Energy Flashcards
Enthalpy
Conservation of energy
Enthalpy change is 🔺H
Exothermic
Heat loss from system to the surroundings
🔺H is negative
Products have a lower enthalpy than reactants
Examples
self-heating cans
Oxidation of fuels for cooking
Respiration
Endothermic
Enthalpy of products is greater than the reactants
Heat is gained from the surroundings
🔺H is positive
Examples
self-cooling beer cans
Thermal decomposition
Photosynthesis
Activation energy
The minimum amount of energy required to start a reaction by breaking bonds
Standard conditions
Pressure of 100kPa or 1 atmosphere
Stated temperature usually 289K or 25oc
And a concentration of 1.0 mol dm -3 for reactions with aqueous solutions
Standard state
Physical state of a substance under standard conditions of 100kPa and 298K
The standard enthalpy change of reaction 🔺Hr
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions and standard states
The standard enthalpy change of combustion 🔺Hc
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions and a standard state
The standard enthalpy change of a formation 🔺Hf
Enthalpy change that takes place when one mole of a compound is formed when its constituent elements in their standard states and standard conditions
Specific heat capacity, c
Energy required to raise the temperature of 1g of a substance by 1oc
q=mc🔺t
Hess’s law
If a reaction can take place by more than one route and the initial and final conditions are the same ,the total enthalpy change is the same for each route
Rates of reaction
Change in concentration of a reactant or a product in a given time
Factors that affect it: Temperature Pressure Concentration Surface area Adding a catalyst
Catalysts
Increases the rate of reaction without being used up in the process Provides an alternative route Require lower temperature, reducing energy demand and emissions Better atom economy Reduce waste Harber process: ammonia is made by reacting nitrogen and hydrogen Nitrogen triple bond has to be broken Enzymes act as a biological catalysts - can work at lower temperatures - biodegradable -eg: washing detergents
The Boltzmann distribution
The distribution of energies of molecules at a particular temperature often shown as a graph
Only molecules with an energy greater than the activation energy, Ea, are able to react
Equilibrium
Reversible reactions
Dynamic equilibrium:
Closed system
Rates are equal
Le Cheatelier’s principle: when a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change
Concentration:
Increase - left
Decrease - right
Pressure:
Increase - fewer molecules
Decrease - more molecules
Temperature:
Increase - endothermic
Decrease - Exothermic
