Topic 5: energetics/thermochemistry Flashcards
Energy definition
ability to do work, ability to move a force through a distance, measured in joules
Energy formula
energy (J) = force (n) * distance (m)
Energy in bond breaking and formation
energy is required to break bonds and released when bonds are formed
What is exothermic reaction
A release of heat energy to surroundings, bonds in products are stronger than reactants
e.g. neutralization and combustion
Endothermic
Energy absorbed, bonds in reactants stronger than bonds in products.
Position of things in enthalpy diagram for exothermic
Products below reactants (with a negative value because products - reactants)
Position of enthalpy diagram for endothermic
Products above reactants
What is enthalpy
The internal energy stored in reactants - we only measure enthalpy change
Heat
measure of the total energy in a given amount of substance
temperature
measure of the hotness of a substance, the average kinetic energy of a substance - independent of the amount
Describe a calorimeter and how it works
Heat evolved in an exothermic reaction used to raise temperature of water or heat absorbed in endo reaction used to lower temperature of water. To compensate for heat loss, temperature against time graph is plotted for exo and extrapolated to what would be the temperature if the reaction was instantaneous.
heat energy formula
heat energy = mass (m) * specific heat capacity (c) * temperature change
mc delta T (MCAT)
what is the heat capacity for water
4.18 kJ kg-1 K-1
requires 4.18 kJ of energy to raise the temperature of one kg by one kelvin
how to calculate enthalpy change with heat evolved in reaction formula
heat evolved in reaction/moles of reactant involved in reaction
What is hess’ law
enthalpy change for a reaction depends only on the difference between the enthalpy of the products and the enthalpy of the reactants, independent of the reaction pathway.
