9- Enthalpy Flashcards
What is Enthalpy (H)?
Measure of the heat energy in a chemical system.
Can enthalpy be mesaured?
No, enthalpy changes can be
How do you work out the enthalpy change (∆H)?
∆H = H (products) - H (reactants)
Can ∆H be positive and negative?
Yes, depending on whether the products contain more or less energy then the reactants
What is the law of conservation of energy?
Energy cannot be created or destroyed, just converted from one form to another.
The amount of energy in a isolated system remains the same
Define enthalpy change?
The heat exchanged with the surroundings during a chemical reaction.
The difference between the enthalpy of the products and the reactants
Define exothermic?
-A reaction in which the enthalpy of the products is smaller then the enthalpy of the reactants
-resulting in heat loss to the surroundings.
-∆H is negative
-Energy transferred from the system to the surroundings
-Temperature of surroundings increases as they gain energy
-enthalpy of products < enthalpy of reactants
Define endothermic?
-a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants
-resulting in heat being taken out of the surroundings
-∆H is positive
-energy transferred from the surroundings to the chemical system
-temperature of surroundings decreases as they loose energy
-enthalpy of products > enthalpy of reactants
Define activation energy?
The minimum amount of energy required for a reaction to take place
What are the standard conditions?
Standard pressure - 101kPa
Standard temperature - 298K (25°C)
Standard concentration - 1 mol/dm^3
Standard state - the physical state of a substance under standard conditions
Define standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactions and products in their standard states
Define standard enthalpy change of formation
(ΔfH⦵)
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 their standard states
elements to 1 mol
Define 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 states
1 mol to combustion products
Define standard enthalpy change of neutralisation
(ΔneutH⦵)
The enthalpy change that accompanies the reaction of an acid by a base to form one mole of H2O, under standard conditions, with all reactants and products in their standard states
What is the equation used to measure an energy change?
Q=mcΔT
Q - energy change with surroundings (J)
m - mass (g)
c - specific heat capacity (J/g/K)
ΔT - change in temperature (K)
How is enthalpy change worked out from the energy change?
ΔH = -Q/n
ΔH - enthalpy change (J/mol)
Q - energy change with surroundings (J)
n - number of moles (mol)
What are the causes for less energy being transferred than expected when working out ΔH(c)?
-Heat loss to surroundings
-Incomplete Combustion
-Evaporation
-Non standard conditions
How can heat loss be accounted for using a graph of temperature against time?
Extrapolate the cooling curve back to when it was added
Enthalpy change of neutralisation always has the same value, what is it?
-57.5 kj/mol
what’s the enthalpy change equation using ‘enthalpy change of combustion’ values?
Δrh = Σ reactants - Σproducts
what’s the enthalpy change equation using ‘enthalpy change of formation?
ΔrH = Σformation of products - Σformation of reactants
what’s the enthalpy change equation using ‘average bond enthalpies’ ?
ΔrH= Σ(bond enthalpies of reactants) - Σ (bond enthalpies of products)
Define average bond enthalpy?
The energy required to break one mole of a specified type of bond into a gaseous molecule
What are some properties of bond enthalpies?
-Energy is always required to break bonds
-Bond enthalpies are always endothermic
-Bond enthalpies always have positive enthalpy values
What happens to bond in chemical reactions?
Break and new bonds form
How do we determine whether a reaction is exothermic or endothermic?
The difference between the energy required for bond breaking and the energy released by bond making
In terms of bond breaking and making when is a reaction endothermic or exothermic?
-Energy is required to break bonds
bond breaking is endothermic
ΔH is positive
-Energy is released when bonds form
bond making is exothermic
ΔH is negative
What does Hess’ law state?
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
What are the limitations of calculating enthalpy changes from average bond enthalpies?
-we use average bond enthalpies, the actual energy involved in breaking and making individual bonds would be slightly different
-all species need to be gaseous molecules
Give an example of an equation which represents standard enthalpy of formation
H2 (g) + 1/2 O2 (g) ➝ H2O (l)
Give an example of an equation which represents standard enthalpy of combustion?
C (s) + O2 (g) ➝ CO2 (g)
Why is Hess’s law useful?
-can apply to all chemical reactions
-allow us to calculate enthalpies that we can’t calculate by experiment
-used in industry to make sure chemical processes are as efficient as possible
-can tell us if a reaction is going to be exothermic or endothermic
What is the symbol for standard conditions?
⦵
What is a chemical system?
Refers to the atoms, molecules or ions making up the chemicals
What is the relationship in general between the size of activation energy and time?
Small activation energies - take place rapidly
(the energy needed to break bonds is readily available from surroundings)
Large activation energies - present a large energy barrier that a reaction may take place extremely slowly or not even at all)
What is the specific heat capacity of water?
4.18 JgK
How do you calculate temperature change?
T(final) - T(initial)
What is the method for using a spirit burner to determine enthalpy change of combustion?
- Using a measuring cylinder, measure out 150 cm^3 of water and pour it into a beaker. Record the initial temperature of water to the nearest 0.5 degrees
- Add methanol to spirit burner. Weigh the spirit burner containing methanol
- Place spirit burner under the beaker (which is held up using a clamp and boss). Light the burner and burn the methanol whilst stirring the water with thermometer
- After about 3 minutes extinguish the flame. Immediately record the maximum temperature reached by water
- Reweigh the spirit burner containing the methanol. Assume that the wick has not been burnt
How do we determine the enthaply change of reaction?
-using plastic cups made of polystyrene foam (cheap, lightweight, waterproof)
-offer some insulation for heat loss to surroundings
-thermometer to record any temperature change allowing heat energy change to be calculated using Q = mcΔT
How are cooling curves used to measure enthalpy change?
This is an adapted method of using a polystyrene cups to correct for the heat loss by using a cooling curve correction
- Pipette 25cm^3 of 1moldm CuSO4 into a polystyrene cup. Weight out an excess of zinc powder
- Start a stop-clock and take the temperature of the solution every 30s until the temperature remains constant
- Add the zinc to the solution and stir the mixture. Record the temperature every 30 seconds until the temperature has fallen for several minutes
- Plot a graph of temperature against time
To correct for the cooling , extrapolate the cooling curve section of the graph back to when zinc was added. Draw a vertical line from the time the solutions were mixed
What are the limitations of average bond enthalpies?
-actual bond enthalpy can vary depending on environment of the bond
