8B. Energetics 1 [SET 2] Flashcards
What does system mean in a chemical reaction?
The atoms and bonds involved in the
chemical reaction
Explain the law of conservation
The amount of energy in an isolated system remains the same. Energy cannot be destroyed or created, It can only be transferred from one form to another
What energy change is breaking bonds associated with?
Energy is taken in to break bonds → endothermic reaction
What energy change is making bonds associated with?
Energy is released to make bonds → exothermic reaction
What is an endothermic reaction?
A reaction with an overall positive enthalpy change (+ΔH) → enthalpy of products > enthalpy of reactants
What is an exothermic reaction?
A reaction with an overall negative enthalpy change (-ΔH) → enthalpy of products < enthalpy of reactants
Describe the diagram for an endothermic reaction
Reactants lower than products
Describe the diagram for an exothermic reaction
Reactants higher than products
What does activation energy mean?
The minimum energy required for a reaction to take place
Which way does the arrow for activation energy point on an enthalpy profile diagram?
Always points upwards
What are the standard conditions?
100 kPa
298 K
What does “in standard state” mean?
The state an element / compound exists at in standard conditions (100 kPa, 298 K)
Define enthalpy change of formation
The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions
Give an example of an equation which represents standard enthalpy of formation
H 2 (g) + ½ O 2 (g) → H 2O (l)
Define enthalpy change of combustion
The energy change that takes place when 1 mole of a substance is burnt in excess oxygen under standard conditions.
Give an example of an equation which represents standard enthalpy of combustion
C (s) + O 2 (g) → CO 2 (g)
Define enthalpy change of neutralisation
The energy change that takes place when 1 mole of water is formed by reacting an acid and alkali under standard conditions.
What does enthalpy change of reaction mean?
The energy change when the amounts of reactants shown in the equation react under standard conditions to give the products in their standard states.
How can you calculate enthalpy change from experimental data?
Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18gJ-1K-1) and ΔT is the change in temperature
Draw a simple calorimeter
A polystyrene cup, a vacuum flask or a metal can.
A plastic lid
A thermometer through the lid.
Why might experimental methods for enthalpy determination not be accurate?
Heat is lost to the surroundings
Not in standard conditions
Reaction may not go to completion
What does average bond enthalpy mean?
The mean energy required to break 1 mole of bonds in gaseous molecules
Why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/formation?
Bond enthalpies are a mean for the same bond across different molecules whereas standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate
How to calculate enthalpy change of reaction using average bond
enthalpies?
ΔH = Σ (bond enthalpies of reaction) - Σ (bond enthalpies of products)
What is Hess’s Law?
The total enthalpy energy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long the initial and final conditions are the same.
Is the enthalpy of reaction exothermic or endothermic?
Can be both
Is the enthalpy of formation exothermic or endothermic?
Can be both
Is the enthalpy of combustion exothermic or endothermic?
Exothermic
Is the enthalpy of neutralisation exothermic or endothermic?
Exothermic
What does enthalpy mean?
The total chemical energy inside a substance.
What is enthalpy also known as?
Heat content
What happens when there is enthalpy change?
A change in chemical energy due to a bond breaking or forming.
When is activation energy shown on a diagram?
When it’s reaction profile diagram not enthalpy level diagram.
What happens in an exothermic reaction?
Heat energy is given off to the surroundings. Hence the temperature of the surroundings increases and the energy of the system decreases.
Why might an exothermic reaction not occur?
When the rate of reaction is too slow.
When is a reaction kinetically controlled?
A reaction in which the product ratio is determined by the rate at which the products are formed.
What happens in an endothermic reaction?
Heat energy is absorbed from the surroundings. Hence the temperature of the surroundings decreases and the energy of the system increases.
What is calorimetry?
Calorimetry is the measurement of enthalpy changes in chemical reactions.
What is the specific heat capacity of a liquid?
The energy needed to increase the temperature of 1g of substance by °C.
What is the specific heat capacity of water?
4.18 J g^-1 K^-1
What other substances can we use the SHC of water for and why?
Aqueous solutions of acids, alkalis, and salts.
They are assumed to be largely water.
What is the formula for ΔH?
ΔH = (-Q)/n = (-(mcΔT))/n
What is a closed system?
A closed system can exchange energy (as heat or work) but not matter, with its surroundings.
What is the total energy present in a closed system?
Always constant
What is bond dissociation enthalpy?
The energy required to break a particular bond.
What is bond dissociation enthalpy also known as?
Bond enthalpy or bond energy.
When are the products more stable than the reactants?
An exothermic reaction
When are the products less stable than the reactants?
An endothermic reaction
What is average bond enthalpy?
The energy needed to break one mole of bonds in a gaseous molecule averaged over similar compounds.
What is the average bond enthalpy of C-H?
The bond dissociation enthalpy for the whole molecule and dividing it by the number of C-H bonds.
Why would each bond enthalpy of C-H bonds in methane be different?
The first bond is easier to break than the second as the remaining hydrogens are pulled more closely to the carbon.
