1.4 Energetics Flashcards
What is enthalpy change?
Heat energy change at a constant pressure
How does a thermometer in a hot substance?
It absorbs heat energy from the liquid so the particles gain more kinetic energy and move apart
The space between the particles increases and the substance therefore becomes less dense
The particles will therefore rise showing a temperature increases
How does a thermometer work in a cool substance?
Little to no heat supplied means the particles lose kinetic energy and move closer together
The energy moves out of the thermometer and into the substance
The space decreases between the particles and become more dense
The particles will then sink showing a decrease in temperature
What are the two systems in a chemical reaction?
Chemical system - reactants and products only
Surrounding system - everything! Hand, beaker, atmosphere, thermometer etc…
What does a thermometer record?
The temperature of the surroundings
What is the endothermic part of a reaction?
Where bonds are broken
The activation energy
What is an endothermic reaction?
Examples?
Reactions which take in heat energy from the surroundings as the reaction takes place
Results in a positive delta H ie increase in enthalpy change
Photosynthesis
What does an endothermic reaction result in?
A decrease in temperature surroundings
What is an exothermic reaction? Examples?
Reactions in which heat energy is released to the surroundings as the reaction takes place
Results in a negative delta H ie decrease in enthalpy change
Respiration and Combustion
What are the standard enthalpy change symbols?
H - enthalpy
Δ - change
θ - standard
ΔH ^θ
What are standard conditions?
Temperature - 298k
Pressure - 100kPa
What is standard enthalpy change? ΔH ^θ
Heat energy change at a constant pressure and under standard conditions
What is standard enthalpy of reaction? ΔH ^θr
The enthalpy change for a chemical reaction under standard conditions and all substances in their standard states
What is standard molar enthalpy of combustion? ΔH ^θc
The enthalpy change that takes place when 1 mole of a substance completely reacts with excess oxygen under standard conditions and all reactants and products being in their standard state
What is standard molar enthalpy of formation? ΔH ^θf
The enthalpy change that takes place when 1 mole of a substance is formed from its constituent elements under standard conditions with all reactants and products in their standard states
What does an exothermic reaction result in?
An increase in temperature surroundings
What is the exothermic part of a reaction?
Bond making - to produce products
What are mean bond enthalpies?
The average energy change required to break 1 mole of a particular covalent bond with all species in the gaseous state averaged over many different molecules
What are bond enthalpies?
The energy change required to break 1 mole of a particular covalent bond with all species in the gaseous state
What is the equation to work out (mean) bond enthalpies?
ΔH = ∑ (mean) bond enthalpies of reactants - ∑ (mean) bond enthalpies of products
What is Hess’ law?
The enthalpy change for a reaction is independent of the reaction pathway taken as long as the starting and ending reaction conditions are the same
Give a general summary of a Hess cycle?
The enthalpy change for a chemical reaction is the same whatever route is taken from reactants to products
ΔH1 = ΔH2 + ΔH3 (as ΔH1 may be too difficult to work out)
When working out enthalpy changes what must we do?
Look at the DATA provided!!!
To determine the equation
(Not the balanced equation)
What equation is used for working out enthalpy of formation?
Using COMBUSTION DATA?
ΔH^θf =
∑ ΔH^θc reactants - ∑ ΔH^θc products
What equation is used for working out enthalpy of combustion?
Using FORMATION DATA?
ΔH^θc =
∑ ΔH^θf products - ∑ ΔH^θf reactants
Points about constructing Hess cycles?
- All arrows go away from elements
- If elements are reactants ‘magical oxygen’ is used to make compounds
- If reactants are compounds it will go back to its elements for ‘underneath’ reaction
- if large product on right arrow will go to many products underneath
What to remember in combustion reactions?
Oxygen is a reactant
What are the parts of an enthalpy level diagram?
Enthalpy in KJ up the side
Time in seconds (s) across the bottom
Activation energy
Overall enthalpy change KJ mol^-1 (+ or -)
What is the calorimetry calculation?
q = mc ΔT
q = heat enthalpy (J) m = mass of water (ml) c = specific heat capacity ΔT = temperature change (degrees C)
What is important to remember about q (heat enthalpy)
You have to put (+ or -) depending on if it was exothermic or endothermic
Look at the ΔT to work out
What is standard enthalpy of neutralisation?
The enthalpy change that accompanies the formation of one mole of water from an acid-base/alkaline reaction with all reactants and products in their standard states and under standard conditions
What are some issues with ‘coffee cup’ calorimetry?
Heat loss
Incomplete reaction
Not in standard conditions
What are some advantages of a bomb calorimeter over a combustion calorimeter?
Well insulated
Pure oxygen = complete combustion
Leads to very accurate combustion values fr food packets
How to calculate ΔT from a graph?
Draw a vertical line up where the reaction started
Draw a line of best fit from the high values to your vertical line (no anomalies) (can be straight or curved)
Draw a line of best fit from the low values to your vertical line
Where your lines of best fit hit the vertical line - ΔT is the difference between those values
If you have to draw a graph to determine ΔT, what features would it have?
Temperature up the side
Time across the bottom
Often broken axis as temp doesn’t need to start at zero
Vertical line where the reaction starts
What do you have to in molar enthalpy of neutralisation calculations?
q = mc ΔT
Then work out the moles (usually from concentration and volume)
Ratio to get the Joules to 1 mole
Convert to KJmol^-1
