Endo&Exothermic
A reaction causes the temperature of the surroundings to increase. Is it exothermic or endothermic?
Exothermic.
Does breaking a chemical bond require or release energy?
Requires energy.
What is activation energy?
The minimum energy that colliding particles must have to react.
In an exothermic reaction, are the products at a higher or lower energy level than the reactants?
Lower.
Dissolving ammonium nitrate in water causes the temperature to fall from 22°C to 15°C. Describe the energy transfer. (from...to...)
Energy is transferred from the surroundings to the reacting system
Does making chemical bonds require or release energy?
Releases energy.
Why does a reaction usually need some energy to start even if it is exothermic?
Reacting particles must overcome the activation energy barrier before reaction can occur.
On a reaction pathway diagram, what does the vertical difference between the reactants and products represent?
The enthalpy change, delta H.
A reaction has delta H=+85 degree Celsius. Is it exothermic or endothermic? Explain what the sign tells you.
Endothermic. Positive delta H means the system takes in energy from the surroundings.
A reaction requires 650 kJ mol⁻¹ to break bonds and releases 820 kJ mol⁻¹ when new bonds form. Calculate delta H.
650-820=-170 kJ mol⁻¹
Two reactions have activation energies of 45 and 95 kJ mol⁻¹. Which one requires less energy to initiate?
The reaction with 45 kJ mol⁻¹.
An energy profile has reactants at 120 kJ mol⁻¹, a peak at 270 kJ mol⁻¹ and products at 80 kJ mol⁻¹. Calculate Ea and Delta H.
Ea=270-120=150; Delta H=80-120=-40kJ/mol
Reaction A has Delta H=-120 degree Celsius, while Reaction B has Delta H=+75 degree Celsius. Compare the direction of energy transfer in the two reactions.
A transfers energy to the surroundings; B transfers energy from the surroundings to the system.
Given: H–H = 436, Cl–Cl = 243, H–Cl = 431 kJ mol⁻¹. Calculate Delta H for {H2 + Cl2 -> 2HCl}.
Bonds broken: (436+243=679). Bonds formed: (2(431)=862). Delta H=679-862=-183 kJ/mol
Explain why increasing temperature increases reaction rate in terms of activation energy.
At higher temperature, a greater proportion of particles have energy equal to or greater than Ea, so there are more successful collisions per unit time.
Sketch an energy profile for an endothermic reaction and label reactants, products, Ea and Delta H.
Products above reactants; curve rises to a peak; Ea from reactant level to peak; Delta H upward from reactants to products.