Energetics Flashcards
Why is the amount of energy involved in a chemical reaction important?
-measure energy values of fuels
-calculate energy requirements for industrial processes
-work out theoretical amount of energy to break bonds and amount of energy released when made
-helps predict if a reaction will occur
-energy may be in different forms
Energy during bond making and breaking
-energy needed to break bonds
-energy given out when new bonds form
Exothermic reaction
-more energy is given out
-give out heat
-example is a neutralisation reaction and displacement reaction
Endothermic reaction
-more energy is taken in
-take in heat
-heating copper sulfate or the breakdown of limestone
Enthalpy change
-measuring heat change at a constant temperature
-measured under standard conditions
-298K
-100KPa
Enthalpy of an exothermic reaction
negative enthalpy as the products end up with less heat energy as the reactants lost heat energy when heating up the surroundings
Enthalpy of an endothermic reaction
positive enthalpy change as products end up with more energy than reactants
How does pressure effect heat energy given out in reactions involving gases?
if gas is given out energy is required to push away the atmosphere.
-higher atmospheric pressure means more energy is needed
-means that less energy remains to be given out as heat by the reaction
-why it is important to do it in standard conditions
How does the physical state of a reaction and product effect the enthalpy?
Standard enthalpy change of formation
the enthalpy change when one mole of substance is formed from its constituent elements under standard conditions and standard states
Standard enthalpy of combustion
the enthalpy change when one mole of substance is completely burnt in oxygen under standard conditions in standard states
Tempreture
-related to average kinetic energy of particles
-as particles move faster kinetic energy increases and temp increases
-temp is independent of the number of particles
-measured with a thermometer
Heat
-measure of the total energy of all particles present in a given amount of substance
-does depend on amount of substance present
-energy of every particle included
-always flows from high to low temp
Measuring enthalpy change
-heat given out/in during a reaction
-arrange for heat to be transferred into a specific mass of a substance often water
-uses a calorimeter which are usually made of polystyrene due to good conductors with low heat capacity so absorb little heat
Equation for enthalpy change
enthalpy change = mass x shc x temp change
Specific Heat Capacity
amount of heat needed to raise the temperature of a 1g of substance by 1K
Flame calorimeters
-improved to reduce heat loss by:
-spiral chimney made of copper
-flame is enclosed
-fuel burns in pure oxygen
Hess’ law
the enthalpy change for a chemical reaction is the same independent of the route taken from reactants to products.
Law of conservation of energy
energy cannot be created or destroyed
Enthalpy changes of combustion in straight chain alkanes
-plot change against number of carbons a straight line graph is produced as enthalpy changes by the same amount for each extra carbon added to the chain
Bond Dissociation enthalpy
enthalpy change required to break a covalent bond with all species in the gaseous state
Mean bond enthalpy
-average value over several compounds so will only give approximate answers
-example if bonds in methane are broken one at a time the energy required is not the same for each bond
