Unit 3.2.1 - Energetics Flashcards
Define what an exothermic reaction is?
A reaction that gives out energy to their surroundings, usually in the form of heat, causing the temperature to increase
Is the ΔH negative or positive for an exothermic reaction and why?
Negative as the products have less energy than the reactants as energy is given out
Define what an endothermic reaction is?
A reaction that takes in energy from its surroundings, usually in the form of heat, causing the temperature to decrease
Is the ΔH negative or positive for an endothermic reaction and why?
Positive as the products have more energy than the reactants as they take in energy
Give an example of an exothermic and endothermic reaction?
Exothermic - Oxidation
Endothermic - Thermal Decomposition
What is the definition of enthalpy change for a reaction?
The heat energy change per mole at a constant pressure
What is the definition of standard enthalpy change of a reaction?
The heat energy change per mole of a reaction under standard conditions of 100KPa and 298K with all the reactants and products in their standard states
What is the definition of standard enthalpy change of ormation?
The enthalpy change when 1 mole of a compound is formed from its elements under standard conditions of 100KPa and 298K with all the reactants and products in their standard states
What is the definition of standard enthalpy change of combustion?
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions of 100KPa and 298K with all the reactants and products in their standard states
What is the name of an experiment to measure enthalpy change ?
Calorimeter
Name two calorimeter experiments you can do?
Coffee cup and copper calorimeter
Describe what happens and what you measure during a coffee cup calorimeter?
You add one reactants to an insulated container and measure its temperature for three mins, at the forth minute you add the other reactant and stir and then measure the temperature again at the fifth minute up to 10 mins. Using this you can calculate the temperature change.
Describe what happens and what you measure during a copper colorimeter?
You add a certain amount of water to a container, and burn a known mass of a reactant underneath it, using the heat to heat up the water. You then measure the temperature change throughout the experiment.
During a coffee cup calorimeter why can’t you measure the temperature when you first mix the two reactants together?
Because this is when the reaction will first start to happen and so the temperature would be increasing at such a fast rate that it would be impossible to get an accurate reading
What are the two main causes of uncertainty in any calorimeter experiment?
- ) Heat loss
2. ) Incomplete reaction
What do ΔH and ΔT stand for?
Enthalpy change and Temperature change
How can you calculate the energy taken in or given out once you have the temperature change from a calorimeter experiment?
q=mcΔT
Once you have calculated q from q=mcΔT how can you calculate ΔH?
As the q you get is in joules you first have to divide it by 1000 to get kj
Then you can use the equation: ΔH = kj/moles
What does Hess’s Law state?
The total enthalpy change of a reaction is always the same and is independent of the route taken
Draw a Hess cycle for the enthalpy change of combustion?
See flash card 1
Draw a Hess cycle for the enthalpy change of formation
See flash card 2
Is breaking bonds an exothermic or endothermic process?
Endothermic as energy is taken in
Is forming bonds an exothermic or endothermic process?
Exothermic as energy is released
In terms of bond strength describe an exothermic reaction?
Less energy is required to break the bonds in the reactants than is released when the bonds are formed in the products. So overall energy is released
