Energetics Flashcards
What is an enthalpy change?
Enthalpy change is the heat energy transferred in a reaction at constant pressure. The units of (delta H) are KJ mol-1
What does the little circle with a line through mean when it is by an enthalpy change value?
It tells you that the substances were in their standard states and the measurement was made under standard conditions. Standard conditions are 100kPa pressure and a stated temperature.
What are exothermic reactions?
They are reactions that give out energy, they have a negative enthalpy change.
What are endothermic reactions?
They are reactions that absorb energy, they have a positive enthalpy change.
Is bond-breaking endothermic or exothermic?
You need energy to break bonds, so bond breaking is endothermic . Stronger bonds take more energy to break.
Is bond making endothermic or exothermic?
Energy is released when bonds are formed, so bond-making is exothermic. Stronger bonds release more energy when they form.
What is bond enthalpy?
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- which is the average energy needed to break a certain type of bond over a range of compounds.
How can you calculate the enthalpy change using mean bond enthalpies?
Enthalpy change of reaction= total energy absorbed - total energy released.
(because they use average values, enthalpy changes calculated using mean bond enthalpies aren’t exact - they are slightly less accurate than enthalpy change values calculated using Hess’s law)
What is the standard enthalpy of formation?
The standard enthalpy of formation is 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?
The standard enthalpy of combustion is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions.
How can you find enthalpy changes using calorimetry?
- you can use calorimetry to find out how much heat is given out by a reaction by measuring a temperature change.
- to find the enthalpy of combustion of a flammable liquid you burn it inside a calorimeter
- as the fuel burns 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, all the heat given out will be absorbed by the water
- but you will probably lose heat to the surroundings even if the calorimeter is insulated
- there might also be some incomplete combustion so less energy can be given out
- flammable liquids are quite volatile so may lose some fuel to evaporation
How do you use calorimetry to measure enthalpy changes in solution?
(could be neutralization, dissolution, or displacement)
- for naturalization add a known volume of acid to an insulated container and measure the temperature
- then add a known volume of alkali and record the temperature of the mixture at regular intervals over a period of time. stirring to make sure it is evenly heated
- Find the temperature change for the experiment. Use it to calculate the enthalpy change of the reaction
What is the equation for the mass of solutions used if they have the same density as water?
density= mass/volume
How do you find the energy change per mole of reactant?
Moles = concentration (mol dm-3) x volume dm3
Energy change per mole = energy/mole
How do you use a graph to get 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 two lines of best fits: one through the points before the reaction started and one through the points after it started
- Extend both lines so they both pass the time when the reaction started
- The distance between the two lines at the time the reaction started is the accurate temperature change for the reaction.
