Rates Of Reactions And Energy Changes

Question 1

How to measure rates of reaction?

Answer 1

Rate of reaction- how quickly a reaction happens.
Rate of reaction = amount of product formed divided by time or amount of reactant used divided by time.
Units of rate depend on what has been measured- they’re in the form ‘amount per time’.
Three ways to measure rate of reaction:
1. Time to form a precipitate
2. Change in mass over time
3. Volume of gas produced over time

Question 2

How do compare rates of reactions?

Answer 2

• steeper lines show a faster rate of reaction.
  -amount of product can be measured in different ways E.g mass, volume of concentration.
• more product can be formed by using more reactant.
• flat lines show the reaction has finished.

Question 3

How to calculate rates from graphs?

Answer 3

• find the rate at a specific point by drawing a tangent to the curve at that point.
• rate = gradient = change in y divided change in x
• use this formula to calculate the rate if the graph is a straight line.

Question 4

What is collision theory?

Answer 4

Reactions happen if particles collide with enough energy.
Activation energy- minimum amount of energy that particles need to react.
Rate depends on:
Collision frequency- the more collisions between particles, the faster the rate of reaction.
Collision energy- the more collisions with at least the activation energy, the faster the rate of reaction.

Question 5

How does temperature affect collisions? How does pressure and concentration affect collisions?

Answer 5

If a high temperature- Particles move faster and collide more frequently with more energy.
Slow rate- cold
Fast rate- hot
Pressure or concentration- more particles in the same volume means more frequent collisions
Fast rate- high pressure/ concentration
Slow rate- low pressure/ concentration

Question 6

How does surface area affect collisions?

Answer 6

Fast rate of reaction if there is more area for particles to collide with- more frequent collisions.
Slow rate of reaction if there are bigger pieces less frequent collisions.
The smaller the piece of solid, the larger the surface area to volume ratio.

Question 7

What are catalysts?

Answer 7

Catalysts- speeds up a reaction without being chemically changed or used up in the reaction, and without changing the products.
Activation energy is lower with catalyst so more collisions have enough energy to react.
Enzymes are biological catalysts.

Question 8

What is endothermic and exothermic?

Answer 8

Endothermic reaction- takes in heat energy from the surroundings (shown by a fall in temperature). Reactants —> products
Exothermic reaction- gives out heat energy to the surroundings (shown by a rise in temperature).
Products —> reactants

Question 9

How to measure temperature change?

Answer 9

You need: thermometer, large beaker, reaction mixture, lid, polystyrene cup, cotton wool (insulation)
- Record initial temperature and maximum/ minimum temp reached
- Then calculate temperature change.
Use this to investigate:
- dissolving salts in water.
- neutralisation
- displacement and precipitation reactions.

Question 10

What is the bond energies in endothermic and exothermic?

Answer 10

Endothermic- bond breaking
Energy used to break bonds is greater than energy released by forming new bonds.

Exothermic- bond forming
Energy released by forming bonds is greater than energy used to break existing bonds.

Overall energy change = total energy needed to break bonds - total energy released by forming new bonds.