Energy Changes - Topic 7 Flashcards
Which changes do changes in heat energy accompany?
- Salt dissolving in water
- Neutralisation reactions
- Displacement reactions
- Precipitation reactions
- When these reaction take place in solution, temperature solution can be measured to reflect the heat changes
Exothermic reaction:
- a reaction where heat energy is given out to the surroundings so the temperature of the surroundings increases
- makes bonds
- temperature increases as chemical energy → thermal/heat energy
Endothermic reaction:
- a reaction where heat energy is taken in from the surroundings so the temperature of the surroundings decreases
- breaks bonds
- temperature decreases as thermal/heat energy → chemical energy
How do you know if the overall heat energy change for a reaction is exothermic?
- E is needed to break bonds and E is released when bonds are formed
- the overall heat energy change for a reaction is exothermic if more heat is released in forming bonds in the products than is required in breaking bonds in the reactants
How do you know if the overall heat energy change for a reaction is endothermic?
- E is needed to break bonds and E is released when bonds are formed
- the overall heat energy change for a reaction is endothermic if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants
Activation Energy:
- the minimum amount of energy that particles must have for a reaction to take place
- it is the minimum amount of energy with which the particles need to collide with to make the collisions successful
What happens if particles do not reach the activation energy?
The particles may still collide, but they will simply just bounce of each other
Is the activation energy different for each experiment?
yes
What do all substances have?
all substances have chemical energy - a form of stored or potential energy
How does energy change in chemical reactions?
when chemical reactions occur and new substances are formed with different amounts of energy - energy can be exchanged with the surroundings and can be released or absorbed as heat
Enthalpy change:
- The difference between the energy of the reactants and the energy of the products - approximately the amount of energy lost or gained during the reaction
- ΔH (“delta H”)
What is the energy change for an exothermic reaction?
- energy change is negative
- as more heat energy has been released
- negative ΔH
What is the energy change for an endothermic reaction?
- energy change is positive
- as heat energy has been absorbed/taken in
- positive ΔH
How do you calculate the energy needed to break bonds and the energy released when bonds are formed?
the energy needed to break bonds and energy released when bonds are formed
can both be calculated from bond energies
How do you calculate the overall energy change for a reaction?
overall energy change = sum of energy taken in to break bonds – sum of energy released to form bonds
How can you tell from the energy change whether a reaction is exothermic or endothermic?
- if the energy out > energy in, the energy change will be negative showing an exothermic reaction
- if the energy out < energy in, the energy change will be positive showing an endothermic reaction
Overall energy change of the reaction:
the difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed
Reaction profile:
- graphs used to show the relative energies of reactants and products
- the activation energy and the overall energy change of a reaction
Energy profile for exothermic reaction pathway:
Energy profile for endothermic reaction pathway:
Energy profile for reaction using catalyst:
How do you compare the temperature changes of a neutralisation reaction when 2 different acids are used to neutralise an alkali solution?
- measure known volume of alkali solution
- add same volume of each of the acids
- stir the mixture
- record the initial and final temperatures/temperature change
