3.2.1 - Enpalthy changes Flashcards
What is a system ?
Is everything that is involved in the reaction (e.g the reactants and products) or in a chemical reaction the system would be all the atoms and bonds involved in the chemical reaction.
Explain the law of conservation of energy.
The amount if energy in a system remain the same. Energy can not be created or destroyed it can only be transferred.
Explain what an exothermic reaction is and draw a diagram to show this.
An exothermic reaction is when a system releases heat energy to the surroundings,
The products have less energy than the reactants
Explain what an endothermic reaction is.
A reaction where the system takes in heat energy from the surroundings.
Which type of reactions is bond breaking ?
Endothermic reaction and has a positive enthalpy change +ΔH where the enthalpy of products is greater than the enthalpy of reactants.
tip : bendo
Explain what type of reaction bond making is.
Exothermic reaction and has a positive enthalpy change -ΔH where the enthalpy of products is less than the enthalpy of reactants.
tip : Mexo
What is the activation energy (Ea) ?
The minimum amount of energy required for a reaction to take place.
What is meant by standard conditions ?
The specified temperature and pressure.
100kPa - Pressure
298K (25oC )- Temp
1mol dm3 - con
Standard state
The state of an element / compound under standard conditions
Define enthalpy change of formation.
The energy change that takes place when 1mole of a compound is formed from its constituent element in their standard state under standard conditions.
What is average bond enthalpy ?
Average bond enthalpy is the average energy required to break one mole of a specific type of bond in a gaseous molecule.
Energy is always required to break bonds, so the value is always endothermic and positive.
Why is average bond enthalpy needed ?
-
Actual bond enthalpies vary depending on
the chemical environment it exists in -
Average bond enthalpy values are used as
it takes an average of all values across
different chemical environments
This means your calculations will often be
different from the actual enthalpy
What do we know when we see this sign.
Standard sates and conditions
measured in kjmol-1
Define enpalthy change of combustion
The enpalthy change when 1 mole of a substance completely combusts in oxygen understandard states and conditions
Define enpalthy change of neutralisation
The enpalthy change when solutions of an acid and alkali react together under standard states and conditions to produce 1 mole of water
How do you calculate average bond enpalthy
ΔH=Energyrequiredtobreakbonds−Energyreleasedfromformingbonds
enpalthy change of reaction formula
ΔH=∑H reactants −∑H products
reaction = reactants first
How do we calculate the enpalthy change of formation use diagram to show this as well
ΔH=∑H products −∑H reactants.
- * Remember that ΔHf of all elements is zero so you don’t need to balance the bottom equation
- Remember that you must balance the top equations.
How do we calculate the enpalthy change of combustion use diagram to show this as well
ΔH=∑H reactants −∑H products
- We always from carbon dioxide and water.
- Use the arrows to help you so
delta H + one of the combustion = the other arrow. - Remember that you must balance the top equations.
Evaluative points for your Q=mct calculations
- polystyrene cup to prevent heat loss
- lid to prevent evaporation of water/alchol
- incomplete combustion
- non-standard conditions
Tips you need to remember for your q=mct questions
- Always divide by 1000 when you find q to get q into kj
- If there are 2 volumes add those together to find grams unless grams is already given
- At the end do q/moles
