chemistry - rates of reaction

on a graph what do we put on the x-axis?

time and quantity of product formed goes on y-axis

what does the solpe of the line tell us?

gives us an idea of the rate of reaction

if the slope is steeper is the reaction faster or slower?

faster

why is the slope in a graph more steep at first becomes less steep?

a large number of reactant molecules so lots of them are reacting and forming product

what are the equations of mean rate of reaction?

- quantity of reactant used/time taken

- quantity of product formed/ time taken

what are the units of rate of reaction?

g/s, cm³/s, or mol/s.

what are the factors that affect the rate of reaction? (Can't Penny Say Thanks)

- the concentrations of reactants in solution

- the pressure of reacting gases

- the surface area of solid reactants

- the temperature and the presence of catalysts.

what are the two conditions that must be met for a chemical reaction to take place?

- Reacting particles must collide with each other

- They must collide with sufficient energy

What is activation energy?

The minimum amount of energy that particles must have in order to react.

Why does increasing the concentration, pressure, or surface area increase the rate of reaction?

It increases the frequency of collisions (more collisions happen every second).

What two things happen to particles when you increase the temperature?

- It increases the frequency of collisions (particles move faster).

- It makes the collisions more energetic (more particles have energy greater than the activation energy).

Why must you use the phrase "frequency of collisions" instead of just "more collisions"?

"Frequency" specifies that more collisions are happening per second / within a given time, which is what actually changes the rate.

what is a catalyst and what happens to it during a reaction?

a substance that increases the rate of reaction but is not used up (remains chemically unchanged).

how do catalysts increase the rate of reaction?

they provide an alternative reaction pathway that has a lower activation energy.

how can you identify a catalyst in a chemical exam question?

- It speeds up the reaction rate.

- It is not included in the chemical equation.

What are biological catalysts called?

Enzymes

What is a reversible reaction and what symbol represents it?

A reaction where the products can react to reform the original reactants. It uses this symbol.

If a reversible reaction is exothermic in the forward direction, what is it in the reverse direction, and what can be said about the energy transferred?

It is endothermic in the reverse direction. The same amount of energy is transferred in both directions.

What two conditions are required for a reaction to reach equilibrium?

- It must take place in a closed system (apparatus that prevents the escape of reactants and products).

- The forward and reverse reactions must occur at exactly the same rate.

State Le Chatelier's Principle.

If a system is at equilibrium and a change is made to the conditions, the system responds to counteract the change.

If you increase the concentration of a reactant, how does the equilibrium respond?

It shifts to the right to make more products (until equilibrium is reached again).

If you increase the temperature of a system at equilibrium, which reaction direction is favoured?

The endothermic reaction (to take in the extra heat energy and cool the system down).

If you decrease the temperature of a system at equilibrium, which reaction direction is favoured?

The exothermic reaction (to release heat energy and warm the system back up).

If you increase the pressure of a gaseous reaction at equilibrium, which side does the position of equilibrium shift toward?

The side with the smaller number of gas molecules (as shown by the balanced symbol equation).

If you decrease the pressure of a gaseous reaction at equilibrium, which side does the position of equilibrium shift toward?

The side with the larger number of gas molecules.

How is surface area to volume ratio affected when a reacting solid is broken into smaller pieces, and how does this affect the rate?

The surface area to volume ratio increases. This exposes more surface atoms, increasing the frequency of collisions and increasing the rate

If a factor (like concentration) doubles the frequency of collisions, what simple idea about proportionality explains what happens to the rate of reaction?

The rate of reaction is directly proportional to the collision frequency (if collision frequency doubles, the rate of reaction doubles)

How do you know a catalyst is not a reactant or product when looking at a chemical equation?

It is not included in the chemical equation for the reaction (it is sometimes written above the reaction arrow instead).

If a reversible reaction transfers 100 kJ of energy to the surroundings in the forward exothermic direction, how much energy is transferred in the reverse endothermic direction?

Exactly 100 kJ is taken in from the surroundings (the same amount of energy is transferred in each case).

When drawing a reaction profile diagram, how do you show the effect of adding a catalyst?

You draw a second curve (usually a dashed line) connecting the reactants to the products that has a lower peak (lower hump) than the uncatalysed reaction line. This lower peak represents the reduced activation energy

In the required investigation of catalyst activity using hydrogen peroxide, what type of compounds are added as catalysts, and how do you measure which one is the most effective?

- You add different metal salts (transition metal compounds).

- The most effective catalyst produces the highest volume of oxygen gas per second (highest frequency of bubbles / fastest rate of reaction).

What two pieces of apparatus can you use to accurately measure the rate of gas production when investigating catalysts like the decomposition of hydrogen peroxide?

- A gas syringe (to measure volume).

- A digital mass balance and a conical flask plugged with cotton wool (to measure mass loss).

if the forward direction of a reversible reaction releases heat to the surroundings (feels hot), what type of reaction is it, and what will the reverse reaction feel like?

The forward reaction is exothermic (releases heat). The reverse reaction will be endothermic and will absorb heat (feels cold).