Energetics - Enthalpy Flashcards
(29 cards)
Describe an enthalpy diagram for an Exothermic change
Describe an enthalpy diagram for an Endothermic change
What are key features to remember for an exothermic reaction?
- Heat energy loss to environment
- Reacting chemicals lose energy
- Increase in temperature
- Bond making
- Enthalpy = -kJmol-1
What is important to remember about the enthalpy of an exothermic reaction?
The enthalpy of the products is less than the enthalpy of the reactants so energy has been released to the surroundings
Examples of exothermic reactions
- Combustion
- Neutralisation
- Respiration
- Displacement
What are key features to remember for an endothermic reaction?
- Heat energy gained from the environment
- Reacting chemicals absorb energy
- Decrease in temperature
- Bond breaking
What is important to remember about the enthalpy of an endothermic reaction?
The enthalpy of the products is greater than the reactants so less energy has been absorbed from the surroundings
Examples of endothermic reactions
- Thermal decomposition
- Photosynthesis
- Dehydration of hydrated copper sulfate
What is activation energy?
The minimum amount of energy which the reactants must have in order to react
What is enthalpy?
Enthalpy relates to the energy of the bonds broken and made during a chemical reaction.
Endo = bond breaking
Exo = bond making
What are the Standard conditions / states?
Standard pressure : 100kPa (1atm)
Standard temperature : 298K
Standard concentration : 1.00mol dm-3
What is Mean Bond Enthalpy?
The energy required to break 1 mol of a gaseous covalent bond averaged across many compounds containing the bond.
Units : kJmol-1
H = Σ MBE bonds broken (reactants) - Σ MBE bonds made (products)
What is the Standard enthalpy of reaction?
This is the enthalpy change when substances react under standard conditions in quantities given by the equation for the reaction.
E.g. CaO(s) + H20 → Ca(OH)2(s) ChangerH° = -63.7kJ mol-1
This means that when 1 mole of calcium oxide reacts with 1 mole of water to form 1 mole of calcium hydroxide, 63.7kJ of heat would be released.
What are calorimetery experiments?
Experiments that measure the heat given out by reactions. These reactions take place in solutions, they can be placed into a polystyrene container and the temperature change recorded with a thermometer.
Why are polystyrene cups used?
They are good insulators and have a low heat capacity. This reduces heat loss through the sides of the container.
What are the issues involving caloirmetry?
- Some heat will be absorbed by the container, rather than going towards heating up the water.
- Some heat is always lost to the surroundings during the experiment (depending on how well the container is insulated)
What calculations are used for direct enthalpy change?
Enthalpy change = -q (J) / n
q = mcAT
Density = mass/volume
What is the density of water?
1 g/cm3
It can be used in the calculation to replace the mass in q = mcAT
This can be used for example when using the mass as the volume of water making up a solution in the reaction.
What is the enthalpy change of formation?
This is the enthalpy change when one mole of compound is formed from its elements under standard conditions, all reactants and products being in their standard states.
What is the Enthalpy change for all elements and why?
Zero, because there is no change in enthalpy
What are the equations for the element formations of methane, water, sulfur dioxide, hydrogen sulfide and nitrogen monoxide?
C(s) + 2H2(g) → CH4(g)
H2(g) + 1/2O2(g) → H2O(l)
S(s) + O2(g) → SO2(g) - Sulfur is a yellow solid
H2(g) + S(s) → H2S
1/2N2 + 1/2O2(g) → NO(g)
What is Hess’s Law?
Hess’s Law states that the enthalpy change for a chemical reaction is the same, what ever route is taken from reactants to products.
What is the diagram for Hess’s Law, which is used to calculate the enthalpy for a reaction?
What can this diagram be arranged to?
ArH° = AfH° products - AfH° reactants
