Energy Flashcards
(16 cards)
Define enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products bring in their standard States
Define standard conditions
A pressure of 1 atmosphere, a stated temperature, usually 25*C and a concentration of 1 mol dm-3
Define enthalpy change of formation
The enthalpy change that takes place when 1 mole of a compound in its standard state is formed from its constituent elements in their standard States under standard conditions.
Define enthalpy change of combustion
The enthalpy changes that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard States.
Define average bond enthalpy
The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of gaseous species.
State le Chatelier’s principle
When a system in dynamic equilibrium is subjected to a change, the position of the equilibrium will shift to minimise the change.
some chemical reactions are accompanied by enthalpy changes. What are these changes?
Exothermic -
describe the importance of oxidation as an exothermic process in the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration.
Oxidation is the most exploited exothermic reaction.
Combustion of fuels is the most common; we rely on it as our main source of energy.
However, we now need to start considering alternatives due to our high carbon emissions and Global Warming
What do endothermic processes require?
an input of heat energy, e.g. the thermal decomposition of calcium carbonate.
Endothermic reactions are ∆ +ve as they require more energy to break the bonds than they do to form the bonds.
construct a simple enthalpy profile diagram for a reaction to show the difference in the enthalpy of the reactants compared with that of the products.
Look up simple enthalpy profile diagrams for endothermic and exothermic reactions
Explain qualitatively, using enthalpy profile diagrams, the term activation energy
The activation energy is the difference in energy between the reactants and the top of the energy curve. It shows the minimum energy required in order to start a reaction by breaking the bonds
calculate enthalpy changes from appropriate experimental results directly, including use of the relationship: energy change = mc∆T
Q=mc∆T
m - is the mass of the surroundings (The surroundings is the substance in which the thermometer is placed in)
c – The specific heat capacity (usually 4.18 Jg-1K-1) ∆T – The change in temperature
When using this equation. If the temperature is increased, the reaction is exothermic, so the final answer must have a negative sign placed in front of it. And vice versa if the temperature decreases.
You must then calculate the number of moles of the surroundings.
After this, divide the Q by the number of moles for the overall enthalpy change per mole.
explain exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds;
When breaking bonds, energy is required. This means bond breaking is endothermic When bonds are formed, energy is releases. So this is exothermic.
calculate an enthalpy change of reaction from average bond enthalpies
To calculate the enthalpy change, ∆H, you must
1. Find the total sum of the bond enthalpies of bonds broken (left side of equation)
2. Find the total sum of the bond enthalpies of bonds made (right side of equation)
3. Sum of bonds broken – Sum of bonds made (left – right)
use Hess’ law to construct enthalpy cycles and carry out calculations to determine:
an enthalpy change of reaction from enthalpy changes of combustion,
When using Enthalpy change of combustion figures ( ∆Hcɵ) figures, the formula: ∆H = Sum of ∆Hcɵ reactants – Sum of ∆Hcɵ Products
Use Hess’ law to construct enthalpy cycles and carry out calculations to determine:
an enthalpy change of reaction from enthalpy changes of formation
When using Enthalpy change of formation figures( ∆Hfɵ) figures, the formula: ∆H = Sum of ∆Hfɵ Products - Sum of ∆Hfɵ reactants
