Chapter 9: Enthalpy + Hess's Law

What is enthalpy?

a measure of heat energy in a chemical system

which law alludes to energy changes?

the law of conservation of energy

directions of energy transfer in an exothermic reaction:

• ∆H is negative

• chemical system loses energy

• surroundings gain energy

• temperature of surroundings increases

directions of energy transfer in an endothermic reaction:

• ∆H is positive

• chemical system gains energy

• surroundings lose energy

• temperature of surroundings decreases

what is the activation energy?

the energy required to break bonds, acting as an energy barrier → E_a

• the minimum energy required for a reaction to take place

in general, reactions with low activation energies are…

fast

list the 4 general standard conditions:

1. pressure → 100kPa

2. temperature → 298K

3. concentration → 1 mol dm^-3

4. state → the physical state of a substance under standard conditions

What is this?

the standard enthalpy change of a reaction → enthalpy change accompanying a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states

what is the standard enthalpy change of formation?

∆_fH^ϴ → enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants + products in their standard states

when balancing equations for enthalpy changes of formation, what do you need to make sure???

that a balancing number isn’t added in front of the product!!

all elements have an enthalpy change of formation of ….

0 kJ mol ^-1

definition of the standard enthalpy change of combustion

∆_cH^ϴ → the enthalpy change that takes place when one mole of a substance reacts completely in oxygen under standard conditions, with all reactants and products in their standard states.

definition for the standard enthalpy change of neutralisation:

∆_neutH^ϴ → the energy change that accompanies the reaction of an acid by a base to form one mole of H₂O (l), under standard conditions, with all reactants and products in their standard states.

what is a specific heat capacity, or c ?

the energy required to raise the temperature of 1g of a substance by 1K.

good conductors of heat, such as metals, have a [what] specific heat capacity?

small

what is the specific heat capacity of water?

4.18 J g^-1 K^-1

units for specific heat capacity:

J g^-1 K^-1

0K = [what] degrees Celsius?

-273^oC

what is the m in q = mc∆t?

the mass that changes temperature! NOT the mass of the reactants!!

units for q = mc∆t:

q = J

m = g

c = J g^-1 K^-1

^{∆T = K/C}

4 reasons why combustion provides an inaccurate Q value:

1. heat loss to surroundings → could be to beaker, but mainly surrounding air

2. incomplete combustion of fuel → CO/C produced instead of CO₂

3. evaporation of methanol (fuel) → burner isn’t weighed quick enough

4. non-standard conditions

lead to a value of ∆_cH that is NOT exothermic enough!

how can a combustion reaction be altered to make it more accurate?

1. draught screens

2. input of oxygen gas

what kind of cup should be used to determine the enthalpy change of a reaction between 2 solutions (or a solid + solution)?

a polystyrene cup

a neutralisation reaction is normally between…

2 solutions

what is an average bond enthalpy?

the energy required to break one mole of a specific type of bond in a gaseous molecule

bond enthalpies are always…

endothermic → positive enthalpy value

limitation of average bond enthalpies:

it varied depending on chemical environment

is bond breaking endo/exothermic?

endothermic → ∆H is positive

is bond making endo/exothermic?

exothermic → ∆H is negative → think BMX!

when given bond enthalpies, how do we work out the ∆H of a reaction?

break - make

what does Hess’s Law state?

if a reaction can take place by 2 routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each