Unit 1: Section 4 - Energetics MDY * Flashcards
Enthalpy changes Calorimetry Hess's law
What is an enthalpy change?
ΔH - heat energy transferred in a reaction at constant pressure
What does ΔH⦵ mean?
The substances were in their standard states and the measurement was made under standard conditions.
What are standard conditions?
100kPa pressure
A stated temperature (e.g. 298 K/ 25*C)
What does exothermic mean?
Exothermic reactions give out energy ΔH is negative Oxidation reactions are usually exothermic e.g. combustion of fuel like methane Oxidation of carbohydrates like glucose
What does endothermic mean?
Endothermic reactions absorb energy
ΔH is positive
e.g. Thermal decomposition of calcium carbonate
What happens to bonds of reactants when they react?
Energy is needed to break bonds, endothermic
Stronger bonds need more energy
What happens to bonds of products when they react?
Energy is released when bonds are formed, exothermic
Stronger bonds release more energy when they form
How to find whether a reaction is endothermic or exothermic?
The enthalpy change for a reaction is the overall effect if the breaking and making of bonds
If more energy is needed to break bonds then released when made ΔH is positive and vice versa
What is bond enthalpy?
The energy required to break bonds
Mean bond enthalpy = the average energy needed to break a certain type of bond over a range of compounds
Bond enthalpy are always positive
What is the overall enthalpy change of a reaction?
Enthalpy change of reaction = total energy absorbed- total energy released
OR
energy to break all bonds - energy to make all bonds
Why aren’t enthalpy changes calculated from bond enthalpy exact?
Average values for bond enthalpy’s are used, so enthalpy changes calculated from them aren’t exact
What is the standard enthalpy of formation ΔfH⦵?
The enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions
What is the standard enthalpy of combustion ΔcH⦵?
The enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions
What is calorimetry?
the process of measuring the amount of heat released or absorbed during a chemical reaction
How can you find the enthalpy of combustion of a flammable liquid?
Burn it inside a flame calorimeter
As the fuel burns in a bottle with a wick, it heats the water. You can work out 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 the water
Ideally heat given out by fuel would be absorbed by the water so you can work out enthalpy change exactly
What is the specific heat capacity of water?
4.18 J g^-1 K^-1
What downsides do calorimetry tests have?
In any calorimetry experiment, you always lose heat to the surroundings. This makes it hard to get an accurate result
Some of the combustion may be incomplete so less energy is given out
Flammable liquids are often quite volatile too, so you may lose some fuel to evaporation
How can calorimetry be used to measure enthalpy changes in solution?
Neutralisation
Dissolution (dissolving)
Displacement
How to find the enthalpy change for a neutralisation reaction?
Add a known volume of acid to an insulated container and measure the temperature
Add known volume of alkali and record the temperature of the mixture at regular intervals over a period of time
Find the temperature change for the experiment. Use it to calculate enthalpy change
What do you need to know to calculate the enthalpy change of a neutralisation reaction?
The mass of the solutions that you’ve used in order to calculate the enthalpy change of the reaction. Assume all solutions have have same density as water
Find number of moles concentration x volume
How to find temperature change in calorimetry experiment?
Subtract start temp from highest temp recorded - not very accurate because heat lost to surroundings
Graphs can be used for an accurate value
How can you use a graph to find the temperature change in a calorimetry experiment?
Record temperature at regular intervals, beginning a couple of minutes before you start the reaction
Plot a graph, draw 2 lines of best fit: 1 through the points before the reaction started and 1 through the points after it started
Extend both lines so they both pass the time when the reaction started
The distance between the 2 lines at the time started is the accurate temperature change for the reaction
equation to calculate enthalpy change from a calorimetry experiment?
q = mcΔT q = heat lost or gained (enthalpy change) (J) m = mass of substance heated (g) c = specific heat capacity (J/g/K) ΔT = change in temperature of substance (K)
what is Hess’s law?
the total enthalpy change for a reaction is independent of the route taken