Unit 1: Section 4 - Energetics Flashcards
What is the definition of enthalpy change?
The heat energy transferred in a reaction at constant pressure, the units are kJmol^-1
What are the standard conditions?
298K and 101KPa
What are exothermic reactions and some examples?
Gives out energy, ΔH is negative
Temperature usually increases
Combustion of fuel like methane
Oxidation of carbohydrates such as glucose (C6H12O6) in respiration
What are endothermic reactions and some examples?
Absorbs energy, ΔH is positive
Temperature usually decreases
Thermal decomposition of calcium carbonate
Main reactions of photosynthesis
How to determine the overall enthalpy change of a reaction?
- Energy is required to break bonds (endothermic), stronger bonds take more energy to break
- Energy is released when bonds are made (exothermic), stronger bonds release more energy when they form
- The enthalpy change from a reaction is the overall effect if these 2 changes
Enthalpy change of a reaction = total energy absorbed - total energy released
What are mean bond enthalpies?
Bond enthalpy is the energy required to break bonds
The energy needed to break a bond depends on the environment it’s in
In calculations, you use mean bond enthalpy- average energy needed to break a certain type of bond, over a range of compounds
What is the standard enthalpy of formation ΔfH?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
What is the standard enthalpy of combustion ΔcH?
Then enthalpy change when 1 mole of a substance is completely burnt in oxygen, under standard conditions
How to calculate enthalpy changes using calorimetry?
- To find the enthalpy change of a flammable liquid, you burn it in a calorimeter
- As the fuel burns, the water is heated, and you can work ot the heat energy that has been absorbed by the water if you know the mass of the water, the temperature change and the specific heat capacity of water
Why is it hard to get an accurate result when using a calorimeter?
- Some of the energy is always lost to the surroundings
- Some incomplete combustion may have occurred meaning less energy is given out
- Flammable liquids are often quite volatile so some fuel may be lost to evaporation
How to use calorimetry to measure enthalpy changes for a neutralisation reaction?
- Add a known volume of an acid to an insulated container (polystyrene cup) and measure the temperature
- Add a known volume (or mass if solid) of alkali and record the temperature of the mixture at regular intervals over a period of time (stir the solution to make sure it’s evenly heated)
- Find the temperature change for the experiment
- Use that to calculate the enthalpy change
- You need to know the mass of the solutions used in order to calculate the enthalpy change of the reaction, you can assume all solutions have the same density as water
How to use a graph to find an accurate temperature change?
- During the experiment record the temperature at regular intervals, beginning a couple of minutes before you start the reaction
- Plot a graph of your results, draw 2 lines of best fit - one through the points before the reaction started and one after
- Extrapolate both lines so they both pass the time where the reaction started
- The distance between the 2 lines at the time the reaction started (before any heat was lost) is the accurate temperature change
What is the equation and units for calculating the enthalpy change?
q=mcΔT
q - heat lost or gained in J (same as the enthalpy change if pressure is constant)
m - mass of water or other solutions in calorimeter in g
c - specific heat capacity of water (4.18) in Jg^-1K^-1
ΔT - the change in temperature of the water or solution in K or °C
What is Hess’ Law?
The total enthalpy change is independent of the route taken
