Enthalpy Flashcards
(33 cards)
How to calculate enthalpy change of combustion
1) Calculate energy change (q) of water
q=mc^t
2) Calculate amount, in mol, of substance burn
3) Calculate enthalpy change of combustion
energy change
————————
moles
How to calculate bond enthalpy
1) Calculate enthalpy of bonds broken
2) Calculate enthalpy of bonds made
3) Subtract bonds broken from bonds made to calculate bond enthalpy
4) Add in sign for value
+ endothermic
- exothermic
What is the standard enthalpy change of a reaction?
Enthalpy change when the amount shown in the chemical equation react under standard conditions with the reactants and products in their standard states
What is enthalpy of neutralisation?
Enthalpy change of a reaction when amounts of an acid and an alkali (as shown in the chemical equation) react under standard conditions
What is enthalpy of combustion?
Enthalpy change when ONE MOLE of substance completely burns in oxygen under standard conditions with the reactants and products in their standard states
What is enthalpy of formation?
Enthalpy change when ONE MOLE of compound forms from its elements under standard conditions with the elements and the compound in their standard states
What is bond dissociation enthalpy?
The enthalpy change on breaking one mole of a particular covalent bond in a gaseous molecule
Steps for calorimeter experiment for enthalpy change of combustion
1) Measure mass of spirit burner
2) Add set amount of cold water into a copper calorimeter
3) Suspend calorimeter at fixed distance from spirit burner
4) Measure starting temperature
5) Heat water for fixed amount of time
6) Record mass of burner to calculate mass of fuel burnt
7) Record temperature change of water
What do you measure the mass of in enthalpy change calculations?
Mass of the substance that changes temperature
NOT REACTANTS
Hess Cycle
Write card
What signs do you put in front of exo and endothermic reactions?
Exothermic -
Endothermic +
What things reduce accuracy of calorimeter experiment?
- Heat loss to surroundings other than water (lid to reduce this)
- Incomplete combustion
- Evaporation of fuel from wick
What affect do the factors reducing accuracy have on the energy change value?
Lead to an enthalpy change value which is less exothermic than expected
Where does the heat go in exothermic reactions?
Release energy to the surroundings
Why do exothermic reactions have a negative enthalpy change?
Because the products have less energy than the reactants
Where does the heat go in endothermic reactions?
Take in heat from the surroundings
Why do endothermic reactions have a positive enthalpy change?
Because the products have more energy than the reactants
What effect does a catalyst have on the value of an enthalpy change?
No effect
What is the shorthand version of writing activation energy?
Ea
What is activation energy?
The minimum energy required for a reaction to take place
What is enthalpy?
The amount of energy that a substance contains
What is the enthalpy change value for all neutralisation reactions?
-57kJmol-1
What equation can all neutralisation reactions be simplified to?
H+ (aq) + OH- (aq) ——> H2O(l)
What is the method of experiment to calculate the enthalpy change of neutralisation?
- Allow equal volumes and conc. of acid and alkali to reach the same temp in separate containers
- Measure temperature of acid and alkali
- Place both solutions in polystyrene cup
- Measure max temperature reached
