enthalpy Flashcards
enthalpy
a measure of the heat energy in a chemical system
why does a change in enthalpy occur in a reaction
reactants and products have different structures and bonds so therefore different energy
enthalpy change when a reaction is exothermic
energy is lost by the material enthalpy change is negative (delta H is negative)
enthalpy change when a reaction is endothermic
energy is taken in by the material. Enthalpy change is positive (delta H is positive)
why does the temperature of an exothermic reaction increase
because the energy of the surroundings, which includes the thermometer, increases due to it having more energy than it did before.
why does the temperature of an endothermic reaction decrease
surroundings have less energy than it did before so temperature of thermometer decreases
activation energy
minimum energy required to start a reaction by breaking the bonds - necessary to break these bonds for the reaction to start
how do you calculate the energy change of a reaction
energy = mass of solution x specific heat capacity x change in temperature Q= mc^T
why is water commonly used as a heat sink
- easy to obtain
- high specific heat capacity
- doesn’t require much energy to heat up
how to experiment direct enthalpy changes
add a known mass of metal e.g. Zinc to a known temperature solution e.g. water add the zinc to the water and measure the highest temperature reached
standard enthalpy change of combustion
enthalpy change when 1 mole of substance reacts completely with oxygen under standard conditions with all reactants and products in standard states
standard enthalpy change of formation
enthalpy change when 1 ole of compound is formed from its constituent elements in their standard states under standard conditions
standard enthalpy change of neutralisations
the enthalpy change that occurs when a acid reacts with a base to form 1 mole of water under standard conditions with all reactants and products in their standard states
temperature, pressure and concentration for standard conditions
25 degrees or 298 K
10 000 00 Pascal’s
1 moldm-3
state Hess’s law
enthalpy change of a reaction is the same, which ever route is taken
