Chapter 9 - Enthalpy Flashcards
What is enthalpy?
A measure of heat energy in a chemical system (H)
What is enthalpy change?
ΔH = H products - H reactants
Measured in kJmol-1
What happens to energy, temperature and products in exothermic reactions?
- Energy is transferred to the surroundings from the system
- Temperature of surroundings increases
- Products have less energy than reactants
- Enthalpy changes is negative
What does the reaction profile of an exothermic reaction look like?
x - axis = reaction progress
y - axis = enthalpy
Reactants above products
Activation energy from reactants to top of the curve
Enthalpy change from reactants to products pointing downwards
What happens to energy, temperature and products in an endothermic reaction?
- Energy is transferred from surroundings to the system
- Temperature of the surroundings decreases
- Products have more energy than reactants
- Enthalpy change is positive
What does the reaction profile for an endothermic reaction look like?
x - axis = reaction progress
y - axis = enthalpy
Reactants below products
Activation energy from reactants to top of the curve
Enthalpy change from reactants to products pointing upwards
What is activation energy?
Minimum amount of energy required to start a reaction
What is activation energy required for?
Energy required to break bonds in the reactants so that the atoms can rearrange to form new products
Why do we have standard enthalpy conditions?
Enthalpy change can vary depending on conditions
Chemists use standard conditions to compare enthalpy changes for different reactions
What is standard enthalpy change and symbol?
ΔH⦵ = an enthalpy change under standard conditions
What are the standard conditions for standard enthalpy change?
Standard pressure - 100kPa
Standard temperature - 298K (25 degrees)
Standard concentration - 1moldm-3
Standard state - the physical state of a substance under standard conditions
What is the definition for standard enthalpy change of reaction?
Standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactants and products in standard states
How does enthalpy change increase if moles are doubled from 1 to 2?
Enthalpy change doubles
What is the definition of enthalpy change of formation?
Standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions with all reactants and products in standard states
What is the enthalpy change of formation to form an element from itself? O2 (g) -> ?
O2 (g) -> O2 (g)
ΔHf = 0kJmol-1
What is the enthalpy change of formation for NO2?
0.5N2 (g) + O2 (g) -> NO2 (g)
What is the definition for standard enthalpy change of combustion?
The standard enthalpy change of combustion is the enthalpy change that takes place when one mole of substance reacts completely with oxygen under standard conditions with all the reactants and products in their standard states
What is the standard enthalpy of combustion for Li?
Li (s) + 1/2 O2 (g) -> 1/2Li2O (s)
What is the definition for standard enthalpy change of neutralisation?
The standard enthalpy change of neutralisation is the enthalpy change that accompanies the reaction of an acid with a base to form one mole of H2O under standard conditions with all reactants and products are in their standard states
What is the equation for the enthalpy change of neutralisation of sulfuric acid and sodium hydroxide?
1/2H2SO4 (aq) + NaOH (aq) -> 1/2Na2SO4 (aq) + H2O (l)
What is significant about the value of the standard enthalpy change of neutralisation?
It is always the same for all neutralisation reactions
-57 kJmol-1
What is the equation for energy change (Q)?
Q (energy change (j) ) = m (g) x c (SHC) x T change (K)
What are the general steps for calculating enthalpy change combustion in an exam question?
Step 1: calculate Q - using mcT
Step 2: convert Q into kJ (divide by 1000)
Step 3: work out the enthalpy change (mass/mr = moles -> Q/moles = enthalpy change)
Step 4: Add + or - depending on if it’s an exothermic or endothermic reaction - can decide which one from if the temperature increases in the reaction or decreases (endothermic)
Why is the experimental value less exothermic than the data book value?
- heat loss to the surroundings other than the water (e.g. air/beaker)
- some incomplete combustion occurring instead of complete
- evaporation of ethanol (not all reacted)
- non-standard conditions
