4. Energetics Flashcards
What is energy transfer during a reaction due to?
The breaking and forming of bonds
What happens to energy when bonds are broken?
Energy is taken in
What happens to energy when bonds are formed?
Energy is released
In terms of bonds, when is energy taken in?
When bonds are broken
In terms of bonds, when is energy released?
When bonds are formed
What is stored energy known as?
Enthalpy
What is the symbol for enthalpy?
H
What is enthalpy?
Stored energy
Can enthalpy be measured?
No
How can enthalpy changes be found?
By measuring the change in temperature during reactions - at a constant pressure
What must be constant when measuring change in temperature to work out enthalpy change?
Pressure
What is the mnemonic to remember energy when breaking/making bonds?
Bendy Mexicans
breaking = endo, making = exo
What happens to energy in an endothermic reaction?
It is taken in
When energy is taken in during an endothermic reaction, what form does it usually take?
Heat
What happens to the temperature of the surroundings during an endothermic reaction?
It decreases
What is higher in an endothermic reaction; the energy to make bonds or to break them?
Energy to break bonds is higher than to make them
What does it mean, in terms of energy, that endothermic reactions need more energy to break bonds than to make them?
There is an overall need for energy for the reaction to take place = positive ΔH value
Why do endothermic reactions have a positive ΔH value?
There is an overall need for energy in order for the reaction to take place - so the system gains energy
Do endothermic reactions have a positive or negative ΔH value?
Positive
What happens to energy in an exothermic reaction?
It is released
When energy is released during an endothermic reaction, what form does it usually take?
Heat
What happens to the temperature of the surroundings during an exothermic reaction?
It increases
Why does the temperature of the surroundings increase during an exothermic reaction?
Energy is transferred to the surroundings
What is higher in an exothermic reaction; the energy to make bonds or to break them?
The energy when making bonds
What does it mean, in terms of energy, that exothermic reactions need less energy to break bonds than to make them?
There is an overall release of energy when the reaction takes place = negative ΔH value
Why do exothermic reactions have a negative ΔH value?
There is an overall release of energy when a reaction takes place - so the system loses energy
Does a system lose or gain energy during an endothermic reaction?
Gains energy
Does a system lose or gain energy during an exothermic reaction?
Loses energy
What do values for enthalpy change depend on?
The pressure and temperature at which the reaction is carried out at
What is the standard enthalpy change of a reaction?
The heat energy exchange with its surroundings at a constant pressure and temperature (under standard conditions)
What are the standard conditions for standard enthalpy change?
- pressure - 100kPa
* temperature - 298K
What is standard enthalpy change measured in?
kJ mol-1
What is the standard enthalpy change of formation?
The enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, with all substances being in their standard states
What is the symbol for standard enthalpy change of formation?
ΔHf
What is the ΔHf of an element?
Zero
What is standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substances undergoes complete combustion under standard conditions, with all substances being in their standard states
What is the symbol for standard enthalpy change of combustion?
ΔHc
Which experimental technique is used to measure the standard enthalpy of combustion?
Calorimetry
What is calorimetry?
An experimental technique used to measure standard enthalpy change
What has to be 1 mole during an equation for standard enthalpy change of combustion?
The reactant
What has to be 1 mole during an equation for standard enthalpy change of formation?
The product
What is the equation for the standard enthalpy change of formation of methane?
C + 2H₂ -> CH₄
What is the equation for the standard enthalpy change of combustion of methane?
CH₄ + 2O₂ -> CO₂ + 2H₂O
What is calorimetry?
The process of measuring enthalpy changes of reactions
What is a calorimeter?
Any container used to measure enthalpy change
Is combustion an endo or exothermic reaction?
Exothermic
Is energy released or taken in during combustion?
Released
How is the enthalpy of combustion measured?
By measuring the heat transferred (when fuel is burnt) to a given volume of water in the calorimeter
Can the enthalpy of combustion be measured directly?
