 298k (Kelvin) (25°c) Pressure = 100kPa Solutions = 1 mol/dm³
What type of reaction is bond breaking ?
The breaking of bonds is endothermic because energy is taken in to break bonds
What type of reaction is bond making ?
The making of bonds is exothermic as energy is released when new bonds are formed
How do you calculate the energy change of a reaction?
Energy needed to break bonds - the amount of energy given out when bonds where made
What is bond enthalpy?
Bond enthalpy is the energy needed to break a particular covalent bond and is called the bod dissociation energy
What is the average bond enthalpy
The average bond enthalpy is the average value of the bond of s particular type of bond in a range of different compounds
What are the 2 enthalpy changes you will deal with ?
Enthalpy of formation and enthalpy of combustion
What is the standard enthalpy change of combustion ?
The standard energy change of combustion is the enthalpy change that occurs when one mole of a substance under god’s complete combustion in excess oxygen at 298k and 100KPa
What is the standard enthalpy change of formation?
The standard enthalpy change of formation is the energy change that occurs when one mole of a compound is formed from its elements in their standard states at 298K and 100KPa
What is Hess’ law ?
Hess’s law states that enthalpy change in converting reactants to products is the same regardless of the routbby which the chemical change occurs, provided that the initial and final conditions are the same
What is the principle of conservation of energy ?
Energy cannot be created or destroyed it is just transferred from on type to another
What equation is used to calculate the enthalpy change of combustion
Enthalpy change = enthalpy of reactants - enthalpy of products
What equation is used to calculate the enthalpy change of formation?
Enthalpy change = enthalpy of products - enthalpy of reactants
What type of reaction is a combustion reaction ?
An exothermic reaction meaning it gives out heat to its surroundings and has a negative enthalpy value
What type of reaction is a formation reaction?
An endothermic reaction meaning it takes in heat from its surroundings and has a positive enthalpy value