Enthalpy Flashcards
what is enthalpy?
a meaure of the energy stored in a chemical system
what is the conservation of energy?
energy cannot be created or destroyed only transferred
How do you calculate the enthalphy change?
product enthalpy - reactants enthalpy
does exothermic reactions have a positive or negative enthalpy change?
negative
does endothermic reactions have a positive or negative enthalpy change?
positive
What does exothermic mean?
the products have less energy than the reactants as heat is given out to the surroundings
what dpes endothermic mean?
the products have more enrgy than the reactants as energy has been taken in from the surroundings
what are examples of exothermic reactions?
condensation reaction
liquid solidifies
neutralisation
vapour condensing to liquid
what are examples of endothermic reactions?
decomposition reaction
liquid vapourising
solids melting
what is the standard state of a substance?
the substance’s physical state under standard conditions
What are the standard conditions?
1 atm / 100000 KPA
25 degrees / 298K
what is the standard enthalpy change of reaction?
the enthalpy change for a reaction in the molar quantities shown in a chemical equation, under standard conditions
what is the standard enthalpy change of combustion?
the enthalpy change for the complete combustion of 1 mole of a substance under standard conditions
What is the standard enthalpy chnage of formation?
the enthalpy change for the formation of 1 mole of a compound from its elements in their standard states, under standard conditions
what is the standard enthalpy change of neutralisation?
the enthalpy change for the formation of 1 mole of water from the neutralisation of an acid by a base under standard conditions
How do you calculate the heat evolved/ absored (q) in a reaction?
q = M x C x (T1 -T2)
m = mass of substance (g)
c = specific heat capacity of water = 4.18 Jg-1K-1
triangleT = temp change (degrees or kelvin)
q (J)
How do you calculate the enthalpy change from q = MCtriangleT
use equation
covert to KJ (divide by 1000)
divide by moles reacting
decide if exo or endo
decide if + or -
What measurements do you take for calorimetry?
weigh mass of spirit burner with and without fuel
mass of beaker with and without water
what are reasons why the calorimetry enthalpy change is different to the calculated enthalpy change?
losing heat to the surroundings
not under standard conditions
incomplete combustion
evapouration of alcohol from wick
What are the assumptions with enthalpy changes of solutions?
the density is the same as water = 1gcm-3
volume (cm3) = mass (g)
the specific heat capacity of the solution is the same as water = 4.18 Jg-1K-1
What is a chemical system?
the atoms, molecules or ions making up the chemicals
How could you minimise errors in calorimetry?
draught screens
input of oxygen gas
Why is a polystyrene cup used in measuring the enthalpy change on reaction?
offers some insulation against heat loss to the surroundings
What is average bond enthalpy?
the energy required to break one mole of a specified type of bond in a gaseous molecule
Are bond ethalpies endothermic or exothermic?
require energy - endothermic = + value
How do you calculate the enthalpy changes of reaction? (combustion)
tot bond enthalpies in reactants - tot bond enthalpies in products
How do you calculate the average bond enthalpies of the enthalpy change of formation?
tot bond enthalpy in products - tot bond enthalpy in reactants
What is Hess’ law?
if a reaction can take place by two routes, and starting and finishing conditions are the same, the total enthalpy change is the same for each route
Which way to the arrows point in a reaction route with formation?
upwards
Which way to the arrows point in a reaction route with combustion?
downwards
Why can you only calculate approximate values for enthalpy changes on reaction?
the average value will not be exactly correct for any specific compound
Why cant you use average bond enthalpies to predict the enthalpy change of formation for water?
the standard state of water is liquid and bond enthalpies can oonly be used for gas phase reactions as they do not take into account intermolecular forces
Would you expect the enthalpy change of formation of water to be more or less exothermic than the calculated enthalpy change of water and hydrogen using bond enthalpies?
the enthalpy change of formation of water will be more exothermic than expected as there will be extra negative contribution to the enthalpy change from the formation of H bonds in liquid water
H bonds are not covered in bond enthalpies
If an enthalpy change is negative, what does that tell you about the bond enthalpies in the reactants and products?
the sum of bond enthalpies in the products is greater than in the reactants
