MODULE 3: Enthalpy Flashcards
What is enthalpy?
Enthalpy, H is the amount of energy that a substance contains (cannot be measure directly)
What is the equation for enthalpy change?
Enthalpy change (ΔH) = enthalpy of products (H) - enthalpy of reactants (H)
What is Hess’ Law?
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change will be the same for each route
What is an exothermic reaction?
Heat energy is transferred from the reaction to the surroundings, enthalpy change is negative therefore energy is lost (bond making)
What is an endothermic reaction?
Heat energy is transferred from the surroundings into the reaction, enthalpy change is positive therefore energy is gained (bond breaking)
What is activation energy?
The minimum energy required to start a reaction
What are standard conditions?
Standard pressure (100kPa), standard temperature (298K), standard concentration (1 mol dm-3), standard state (state in 101kPa and 298K)
What is enthalpy change of a reaction? (ΔrHᶿ)
The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state
What is enthalpy change of formation? (ΔfHᶿ)
Enthalpy change that takes place when one mole of a compound is formed from its elements at standard conditions
What is enthalpy change of combustion? (ΔcHᶿ)
The standard enthalpy change of combustion is the enthalpy change when 1 mol of a substance
is burnt completely, in an excess of oxygen, under standard conditions of 298K and 101kPa
How is the enthalpy change of a reaction measured? (ΔrHᶿ)
Using the equation ‘q = m x c x ΔT’, a Styrofoam cup (to prevent heat loss), and a temperature to record the temperature change
What is the equation for enthalpy change of a reaction? (ΔrHᶿ)
∆rHᶿ = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)
What is average bond enthalpy?
Energy required to break one mole of bonds in a gaseous molecule
What is the equation for average bond enthalpy?
ΔrHᶿ = Bond enthalpies in reactants - bond enthalpies in products
What is enthalpy change of neutralisation? (ΔneutHᶿ)
The standard enthalpy change of neutralisation is the enthalpy change when an acid and alkali react together to form 1
mol of water, under standard conditions of 298K and 101kPa
What is the equation for measuring energy change?
q= mcΔT
In the equation for energy change, what is ‘m’?
Mass, grams
In the equation for energy change, what is ‘c’?
Specific heat capacity, g-1 K-1
In the equation for energy change, what is ‘ΔT’?
Temperature change, kelvin
Why may testing for enthalpy change of combustion (ΔcHᶿ) be inaccurate?
Heat loss to surroundings, incomplete combustion, evaporation of methanol from the wick, non-standard conditions
What equipment do you use for enthalpy change of combustion (ΔcHᶿ) be inaccurate?
Thermometer, beaker full of water, spirit burner full of fuel
How is enthalpy presented?
Positive if endothermic, negative is exothermic
What are the limitations of bond enthalpies?
Bond enthalpy can vary depending on the chemical environment of the bond
What is the general equation used for Hess’ Law?
A + B = C
What does an exothermic energy profile look like?
Energy of reactants is above the energy of products; enthalpy change is negative
What does an exothermic energy profile look like?
Energy of reactants is below the energy of products; enthalpy change is positive
