3.2.1 Enthalpy Changes Flashcards
What’s enthalpy
The heat change in a reaction (hotter/colder)
Define enthalpy change (🔺H)
Heat energy change at constant pressure when a chemical process occurs in molar amounts
What does the triangle symbol mean
Change in
What’s the standard enthalpy change
Enthalpy change occurring under standard conditions
298K (25*c room temperature)
100kPa (1 atm)
What’s the units of standard enthalpy change
Jmol-1/KJmol-1
When is an element/compound in its standard state
E.g
When it’s under standard conditions
E.g H2O(l), CO2(g), H2(g)
What do exothermic reactions do as they proceed
E.g
Give out heat
E.g neutralisations, combustions
What symbol shows reaction taken place under standard conditions
Circle with line through it
What symbol shows reactions is exothermic
Why
A minus (-) As heat energy is lost to the surroundings (negative absorption)
What’s an exothermic enthalpy diagram always like
Reactants always higher than products
What do endothermic reactions do
Absorb heat
What type of reactions are most common
Exothermic
What symbol shows the reaction is endothermic
Why
Plus (+)
As heat energy is being gained (absorbed) by chemicals
What’s an endothermic diagrams always like
Reactants always lower enthalpy than products
What is the method used to find the enthalpy change in a chemical reaction
Calorimetry
Why can we use water to measure heat changes
Due to knowing specific heat capacity
Define specific heat capacity
Amount of energy needed to raise temperature of 1kg of substance by 1*c
What’s the specific heat capacity of water
4.18Jg-1K-1
What equation can we use to work out the amount of heat released/absorbed by the reaction if we find out the temperature change
q = mc🔺t
q - heat energy (J)
m - mass of water (g)
c - SHC of water
🔺t - change in temp (K)
2 ways water can absorbed heat from a reaction
Directly
Indirectly
What’s a direct method of water absorbing heat
Chemicals are dissolved in water + transfer their energy directly to water causing temperature changes
What’s an indirect method of water absorbing heat
Chemicals aren’t in direct contact with water, but heat is transferred through the surrounding air to the water, causing a temperature chance
What’s a problem with an indirect method of heating water
It’s inefficient
What are the 2 direct reactions
- Calculating standard enthalpy change of a MASH reaction
2. Calculating standard enthalpy neutralisation
What’s a MASH reaction
Metal + Acid -> Salt + Hydrogen
What can we measured in a MASH reaction (direct)
The water temperature rising as heat energy is generated
Equipment for direct MASH reaction
Polystyrene cup Lid HCl (aq) Magnesium metal Thermometer
Method of direct MASH reaction (4)
- Measure + record the mass of magnesium ribbon
- Measure + record the temperature of the HCl(aq) in the cup (with the Mg added)
- Add the metal to the acid + stir gently
- Measure the maximum temperature rise on the thermometer
What type of reaction is the direct MASH reaction
Exothermic
Define 🔺H* neutralisation
The enthalpy change when an acid reacts with a base to form 1 mole of water, under standard conditions and all reactants and products are in standard state
Equipment for 🔺H* neutralisation
2 polystyrene cups and lids
Thermometers
50cm3 HCl
50cm3 NaOH
Method for calculating 🔺H* neutralisation (4)
- Ensure temperatures of each solutions are the same (put in same room overnight)
- Record HCl temperature for 3 minutes every minute
- Add NaOH on 4th minute (don’t take temp reading for this)
- Continue to record temperature every minute
Why should a polystyrene cup be used
Why should a lid be used
Insulating material
Lid prevents heat loss
Ionic equation for 🔺H* neutralisation
H+(aq) + OH-(aq) -> H2O(l)
What’s the indirect method
Calculating 🔺H* combustion of methanol
What can methanol be used as
A fuel
Equipment for indirect combustion of methanol
Clamp + stand
Alcohol burner with methanol
100cm3 water
Beaker (often copper)
Method of indirect combustion of methanol (6)
- Measure + record mass of the spirit burner before heating
- Place 100cm3 water in the copper beaker
- Measure + records water’s temperature
- Light burner and stir the water gently
- Extinguish the burner once temperature has risen by 20*c
- Measure mass of burner after heating
What is more common in the indirect method
Why
Errors like heat loss
as the thermometer is in direct contact with the reaction mixture
Define standard enthalpy of combustion (🔺H*c)
The enthalpy change when 1 mole of an element/compound is completely burned in oxygen under standard conditions, with all reactants and products in their standard states
Define standard enthalpy of formation (🔺H*f)
The enthalpy change at constant pressure when 1 mole of a substance is formed from its elements with all reactants + products in their standard states. All reactants and products are under standard conditions (100kPa, 298K)
Which standard enthalpy is backwards as we know products
Formation
What is Hess’s law
In a reaction, the total enthalpy change is independent of the route taken
4 steps to calculating enthalpy changes using Hess’s law
- Write desired chemical equation
- Write down the data chemical equations
- Construct a cycle with desired equation on top line
- Identify where all arrows start and end + then use Hess’s law
When do arrows point up/down
Down - combustion
Up - formation
When does the sign become positive using Hess’s law
If going in the opposite direction of the arrow
What’s the short cut equation for Hess’s law combustion
Acronym
total🔺Hc reactants - total🔺Hc products
CR-P
Combustion = reactants-products
What’s the short cut equation for Hess’s law formation
Acronym
Total🔺Hf products - total🔺Hf reactants
FP-R
Example of an endothermic reaction
Thermal decomposition
What’s bond dissociation energy
Energy needed to break a particular covalent bond
Define average bond enthalpy
Enthalpy needed to break 1 mole of bonds in gaseous molecules averaged over different molecules
What’s the value of both bond dissociation energy and average bond enthalpy always
Why
Always positive in value
As we need to put energy in to a chemical to break a bond (endothermic)
Bond enthalpy shortcut
Broken - formed
