enthalpy Flashcards
what is enthalpy
Enthalpy(H) is a measure of the heat energy in a chemical system. Enthalpy can’t be measured, enthalpy change can
What does system mean in a chemical reaction
The atoms and bonds involved in the chemical reaction
What is enthalpy change H
H(product) – H(reactant)
Describe an exothermic reaction
- Energy goes from system to surroundings
- the change in enthalpy is negative
- products have less energy than the reactants
- There is a temperature rise in surroundings
Describe an Endothermic reaction
- Energy goes from surroundings to system
- the change in enthalpy is positive
- products have more energy than reactants
- There is a temperature drop in surroundings
What is the law of conservation of energy
The law states that The amount of energy in an isolated system remains the same
Energy can’t be created/destroyed only transfered
What is activation energy
the minimum energy needed for a reaction to take place. It acts as a barrier to the reaction
The smaller it is the quicker a reaction occurs.
what is Standard Enthalpy change of reaction/\rH’
the enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation with all reactants & products in standard state And under standard conditions
what is Standard Enthalpy change of formation /\fH’
the enthalpy change that takes place when 1 mole of a compound is formed from its element under standard conditions with all the reactants and products in their standard state.
what is Standard Enthalpy change of combustion /\cH’
the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their Standard state.
what is Standard Enthalpy change of neutralisation /\neutH’
the energy change that takes place when an acid reacts with a base to form 1 mole of H2O under standard conditions with all reactants and products in their standard states
What are standard condition and what does in standard state mean:
100kPa
25’C / 298K
1 mol/dm3 for chemicals in solution
The compound exists in standard conditions
how do we work out enthalpy change
Jq = mc/\T
Jq ‘/. 1000 = KJ
q/n = /\H KJ mol-1,
What is mean bond enthalpy?
A measure of the energy required to break one mole of a covalent bond measured in gaseous state in KJ/mol averaged across many compounds containing the bond
Is Bond breaking endothermic or exothermic?
endothermic (Positive enthalpy change)
Is bondmaking exothermic or endothermic?
Exothermic (negative enthalpy change)
What does it mean if it has a very high bond enthalpy value?
It’s a shorter bond and The covalent bond is very strong
What type of bonds are the shortest and therefore the strongest?
Triple bonds, then double bonds then single
How to calculate overall enthalpy change From bond enthalpies
sum of Bond energy in reactants - products,
if it’s negative it’s an exothermic reaction,
if positive - endothermic
What is hess’ law
The enthalpy change for a chemical reaction is independent of the route taken and depends only on the initial and final states.
States That if a reaction can take place by two routes and the start and finish conditions are the same, the total enthalpy change is the same for each route
Why might experimental methods for enthalpy determination not be accurate
Heat is lost to surroundings
reactants not in standard conditions
incomplete reaction
Why will using average bond enthalpy not be as accurate as using standard enthalpy of formation Or combustion
average bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion or formation applies just to that molecule in that reaction therefore they are more accurate
How to work out standard enthalpy change of reaction/ combustion given standard enthalpy changes of formation
products - reactants
How to work out standard enthalpy change of formation given standard enthalpy changes of Combustion
reactants - products
In terms of bond breaking and bond making explain why x is endothermic
More energy is required for bond breaking than is released by bond making
In an enthalpy profile diagram what are the key things to label
Reactants and products with state symbols
activation energy Ea
enthalpy change /\H
How to reduce percentage uncertainty
Use a greater mass for a larger temperature change
