3b.2 - Energy Flashcards
What is an exothermic reaction? (2)
- Gives out energy
- Shows with rise in temperature
What is an endothermic reaction? (2)
- Takes in energy
- Shows with decline in temerature
What happens during a chemical reaction?
Old bonds are broken and new bonds are made
Is bond breaking exothermic or endothermic?
Endothermic
Why is endothermic reactions bond breaking?
Energy must be supplied to break existing bonds
Is bond making exothermic or endothermic?
Exothermic
Why is bond making exothermic?
Energy is released when bonds are made
What makes an endothermic reaction an endothermic reaction?
The energy required to break old bonds is greater than the energy released when new bonds are formed
What makes an exothermic reaction an exothermic reaction?
The energy released in bond formation is greater than the energy used in breaking old bonds
How can energy transfer be measured? (3)
- Taking the temperature of the reagents and making sue they’re the same
- Mixing them in a polystyrene cup
- Measuring the temperature of the solution at the end
What is the biggest problem with measuring energy?
Energy is lost to the surroundings
How can you reduce the amount of energy lost to surroundings when measuring energy transfer? (2)
- Insulating the polystyrene cup
- Put a lid on it to stop evaporation
What is fuel energy energy calculated using?
Calorimetry
What is the method of calculating fuel energy? (6)
- Put 50g water in the copper can and record it’s temperature
- Weigh the spirit burner and lid
- Put spirit burner underneath the can, and light the wick
- Stir the water, stirring constantly until it reaches about 50C
- Put out the flame using the butner lid and measure the final temperature of the water
- Weigh the spirit burner and lid again
What is the equation for the amount of energy transferred?
Energy transferred (J)= Mass of water (g) x specific heat capacity (C) x change in temperature (degrees)
What type of reaction has a negative energy change?
Exothermic
What does the energy level diagram look like for an exothermic reaction?
- Straight line at top of graph
- Curve up a little
- Falls
- Line
What the initial rise of the line represent on an energy level diagram?
The activation energy
What type of reaction has a positive energy change?
Endothermic
What does the energy level diagram look like for an endothermic reaction?
- Straight line at bottom of graph
- Curve up
- Decreases a little
- Line
What does the difference of height represent on an endothermic energy level diagram?
Energy taken in per mole
What is the activation energy?
Minimum amount of energy needed to break the bonds between reacting particles
Using known bond energies calculate the energy change for this reaction: H---H + Cl---Cl -> 2H---Cl H2 + Cl2 -> 2HCl Bond energies: H---H: +436 kJ/mol Cl---Cl: +242 kJ/mol H---Cl: +431 kJ/mol
1) Calculate how much energy to break bonds:
436 + 242 = 678 kJ
2) Calculate how much energy to make bonds:
2 x 431 = 862 kJ
3) Overall energy change:
Energy to break - energy to make
678 - 862 = -184 kJ/mol
Why is hydrogen used to power vehicles?
- Burns to form water
- Exothermic reaction so gives out energy
