chemistry - unit 1 - section 4: energetics

what is enthalpy?

when a chemical reaction occurs there is normally a change in energy

the enthalpy change in reaction

the heat change in a reaction at constant pressure

symbol and units of enthaply change of a reaction

symbol = delta H with underground symbol

units = KJmol^-1

underground symbol stands for standard conditions

standard conditions:

1. 100KPa

2. 298K

endothermic reaction

reactions that absorb energy from the surroundings

energy profile of an endothermic reaction

example of endothermic reaction

exothermic reaction

reactions that reales energy to the surroundings

energy profile of exothermic reactions

example of exothermic reactions

bonds broken

to break a bond energy needs to absorbed.

bonds are broken in the reactants and this is an endothermic process so delta H is positive

if more energy is needed to break bonds than energy given out when bonds are formed the reaction will be endothermic

bonds made

when bonds are formed energy is released

bonds are made when products are being produced and this is an exothermic reaction so delta H is negative

if more energy is released when bonds are formed than what was needed to break initial bonds the reaction will be exothermic

mean bond enthalpy

bonds of the same type don’t all have the same amount of energy

enthalpy change formula

enthalpy change = total energy to break bonds - total energy released forming bonds

calorimetry

used to work out the enthalpy change of combustion

calorimetry - method

1. a fuel is burned to raise the temperature of the water by a specific amount.

2. weight the fuel before and after burning to work out the mass of fuel burned

3. a lid is placed on top to prevent heat loss and windshields placed by the flame to prevent a draught moving the flame to prevent a draught moving the flame.

calorimetry formula

q=m x c x delta T

q = (J)

m = (g)

t = (k)

hess’s law

the total enthalpy change of a reaction is independent of the route taken

hess’s law - formation

Hess’s law - combustion