Define the term ‘Enthalpy’ ?
Enthalpy - The measure of heat energy in a chemical system
Define the term ‘Activation energy’ ?
Activation energy - The minimum amount of energy required for a chemical reaction to occur
What is an exothermic reaction ?
A reaction where energy is given out from the system into the surroundings
What is an endothermic reaction ?
A reaction here energy is absorbed from the surroundings into the system
What is considered the system and the surroundings ?
- The system refers to atoms, molecules and ions that make up chemicals
- The surroundings refer to everything beside the system
Draw an enthalpy profile for an exothermic reaction ?
Draw an enthalpy profile for an endothermic reaction ?
What standard conditions are standard enthalpy changes measured under ?
- Standard temperature of 298K or 25oC
- Standard pressure of 100 kPa
- Standard concentration of 1moldm^-3
- Standard state which is the physical state of a substance under standard conditions
Define the term ‘Enthalpy change of formation’ ?
Enthalpy change of formation: The enthalpy change when one mole of any substance is formed from its element in their natural state, measured at STP
Define the term ‘Enthalpy change of combustion’ ?
Enthalpy change of combustion : The enthalpy change when one mole of a substance is completely combusted ( a plentiful supply of oxygen ) at STP
Define the term ‘Enthalpy change of neutralisation’ ?
Enthalpy change of neutralisation : The enthalpy change when an acid and base react to form one mole of water measured at STP
Define the term ‘Enthalpy change of reaction’ ?
Enthalpy change of reaction : The enthalpy change when molar quantities ( moldm^-3 ) of reactants , as written in balanced equation, react under STP
How can enthalpy changes be calculated ?
- It can be calculated by observing the temperature change of some water placed above or around the reaction
- It can be calculated by observing the temperature change of the solution
Why may the calculated enthalpy change of combustion for a given experiment be inaccurate ?
- Heat loss to the surroundings other than the water.
- Incomplete combustion of reactant
- Evaporation of reactant from the wick.
- Non-standard conditions ( The data book value is a standard value. The conditions for this experiment are unlikely to be identical to standard conditions )
How can an experiment measuring the enthalpy change of combustion be improved ?
- Drought screens to minimise heat loss to the surroundings other than water
- Input of oxygen gas to ensure complete combustion od reactants
What assumptions are made when calculating enthalpy changes ( investigating temperature difference of solution ) ?
- Density of solution is 1gcm^-3 / the same as water
- The specific heat capacity of solution/ c is the same as that of water
- Ignore the mass/ specific heat capacity of reactant ( eg Zinc ) added to solution
- No heat is lost to the surroundings
- Reaction goes to completion
- Reaction is completed under standard conditions
Explain how improvements to experiment can be made when calculating enthalpy changes ( investigating temperature difference of solution ) ?
- Polysterne cup/ thermos flask
- Use a lid
- High resolution/ more precise thermometer
- Higher resolution/ more precise balance
- Measure mass of solution
- Use a cooling curve to achieve accurate temperature change
- Use standard conditions
If all apparatus were to be kept the same, how could the experiment be modified ( measuring temp of solution ) to reduce percentage error ?
- Use a greater mass of reactant ( eg. Nv2Ov2 )
- This will result in a greater temperature change which will reduce percentage error
what are the equations to calculate the enthalpy change ?
and 1/q x n when scaling up for one mole
Do you know how to calculate the enthalpy change of a reaction ?
Yes
What are the units for enthalpy change ?
kJ/mol or kJmol^-1
Define the term ‘average bond enthalpy’ ?
Average bond enthalpy : The breaking of one mole of bonds in gaseous molecules
How is energy involved in bond breaking/ making ?
- Energy is required to break bonds, bond breaking is endothermic and results in a positive enthalpy change
- Energy is released when bonds form, bond making is exothermic and results in a negative enthalpy change
Explain what an exothermic reaction is in terms of energy involved in breaking/ making bonds ?
During an exothermic reaction, less energy is required to break bonds in reactants than is used to make bonds in products
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1.1 - Practical skills assessed in a written examination0
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1.2 - Practical skills assessed in the practical endorsement0
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2.1 - Atoms, compounds, molecules and equations35
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2.2 - Amount of substance26
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2.3 - Acid–base and redox reactions29
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2.4 - Electrons, bonding and structure42
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2.5 - Shapes of molecules and intermolecular forces47
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3.1 - The periodic table and periodicity71
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3.2 - Group 2 and the halogens52
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3.3 - Qualitative analysis22
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3.4 - Enthalpy changes32
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3.5 - Reaction rates and equilibrium (qualitative)38
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4.1 - Basic concepts44
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4.2 - Hydrocarbons37
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4.2 - Alkenes52
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4.3 - Alcohols and haloalkanes53
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4.4 - Organic synthesis12
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4.5 - Analytical techniques (IR and MS)17
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5.1 - Reaction rates and equilibrium (quantitative)71
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5.2 - pH and buffers60
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5.3 - Enthalpy, entropy and free energy48
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5.4 - Redox and electrode potentials57
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5.5 - Transition elements64
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6.1 - Aromatic compounds59
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6.2 - Carbonyl compounds29
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6.3 - Carboxylic acids and esters46
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6.4 - Nitrogen compounds49
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6.5 - Polymers20
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6.6 - Organic synthesis23
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6.7 - Chromatography and spectroscopy (NMR)13
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3.3 - Qualitative analysis COPY20
