Ch - 7 (Chemical Energetics) Flashcards
When do exothermic and endothermic reactions take place?
Chemical bonding
What does chemical bonding involve?
The transfer of thermal energy in or out.
What are the 2 key terms involved in the transfer of thermal energy?
- System (What happens in the chemical reaction)
- Surroundings (anything other than the chemicals reacting)
What happens in an exothermic reaction?
- Thermal energy is transferred from the system to the surroundings.
What happens to the surrounding temp. in an exothermic reaction?
Temp. increases
Why is the energy change in an exothermic reaction negative?
- The energy is transferred from the chemical energy store of the chemical system to the surroundings
- Energy of the system falls.
What are examples of an exothermic reaction?
- Combustion
- Oxidation
- Neutralisation reactions.
- Handwarmers
- Self-heating cans of food n drinks
What happens in an endothermic reaction?
Thermal energy is transferred from the surroundings into the systems
why is the energy change in an endothermic reaction positive?
This energy is transferred to the chemical energy story of the chemical systems
- Energy of the system increases.
what are examples of endothermic reactions?
- Electrolysis
- Thermal decomposition reactions
- Photosynthesis First stages
- Cold packs
What are reaction pathways diagrams or energy level diagrams?
Graphical representation of relative energies of the reactants and products in chemical reactions.
What represents the overall energy change of a reaction?
The diff. in height between the energy of reactants and products
What represents the activation energy (Ea)?
The initial increase in energy.
Define Ea.
Minimum energy that colliding particles must have in order to react.
How can we identify a reaction pathway diagram for an exothermic reaction?
- Energy of the product is lower than the reactants
- (As thermal energy has been transferred to the surroundings)
How can we identify a reaction pathway diagram for an endothermic reaction?
- The energy of the product is higher than the reactants.
- (As thermal energy has been taken in from the surroundings.
What is an easy way to understand the direction of thermal energy in endo n exothermic reactions?
EXothermic - EXits
ENdothermic - ENters
What must atoms or particles do to react with each other in a chemical system?
come into contact in a collision
What factors come into play when analyzing collisions?
- Energy
- Orientation
- No. of collisions per sec (Frequency)
What is the transfer of thermal energy during a reaction called?
Enthalpy change, delta H
Which reaction is bond breaking?
Endothermic (energy needs to be taken in to break bonds)
What reaction Is bond making?
Exothermic reaction (Energy is transferred to the surroundings as the new bond is formed)
what is it called if more energy is released than absorbed?
Exothermic
what is it called if more energy is absorbed than released?
Endothermic
What is the formula for enthalpy change? (delta H)
Enthalpy change(^H) = Energy taken in - Energy given out.
What is bond energy calculations useful for?
used to calculate how much heat is released or absorbed.
what is the method to solve bond energy calculations?
- Write the balanced equation
- Draw displayed formula to identify type & no. of bonds (Optional)
- Add bond energy in reactants (Energy in)
- Add bond energy in products (Energy out)
- Calculate enthalpy change.
What are physical changes?
- Such as melting or evaporating.
- Do not produce any new chemical system.
- Easy to reverse.
What are chemical changes?
- New chemical substances are formed.
- have different properties to reactants
- cannot be reversed
What are signs to identify a new substance is formed in chemical change?
- color change
- precipitate
- bubbles of gas being formed
