Unit 5 Energetics Flashcards
What is an enthalpy
term used for the heat released or absorbed by a system at constant pressure
What symbols represents the change in enthaply
Delta H
What do you call the enthaply change measured under standard conditions
standard enthalpy change of reaction (ΔHƟ).
Exothermic and endothermic reactions are
Exo: release heat
Endo: absorb heat
Examples of exothermic reactions
Combustion and Neutralisation reactions
Explain energy profile of exothermic reaction
the enthalpy change (∆H) is negative because heat has been transferred from the system to the surroundings, so the system has lost heat.
The products of the reaction have lower enthalpy than the reactants, so they are more energetically stable.
Examples of endothermic reaction
thermal decomposition of calcium carbonate.
Explain energy Diagram of endothermic reaction
The enthalpy change is positive because the reactants gain heat from their surroundings. For this reason, the products have higher enthalpy than the reactants and are therefore less energetically stable.
How are enthalpy changes calculated:
Q= mc deltaT, where…
q is heat absorbed or released in J
C is specific heat capacity of solution
M is mass of solution in gr
DeltaT is change in Temperature in C or K
Add or remove minus to results depending on exothermic or endothermic reaction.
Calculate enthalpy change (deltaH) for a reaction
Ex:
50.0 cm3 of a 0.500 mol dm−3 solution of aqueous copper(II) sulfate was reacted with 3.00 g of zinc powder, according to the equation:
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
The results are shown in Table 1 below. Use this data to determine the enthalpy change (ΔH) for the reaction.
Volume of 0.500 mol dm-3 CuSO4 (aq) used (cm3) =50.00
Mass of Zn powder added (g) = 3.00
Initial temperature of solution (C) = 21.0
Maximum temperature of solution (C) = 43.5
- Determine the change in heat (q) for the reaction using the equation q=mc deltaT with values from table
- delta T: 22.5
- mass is 50.00 cm3 which is 50.00 gr
-assume heat capacity is 4.18 - Determine limiting reactant in reaction
- calculate mol of each reactant
- divide each amount in mol by the coefficient in balanced equation
- lower value is limiting reactant - Calculate the enthalpy change per mole of limiting reagent.
- DeltaH divided by moles of limiting reagent
What is the standard enthalpy of combustion or the molar enthalpy of combustion
ΔHƟc), for a substance is defined as the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.
Always exothermic
How to calculate enthalpy change of combustion of hexane
Ex. In an experiment to measure the enthalpy change of combustion of hexane (C6H14), a student heated a copper calorimeter containing 400.0 cm3 of water with a spirit lamp and collected the following data:
Initial temperature of water: 20.0 °C
Final temperature of water: 77.9 °C
Mass of hexane burned: 2.49 g
Use this data to calculate the enthalpy change of combustion, ΔHc, of hexane.
- Calculate mol of hexane burned
- Calculate q using mc deltaT
- Divide heat released by amount in mol of hexane burned.
How do you calcite the experimental error
Percentage error=
(experimental value- theoretical value) divided by theoretical values times 100.
What is Hess’s law
states that the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place, as long as the initial and final conditions are the same.
What is Hess’s law used for?
be used to calculate enthalpy changes for reactions that cannot be determined experimentally
Ex, enthalpy change of formation of methane bc hydrogen doesn’t react with carbon under standard conditions