physical chemistry Flashcards
What is chemical energy?
a special form of potential energy that lies within chemical bonds
What are chemical bonds?
The force of attraction that bind atoms together in compounds.
During a reaction, bonds break and new bonds form. This process changes the what?
chemical energy of the atoms
What is H?
enthalpy- the heat content that is stored in a chemical system
Explain the law of conservation of energy
If heat is released, the amount of energy that leaves a chemical system is exactly the same as the amount that goes into the surroundings so no heat energy is lost.
what does a heat loss in a chemical system mean?
temperature increase to the surroundings
explain what ΔH means
heat exchange with the surroundings during a chemical reaction, at a constant pressure
What is an exothermic reaction?
The enthalpy of the products is smaller than the enthalpy of the reactants. There is less heat loss from the chemical system to the surroundings, ΔH has a negative sign because heat is lost.
What is an endothermic reaction?
the enthalpy of the products is greater than the enthalpy of the reactants. There is heat gain to the chemical system from the surroundings. ΔH is positive..
Name 2 important exothermic reactions and why they are exothermic
oxidation of fuels- products have less enthalpy than the reactants so the excess energy is released as heat. and respiration- sugars, such as glucose are oxidised to carbon dioxide and water
equation of respiration
C6H12O6 + 60^2 = 6CO^2 + 6H20
ΔH= -2801 KJ mol-1
equation of oxidation of fuels
CH4 + 2CO2 = CO2 + 2H20 ΔH= -890 KJmol-1
name two processes of endothermic reactions
Thermal decomposition of limestone and photosynthesis
explain the thermal decomposition of limestone
limestone contains calcium carbonate. the decomposition of calcium carbonate by heat is used to make calcium oxide.
CaCO3 = CaO + CO2 ΔH= +178 KJ mol-1
equation of photosynthesis
6CO2 + 6H20 = C6H12O6 + 6O2 ΔH= + 2801 KJ mol-1
what is the activation energy?
the minimum energy required for reaction to take place
how is the activation energy always calculated?
the input energy from reactants to the top of the energy barrier
what are standard enthalpy conditions?
pressure of 100Kpa, temp of 298K, concentration of 1moldm-3, physical state of a substance under standard conditions
what Is standard enthalpy change shown by?
ΔH°
give an equation for standard enthalpy change of reaction
H2 + 0.5O2 = H2O ΔH°= -286 kjmol-1
what is the standard enthalpy change
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in standard units.
What is the standard enthalpy change of combustion?
The enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in standard units.
C2H6 + 3.5O2 = 2CO2 + 3H20 ΔcH°= -1560 JKmol-1
What is the standard enthalpy change of formation?
The enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.
H2 + 0.5O2 = H20 ΔcH°= 286 KJmol-1
What is the standard enthalpy change of neutralisation?
The energy change that accompanies the reaction of an acid by a base to form 1 mole of H2O under standard conditions, all reactants and products being in standard units.
