3 Energetics Flashcards
What is an exothermic reaction?
It is a reaction in which heat energy is given out to the surroundings.
What is an example of an exothermic reaction?
Adding water to calcium oxide. If you add water to solid calcium oxide, the heat produced is enough to boil the water and produce steam. Calcium hydroxide is produced.
What has more chemical energy in an exothermic reaction, the reactants or the products?
The reactants.
Why do the reactants have more chemical energy?
This is because the chemical energy in the reactants is converted to heat energy during the reaction, which is released to the surroundings. This means that the temperature of the mixture and its surroundings go up.
What is a diagram that shows how heat is lost during an exothermic reaction?
What are some examples of exothermic reactions?
- Any reaction in which a flame is produced (combustion).
- The reactions of metals with acids.
- Neutralisation reactions.
- Displacement reactions.
What is an example of combustion?
When hydrogen burns in oxygen - producing water and lots of heat.
What is an example of a reaction of metals with acids?
When magnesium reacts with dilute sulfuric acid - the mixture gets very warm.
What is an example of a neutralisation reaction?
When sodium hydroxide solution reacts with dilute hydrochloric acid - the temperature rises.
What is an example of displacement reactions?
The reactions between powdered aluminium and iron oxide, this reaction releases a large amount of heat.
How we measure the amount of heat energy taken in or released in a chemical reaction?
By the enthalpy change.
What is the enthalpy change?
It is the amount of heat energy taken in or given out in a chemical reaction. IT is the difference between the energy of the products and the energy of the reactants.
What symbol is given to the enthalpy change?
ΔH.
How do you show whether heat is being given out or absorbed by the reaction?
By using + and - signs following the answer found for ΔH.
How do we determine whether or not we put a plus or a minus sign?
You look at it from the point of the reactants.
When do we use a - sign?
When the reactants lose energy.
When do we use a + sign?
When the reactants gain energy.
What are the units for ΔH?
KJ/mol (kilojoules per mole).
What is an an endothermic reaction?
A reaction that absorbs heat from the surroundings is said to be endothermic.
What has more chemical energy in an endothermic reaction, the reactants or the products?
The products.
Why do the products have more chemical energy?
In order to supply the extra energy that is needed to convert the reactants (lower energy) to the products (higher energy) - heat energy needs to be absorbed from the surroundings. This heat energy is then converted to chemical energy which is then stored in the bonds of chemicals.
What happens to the temperature of the reaction mixture and surroundings?
They go down because heat energy has been converted into a different form of energy.
What is the specific heat capacity?
It is the amount of heat needed to raise the temperature of 1g of substance by 1*C.
What is the amount of heat energy required directly proportional to?
The mass and the temperature change.
How do you represent the temperature change?
ΔT.
What is the equation for energy change?
Q = m x c x ΔT
What is Q in this reaction?
The heat energy change.
What is m in this reaction?
The mass.
What is c in this reaction?
Specific heat capacity.
What is ΔT in this reaction?
The temperature change?
What is the value that we use for specific heat capacity?
4.18J/g/c. Which is the specific heat capacity of water.
How can we measure the amount of heat absorbed or given out in reactions or physical changes?
By using calorimetry.
What is calorimetry?
It is measuring the heat given out or taken in by a chemical reaction.
What is the idea of calorimetry based on?
That if we use the heat from a reaction to heat another substance, such as water, we can then use the equation for heat energy change to calculate the amount of heat released.
What is ΔH?
The molar enthalpy change.