Energy Changes Flashcards
(36 cards)
What is the conservation of energy?
The amount of energy in the universe is the same before and after a chemical reaction takes place - it is conserved
What is an exothermic reaction?
A reaction that transfers energy to the surroundings so the temperature of surroundings increases
Give examples of exothermic reactions?
Combustion, many oxidation reactions, neutralisation
What is an endothermic reaction?
A reaction that takes in energy from the surroundings so the temperature of the surroundings decreases
List examples of endothermic reactions
Thermal decomposition, reaction of citric acid and sodium hydrogencarbonate
How much energy do the molecules have before and after a reaction if it is exothermic?
Product molecules have less energy than the reactants, as some of the energy has been transferred to the surroundings
What are some everyday uses of exothermic reactions?
Self-heating cans and hand warmers
What is an everyday use of endothermic reactions?
Some sports injury packs
Describe the practical for measuring energy change
e.g. neutralisation in polystyrene cup
1. Measure volume of hydrochloric acid with pipette or measuring cylinder
2. clamp thermometer into place making sure bulb is immersed in liquid
3. Measure initial temp
4. add 5cm^3 sodium hydroxide
5. stir and measure temp
6. Do this until clear decrease in temp
For measuring energy change, why do we use a polystyrene cup in a beaker?
- good thermal insulator so it helps reduce heat loss or gain from the surroundings
- beaker is used for support
- polystyrene is also cheap, doesn’t react with most chemicals and is disposable
Explain this graph
What is required for chemical reactions to occur?
Reacting particles must collide with each other and with sufficient energy
What is the activation energy/
The minimum amount of energy required for particles to react
Why do we use reaction profiles?
They show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction
What is energy used for in a chemical reaction?
- energy must be supplied to break bonds in the reactants
- energy is released when bonds in the products are formed
What is true of exothermic reactions concerning energy and bonds?
In an exothermic reaction, the energy released from forming new bonds in greater than the energy required to break existing bonds
What is true of exothermic reactions concerning energy and bonds?
In an endothermic reaction, the energy required to break existing bond is greater than the energy released from forming new bonds
What is the overall energy change in a 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
What is the calculation for energy change of a reaction?
Energy change of reaction = sum of bonds energies broken (reactants) – sum of bonds energies made (products)
What are cells
Devices containing chemicals which react to produce electricity
What is the voltage a cell is dependent on?
Number of different factors including type of electrodes, difference in reactivity of electrodes and the electrolyte
What is the relationship between the reactivity of the electrodes and the voltage produced in a cell?
The larger the difference in reactivity between the two metal electrodes, the larger the voltage
Why can’t non-rechargeable cells be recharged?
The chemical reaction stop when one of the reactions has been used up
Can alkaline batteries be recharged?
No, they are non-rechargeable as the chemical reactions stop when one of the reactants has been used up
