Confused on which formula to use where

In summary, the conversation discusses confusion about which formula to use in various situations. The first formula, q = mc(delta)t, is used to find the amount of energy needed to change the temperature of a substance. The second formula, (delta)H = n(delta)Hx, is used to find the net enthalpy change in a reaction. The third formula, n(delta)H = mc(delta)t, is not recommended as it becomes complicated to use with multiple terms. Instead, the combined equation 0 = nHx + mc(delta)T can be used to account for all energy changes in a closed system. It is important to remember that all terms are added, change is calculated as final - initial, exother
  • #1
Roxy
52
0
I keep getting confused on which formula to use where.
q = mc(delta)t
(delta)H = n(delta)Hx
n(delta)H = mc(delta)t

I don't know which ones to use, I keep using the wrong formula.. :cry:
How do I know the difference?
 
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  • #2
Roxy said:
q = mc(delta)t
The first equation there looks like it is Energy Transferred. This should be used to find the amount of energy a substance needs to change a specific degree of temperature.

E.g. 50g of water needs 5212.5J of energy to change 25°C or °K as
E = 4.17 x 50 x 25 = 5212.5J

The others I can't say I have seen before.

The Bob (2004 ©)
 
  • #3
Q : How do you decide, in general, what is the correct formula to use ?

A : Usually, if all the quantities but one, in a given equation/formula, are known (or have been found using other formulae), then the remaining quantity can be determined from this formula. If this is the quantity you are required to determine/calculate, then this is the formula to use.

Do you know what quanties each of the symbols in those formulae represent ?
 
  • #4
To my knowledge the third isn't even correct, enthalpy pertains to open systems and is certainly not equivalent to q, heat.

with closed/isolated systems use the first equation, in gen chem. this is almost exclusively with problems relating to calorimeters.

The second equation is usually used in finding the net standard enthalpy of a reaction.
 
  • #5
Thank you :biggrin:

Gokul43201 - yes I do
 
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  • #6
Roxy said:
q = mc(delta)t
This one says the energy change in a system (heat) is proportional to the temperature change of a material, the mass of the material, and the heat capacity of the material. q is in terms of energy of the object you are looking at, so increasing in temperature means q is positive.

example: A frying pan is cooled from 100 to 0 C and you want to find how much energy was released. Since this involves the amount of energy in the frying pan decreasing, q will be negative.

(delta)H = n(delta)Hx
This one says the energy change in the system is proportional to the molar enthalpy of a reaction times the moles in said reaction. Enthalpy Hx is in terms of stored energy in the compound, so exothermic reactions will have a negative Hx (energy is leaving the compounds in the form of heat). Likewise, H will be negative for exothermic reactions

example: Ethanol has a molar enthalpy of combustion of -1366.8 KJ/mol, what is the total enthalpy change when 5 moles of ethanol is burned? Since combustion is exothermic, H will be negative.


n(delta)H = mc(delta)t
This is a HORRIBLE equation. Whoever told you this wants you to fail. It's technically right, but it becomes impossible to use when you start adding too many terms. Can you imagine what this equation would be if you had 10 things heating and cooling? What would the signs be?

Break it down like this: the sum of energies is always constant. Suppose you have 2 things happening at once. You are burning ethanol in a frying pan, and just suppose that all of the energy is between the ethanol and the frying pan.

ethanol combustion
energy change = nHx

frying pan heating
energy change = mc(delta)T

Remember that I said energy is always constant... then wouldn't that mean the sum of all changes is 0? If I add the two above equations together I get one ultimate equation
0 = nHx + mc(delta)T
This equation will absolutely never fail. The ethanol is burning... so the ethanol itself is actually losing stored energy. The frying pan is exposed the released heat from the burning ethanol, so it's gaining energy.
You can tack an infinite number of terms to that and it will always work. You could have 10 different materials heating and cooling at the same time and 10 chemical reactions happening at the same time, and as long as you add them all together in a closed system, the sum is always 0.

Just remember these things with that above equations
-all terms are always added
-change is always final - initial
-exothermic terms are always negative
-endothermic terms are always positive
 
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  • #7
wow thanks Shawn. It explains a lot :biggrin:
 

Related to Confused on which formula to use where

1. What are the basic steps to determine which formula to use?

The first step is to identify the type of problem you are solving. This will determine the category of formula you need to use. Then, carefully read and understand the given variables and how they relate to each other. Next, look for patterns or key words in the problem that may indicate a specific formula to use. Finally, double check your work and ensure that the formula you have chosen makes sense for the problem.

2. How do I know if I should use a specific formula over another?

It is important to understand the purpose and assumptions of each formula. Some formulas may be more appropriate for certain situations or may only apply to specific types of data. Additionally, the given variables in the problem may only fit into one specific formula. If you are still unsure, it is always best to consult a reliable source or seek help from a mentor or teacher.

3. Can I use multiple formulas to solve a problem?

In some cases, using multiple formulas may be necessary to fully solve a problem. However, it is important to make sure that the formulas are compatible and do not contradict each other. Also, be cautious of using too many formulas as it may lead to errors or confusion.

4. What should I do if I cannot find a formula to solve my problem?

If you are unable to find a specific formula, try breaking down the problem into smaller parts and see if there are any formulas that can be applied to each part. You can also try searching for similar problems or consulting with a colleague or mentor for assistance.

5. How can I improve my ability to choose the right formula?

Practice and familiarize yourself with different formulas and their applications. Understand the concepts and principles behind each formula to better determine when it is appropriate to use them. It is also helpful to keep a list or reference guide of commonly used formulas for quick reference.

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