No - instead it is measured by the heat transferred to a given volume of water
Should a calorimeter be a good conductor?
Yes
Why should a calorimeter be a good conductor?
Enthalpy is measured by the heat transferred to a given volume of water in the calorimeter
For neutralisation and displacement, should the calorimeter be a good conductor?
No - it should be an insulator
What can be used as a calorimeter in neutralisation and displacement reactions?
A polystyrene cup
What can be used to measure more accurately the volume of acids and alkalis for neutralisation reactions?
A burette
What is specific heat capacity?
The amount of energy required to raise the temperature of 1g of a substance by 1K
How much energy is required to raise the temperature of one gram of water by one kelvin?
4.18J
What is the specific heat capacity of water?
4.18 J g⁻¹ K⁻¹
What is the equation used to calculate heat released from a reaction?
q=mcΔT
What does the ‘q’ stand for in q=mcΔT?
Heat released (in Joules)
What does the ‘m’ stand for in q=mcΔT?
Mass of substance being heated - often water (in grams)
What does the ‘c’ stand for in q=mcΔT?
Specific heat capacity in J g⁻¹ K⁻¹
What is the equation used to calculate enthalpy change?
ΔH=q/n
What is the unit for enthalpy change?
kJmol⁻¹
What does the ‘q’ stand for in ΔH=q/n?
Heat released in kJ
What is the heat released in the equation ΔH=q/n measued in?
kJ
What does the ‘ΔH’ stand for in ΔH=q/n?
Enthalpy change
What does the ‘n’ stand for in ΔH=q/n?
The number of moles of substance
What is the mass of water equal to?
The volume of water being heated
What equations are used to calculate enthalpy change?
- q=mcΔT
* ΔH=q/n
How is the number of moles of fuel (used to calculate enthalpy change of combustion) calculated?
- if fuel or solid: n=mass/mr
* if solution: n=cv (v in dm³)
When working out the enthalpy of neutralisation, what are the density and specific heat capacities of the acid and base taken as?
The same as for pure water
When measuring the energy transferred for other reactions (other than combustion), what is the mass of water assumed to be equal to?
The total volume of the solution used
Which substance is used to calculate the enthalpy change of a reaction?
Whichever substance is not in excess
What are you required to do on graphs when reagents have just been mixed so a temperature cannot be recorded?
The graphs must be extrapolated
What can you calculate after extrapolating graphs with axes time and temperature?
The maximum temperature change
What law enables us to calculate enthalpy changes uses known enthalpy changes?
The law of conservation, stating that energy cannot be created or destroyed
What does Hess’s law state?
That the overall enthalpy change for a reaction is independent of the route the reaction takes
What is the enthalpy of formation of an element?
0 kJ mol⁻¹
What is the enthalpy of combustion of carbon equal to?
The enthalpy of formation of CO2
What is the enthalpy of combustion of hydrogen equal to?
The enthalpy of formation of H2O
Does breaking bonds require or release energy?
Requires energy
Is breaking bonds exo or endothermic?
Endothermic
Does making bonds require or release energy?
Releases energy
Is making bonds exo or endothermic?
Exothermic
What is mean bond enthalpy?
The average bond dissociation enthalpy for a particular bond in a range of different compounds
Does the bond enthalpy of the same bond ever vary?
Yes, slightly
Why might the bond enthalpy of the same bond vary?
Depending on the molecule it is in
What are mean bond enthalpies used to calculate?
An approximate value for enthalpy changes of a reaction
Why do enthalpy values calculated using mean bond enthalpies differ from those determined using Hess’s law?
Mean bond enthalpies are approximate values whereas those determined using Hess’s law are actual values
What method of calculating enthalpy change should be used if you are given enthalpies of formation/combustion?
Hess cycle
How to calculate enthalpy change using mean bond enthalpies?
- Add up values for each bond in reactants to calculate total energy required to break bonds.
- Add up values for each bond in products to calculate total energy released when bonds formed
- Use equation BB - MB = ΔH
