Energetics Flashcards
What is the definition of Enthalphy change
the heat energy change in a system measured under conditions of constant pressure.
Name examples of exothermic reactions
- neautralising an acid
- oxidation reactions
Name an example of an endothermic reaction
limestone
breakdown of limestone = because it needs heat to be supplied
Why do exothermic reactions have a negative enthalphy change?
the products end up with less heat energy than the starting materials = they have lost heat energy when they heat up they surroundings
Why do endothermic reactions have a positive enthalphy change?
because the products have to take in eat energy from the surroundings, so they end up with more energy than the starting materials.
What are the standard conditions for measuring enthalpy changes?
- pressure of 100 kPa
- temperature of 298 K (25 Celsius)
Reactions are not thought of as being over until what?
the products have cooled back or warmed to 298K
Define the enthalpy change of combustion
the enthalpy change when ONE mole of a substance reacts completely with oxygen under standard conditions and all of the substances are in their standard state
Why is the standard enthalpy change for neutralization always constant for a strong acid and a strong base?
We make the assumption that strong acids and strong alkalis are fully ionised in solution, and that the ions behave independently of each other.
The equation for any strong acid being neutralised by a strong alkali is essentially just a reaction between hydrogen ions and hydroxide ions to make water. The other ions present (sodium and chloride, for example) are just spectator ions, taking no part in the reaction.
The reaction is always the same in each case of a strong acid and a strong alkali
Why is the standard enthalpy change for neutralization different for a weak acid and a weak base?
In a weak acid, the acid isn’t fully ionised. That means that the enthalpy change of neutralisation will include other enthalpy terms involved in ionising the acid as well as the reaction between the hydrogen ions and hydroxide ions.
And in a weak alkali, again, there will be other enthalpy changes involved apart from the simple formation of water from hydrogen ions and hydroxide ions.
Describe the difference between heat and temperature?
Temperature is related to the average kinetic energy of the particles in a system, independent of the number of particles present.
Heat =a measure of the total energy of all the partiles present in a gven amount of substance. therefore it does depend on how much of the substance is present.
What is the definition of specific heat capacity?
the amount of heat needed to raise the temperature of 1g of substance by 1K
How would you measure the enthalpy of a neutralization reaction? (practical)
1) add an equal volume and concentration of acid and alkaline in two different polystyrene cups
2) use a thermometer to take the temperature pf each solution every minute to confirm that both solutions remain at the same temperature, that of the lab.
3) pour one solution into the other and stir, continuing to record the temperature every 30 seconds for a further 10 minutes
Why is the calorimeter experiment extremely inaccurate when done in a class lab? What are the problems with the experiment in general?
because all of the heat energy does not go to the water : some heats up the air, some heats up the gauze, some heats the tripod = huge amount of heat loss
incomplete combustion = reduction in the temperature change measured
flammable liquids can evaporate during the mass measurings
How can heat loss in calorimetry be overcome?
Heat loss to surroundings can be minimised by improving insulation such as adding a lid
heat loss to container = polystyrene cup
put the calorimeter close to the heat source
use a copper calorimeter
Define Hess’s law
the enthalpy change for a chemical reaction is the same, whatever route is taken from reactants to products
Define bond dissociation enthalpy
the enthalpy changed required to break 1 mole of bonds in 1 mole of gaseous molecules (averaged over a number of compounds) to form gaseous atoms
Define mean bond enthalpy
The enthalpy change needed to break that covalent bond, averaged over a range of compounds
Definition of enthalphy change of neutralization
Neutralization = the enthalpy change when ONE mole of water is formed in a neutralization reaction under standard conditions
Define the standard enthalphy change of formation
Formation = the enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions and all of their reactants and products are in standard states
Define the standard enthalpy change of reaction
Reaction = the enthalpy change in the molar quantities shown by the balanced chemical equation under standard conditions
Suggest how the student’s method, and the analysis of the results, could be improved in order to determine a more accurate value for the enthalpy of reaction, Justify your suggestions. Do not refer to the precision of the measuring equipment. Do not change the amounts or the concentration of the chemicals.
- Insulate the beaker or use a polystyrene cup or lid = to reduce heat loss
- Record the temperature for a suitable time before adding the metal = to establish an accurate initial temperature
- record temperature values at regular time intervals - to plot the temperature results against time on a graph
- extrapolate the cooling back to the point of addition = to establish a theoretical maximum temperature OR temperature change (e.g. 4th min)
Explain why your bond enthalphy value is different from the data book
data book value derived from a number of different compounds
mean bond enthalpies are not specific for these reactions
Suggest one reason why the value for H cannot be measured directly
- imposible to add a precise amount of water
- very difficult to measure the temperature rise of a solid
- difficult to prevent olid dissolving
- coppersulfate solution will form
State, why the value quoted in part (c) for the standard enthalpy of formation of CO (g), is the same as the value for the standard enthalpy of combustion of carbon
the same number and same type of bonds are broken and formed
Name examples of exothermic reactions (4)
- neutralization
- combustion
- redox
- respiration
Name examples of endothermic reactions (3)
- thermal decomposition
- cracking
- photosynthesis
What is the equation for enthalpy change when calculating bond energies
sum of reactants - sum of products
