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
why is Hess’s law useful?
for working out enthalpy changes that you can’t find directly by doing an experiment
how to use Hess’s law from enthalpies of formation?
the reactants and products have the same constituent elements
the energy required to form each mole of reactants + the enthalpy change of the reactants is the same as the energy required to form each mole of product
ΔfH⦵(reactants) + ΔrH⦵ = ΔfH⦵(products)
what is the enthalpy of formation of an element?
0 - the element is being formed from an element which is itself so no energy is required
how to use Hess’s law from enthalpies of combustion?
both reactants and products burn to form the same products
reaction energy + energy to burn products = energy to burn reactants
ΔcH⦵(products) + ΔrH⦵ = ΔcH⦵(reactants)
what is thermochemistry?
the study of enthalpy changes during chemical reactions
what is enthalpy?
a measure of the heat content of a substance (H)
what does isothermic mean?
energy in a reaction stays the same
what happens in an endothermic change?
energy is absorbed from the surroundings by the reactants to form the products
enthalpy of products increases so ΔH is positive
what happens in an exothermic change?
energy is released into the surrounding by the reactants to form the products
enthalpy of products has decreased so ΔH is negative
what is a transition state in a reaction?
a point where some bonds are being broken and some being made. it is high energy and unstable
what is the standard enthalpy of reaction?
ΔrH⦵, the enthalpy change for a reaction with the quantities shown in the chemical reaction
what is the standard enthalpy change of neutralisation?
ΔneutH⦵, enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali with standard states under standard conditions
how to carry out a calorimetry question?
- check sig figs
- check units
- calculate q using q=mcΔT
- calculate moles of whatever is reacting to raise temp
- use ΔH=q/moles
- use - sign if reaction was exothermic
- answer to correct sig figs
equation to find enthalpy change per mole?
ΔH = q/ number of moles reacting
how to use calorimetry of reactions taking place in solution?
reaction carried out in insulated beaker and the temp change measured
reaction must be fast so max temp is reached quickly
specific heat capacity of the solution must be taken as being 4.18 J/g/K the mass of water (not solution) is usually used
how to find enthalpy of formation from enthalpies of combustion?
the constituent elements of a substance burn to form the same products so the energy required to burn the elements - energy needed to burn product = enthalpy of formation
what is bond energy?
the amount of energy needed to break a bond in the gaseous state
according to Hess’s law it is the same amount of energy needed to form a bond
what is bond dissociation energy?
bond energy for diatomic molecules is breaking the only bond in the substance. this is called bond dissociation energy
what is mean bond enthalpy?
the enthalpy to break 1 mol of named bonds
averaged over a range of compounds/ molecules
in the gaseous state
how to find the enthalpy of a reaction using bond energy’s?
if molecules aren’t in gaseous state, include enthalpy of vaporisation then find bond enthalpies for all reactants and products and subtract energy for products from reactants
