mid test term 2 2019

Question 1

Describe Chemical Change

Answer 1

• A chemical change occurs when one substance is transformed into one or more new products via a chemical reaction
• In a chemical change, the number and type of atoms remain constant, but their arrangement is altered
• Most chemical changes are not reversible, except via another chemical reaction

Question 2

What is the law of conservation of mass and an example

Answer 2

• The law of conservation of mass means that mass can neither be created nor destroyed in a chemical reaction. Thus, the amount of matter cannot change
• This is the most important law in chemistry
• For example, when wood burns, the mass of the soot, ashes, and gases, equals the original mass of the charcoal and the oxygen when it first reacted.

Question 3

What is the role of energy in chemical reactions

Answer 3

• All chemical reactions involve energy
• Energy is used to break bonds in reactants and energy is released when new bonds form in products.
• A chemical reaction releases or absorbs energy in the form of heat, light or sound.

Question 4

Describe bond breaking

Answer 4

• Breaking a bond is endothermic. Energy is taken in to break a chemical bond
• When a bond is strong, there is a higher bond energy because it takes more energy to break a strong bond

Question 5

Describe bond forming

Answer 5

• Forming a bond is exothermic. Energy is released when a chemical bond is formed.
• Atoms bond together to form compounds because in doing so they attain lower energies than they possess as individual atoms.
• When atoms combine to make a compound, energy is always given off, and the compound has a lower overall energy.

Question 6

Describe endothermic reactions

Answer 6

• Chemical reactions where the energy of the products is greater than the energy of the reactants
• This means the energy in the chemicals goes up
• This energy comes from the surroundings (environment around chemical reaction) and this makes the surroundings cooler. Temperature goes down

Question 7

Describe exothermic reactions

Answer 7

• Chemical reactions where the energy of the products is less than the energy of the reactants
• This means the energy in the chemicals goes down
• This makes the surroundings hotter. Temperature goes up.
• The energy goes out to the surroundings

Question 8

Define combustion

Answer 8

Combustion (burning) is a chemical process in which fuel reacts with an oxidant to give out heat. The release of heat can produce light in the form of flame

Question 9

How do you identify chemical change

Answer 9

Identified by colour change, temperature change, light, bubbles, precipitate forms, odour and irreversible