El chemistry testo one-o

let's pass ms kearney's chudmistry!! (uncfinished)

Define the term ‘rate of reaction’

Change in concentration per unit time of any one reactant or product.

How to calculate average rate of reaction

Overall change in concentration of a reactant or product over time

Calculate initial/instantaneous rate of reaction

Draw the tangent to the curve at your desired point and find the slope of the line.

Investigate the effect of a catalyst on the rate of a reaction

A catalyst lowers the activation energy required for a chemical reaction to occur without being used up. It is likely to speed up the rate of reaction.

Collision theory

Particles must collide. Particles must have the minimum energy required and must be in the correct orientation for products to form.

Activation energy

The minimum energy colliding particles must have in order to react.

Rate doesn’t just depend on the number of collisions per second but also…

the fraction of which are effective

Effective collision

One that results in the formation of products.

Examples of fast chemical reactions

Silver nitrate and sodium chloride, firework exploding

Examples of slow reactions

Nail rusting, Photosynthesis in plants.

5 Factors that affect rate of reaction

Nature of reactants, Particle size, Concentration, Temperature, Presence of catalysts, Pressure

Using collision theory explain how temperature affects rate of reaction.

Higher temp-faster rate. Faster moving particles-higher chance of collision. More collisions-higher proportion of them will have required activation energy

Using collision theory explain how surface area affects rate of reaction.

Smaller reactant particles expose a greater surface area, which leads to more frequent effective collisions per unit time and a directly proportional, faster rate of reaction

Using collision theory explain how concentration affects rate of reaction.

higher concentration-faster rate of reaction. increased concentration-more particles-increase in collisions-increase in effective collisions- directly proportional

Using collision theory explain how pressure affects rate of reaction.

same effects as increasing concentrations except it is for gases. Increasing pressure-lower volume-increase in the number of particles per unit volume.

How catalyst affects rate of reaction

A catalyst is a substance that alters the rate of reaction by lowering the activation energy but is not consumed by the reaction

General properties of catalysts

Catalysts are recovered unchanged at the end of a reaction(won’t change chemically, might physically). Tend to be specific. Only need to present in small amounts

Enzymes

A substance that is produced by a living cell and acts as a biological catalyst e.g. amylase breaks down starch

3 limitations of collision theory

Orientation, catalysts, concentration

Orientation

Collision theory simplifies the idea of orientation, making it less accurate at predicting rates for reactions involving complex molecules.

Catalysts that alter rate of reaction

Saturated and poisoned

Saturated Catalysts

If the surface of the catalyst is saturated e.g. fully occupied by reactant molecules, increasing conc will have no effect on the ror.

Saturated Catalysts 2

Products must desorb from the surface of the catalyst before more reactants can adsorb.

Poisoned catalysts

Although catalysts can be reused they can be poisoned by poisons that permanently adsorb into the surface of the catalyst. These poisons accumulate and cause it to be less effective or stops it from working entirely

Concentration

An limitation of collision theory is saturated catalysts and limiting reagents.

Concentration(Saturated catalysts)

If a catalyst is saturated, an increase in concentration of one of the reactants doesn’t lead to an increase in ror.

Concentration(Limiting Reagents)

When a reaction involves multiple reactants, just increasing the concentration of one reactant may not alter the ror unless there is a sufficient amount of the other reactants.

Intermediate formation theory

Arrhenius suggested that a catalyst only works by forming an intermediate compound that only exists for a short time and may be difficult to detect. For homogeneous

Intermediate formation theory pt 2

The intermediate then reacts with the other reactant to give the final product and regenerate the catalyst.

Intermediate formation theory example.

Potassium iodide KI to speed up the decomposition of hydrogen peroxide.

Absorption

When one substance moves to another eg. sponge absorbing water

Adsorption

The accumulation of substances only at the surface of another solid. eg. gas molecules adsorbing into the surface of a solid. for heterogenous.

Platinum catalyst

Platinum can catalyse the reaction between hydrogen and oxygen to form water. Reactant molecules adsorb onto its surface.

Adsorption stage

Hydrogen and oxygen molecules settle onto the surface of the platinum catalyst and are held there by temporary bonds.

Vacant d orbitals

Platinum is a transition metal with vacant d orbitals. These allow reactant molecules to form temporary bonds with the catalyst surface.

Reaction on surface

Due to the high concentration, it is more likely molecules will collide. Bonds are broken and new bonds are formed.

Desorption stage

More reactants can then adsorb onto the surface of the catalyst and the process repeats again.

Example of adsorption theory

Hydrogen and oxygen catalyzed by platinum to form water.